polynuclear niii and mnii azido bridging complexes. structural trends and magnetic behavior
DESCRIPTION
Review on MnII and NiII azidesTRANSCRIPT
CoordinationChemistryReviews193195(1999)10271068PolynuclearNiIIandMnIIazidobridgingcomplexes. Structural trendsandmagneticbehaviorJoanRibasa,*, AlbertEscuera, MontserratMonforta,RamonVicentea, RobertoCortesb, LuisLezamab,Teo loRojobaDepartamentdeQu micaInorga`nica, Uni6ersitatdeBarcelona, Diagonal, 647, 08028-Barcelona,SpainbDepartamentodeQu micaInorganica, Uni6ersidaddelPaisVasco, Apartado, 644,48040-Bilbao,SpainReceived26October1998; receivedinrevisedform8December1998; accepted15February1999ContentsAbstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10281. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10282. Dinuclearcomplexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10302.1 With1,3-azidobridgingligands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10302.2 With1,1-azidobridgingligands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10353. Trinuclearcomplexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10374. Tetranuclearcomplexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10395. One-dimensional uniformsystems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10405.1 With1,3-azidobridgingligands(AF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10415.2 With1,1-azidobridgingligands(F) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10466. One-dimensional alternatingsystems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10476.1 Withonly1,3-N3bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10476.2 With1,3-N3and1,1-N3bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10506.3 Otheralternatingchains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10557. Two-dimensional systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10567.1 Withonlyazidoasbridgingligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10567.1.1 WithsingleEEbridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1056www.elsevier.com/locate/ccr* Correspondingauthor. Tel.: +34-93-4021264; fax: +34-93-4907725.E-mail address: [email protected](J. Ribas)0010-8545/99/$-seefrontmatter1999ElsevierScienceS.A. All rightsreserved.PII: S0010- 8545( 99) 00051- X1028 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)102710687.1.2 EEandEObridges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10587.2 Compoundswithazideandasecondbridgingligand . . . . . . . . . . . . . . . . . . . 10608. Three-dimensional systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1061Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065AbstractThe azide anionis a goodbridging ligandfor divalent metal ions, mainly CuII, NiIIandMnII. It maygiveend-to-end(1,3) orend-on(1,1) coordinationmodes. Asageneraltrend, the1,1modeexhibitsferromagneticcouplingwhilethe1,3modecreatesantiferro-magnetic coupling. This reviewfocuses on polynuclear NiIIand MnIIazido bridgingcomplexes. Polynuclearstructuresknowntohavethesetwocationsare: discrete(normallydinuclear), one-, two- andthree-dimensional nets. Themaincharacteristicsof thesestruc-turesarereportedtogetherwiththeirmagneticbehavior. Fromalargenumberof knownstructures, magneto-structural correlations are made. Taking into account that MN3distancesarealwayssimilar, theangleswithintheM(N3)nMunitarethemaindetermi-nant of the type andmagnitude of the exchange coupling. Moreover, some one-, two-andthree-dimensional complexes exhibit cooperative effects (long-range magnetic order),behavingas molecular magnets. This behavior is alsoanalyzed. 1999Elsevier ScienceS.A. All rightsreserved.Keywords: Polynuclear NiIIand MnIIazido bridging complexes; Magnetic behavior; Molecular magnets1.IntroductionAmarkedevolutionhas takenplace recently inthe eldof molecular mag-netism. Research has shifted focus fromthe synthesis and study of discretepolynuclearmoleculesinanattempttoimproveourunderstandingofthemecha-nisminvolvedinmagnetic coupling [1,2] andthe productionof newparamag-neticclusterswithhighspinandstronganisotropy(superparamagneticmolecules)[3,4] tothesynthesisandcharacterizationofnewlow-dimensional materials(one-or two-dimensional) that may showferromagnetic long-range cooperative phe-nomena(molecularmagnets)[59].Pseudohalide anions are excellent ligands for obtaining discrete, one-dimen-sional, two-dimensional or three-dimensional systems. Among these, the azidoligandisthemostversatileinlinkingdivalentmetal ions. IfweconsiderNiIIandMnIIions, a large number of polynuclear MIIcomplexes with azido bridgingligands has been reported in the literature. These complexes can be dividedaccording to their dimensionality: discrete molecules, one-dimensional, two-di-mensional andthree-dimensional systems. WhentheN3anionacts as bridgingligand there are two typical coordination modes: end-to-end (EEor 1,3) andend-on(EOor1,1).1029 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Normally, theend-to-end(EE)coordinationmodegivesantiferromagneticcou-pling while the end-on (EO) gives ferromagnetic coupling, but for very largeM-m1,3-N3Mbond angles the magnetic coupling may be reversed. When theazido bridges are exclusively end-to-end or end-on it does not mean that allbridgesarestructurallyandcrystallographicallyidentical. Withinthesamesystem(one or two-dimensional) two or more different structural bridges may bepresent, owingtodifferent distancesand/oranglesfoundinthestructure. Thesesystemsareantiferro-orferromagnetic,buttheybelongtotheso-calledalternat-ing magnetic systems, indicating the needtointroduce twoor more exchangecouplingparameters(J) soastot theexperimental resultsincontrast withtheuniformmagnetic systems inwhichall azidobridges have the same coordina-tionmodeandthesamestructural parameters. Finally, forlow-dimensional com-plexes, highly unusual systems containing simultaneously both kinds ofcoordinationmode(EEandEO) havebeenreported. Theyarealsoof consider-able interest in terms of their behavior as alternating ferroantiferromagnetic(FAF) coupled systems. Synthetically it is impossible to determine, with ourpresentstateofknowledge, whichcoordinationmodewill beadopted.Inthisreviewwedividethesesystemsaccordingtotheirnuclearityanddimen-sionality.Ineachcaseweseparate,wherepossible,thereportedcomplexesonthebasis of their magneticbehavior: antiferromagnetic, ferromagneticor alternatingsystems.Theoretically, justwhatconstitutesthemostsuitablemethodforanalyzingthesuperexchange interactionthroughanazidobridge has beenmuchdiscussedinrecent years. The rst approach was undertaken by means of MOextended-Hu ckel calculations [10] andin1986Kahnandcoworkers [11a] notedthat thespinpolarizationmodel woulddescribe the superexchange for the azidoligandmoreaccurately, particularlyfor theEOcoordination. Morerecently, spinden-sitymapsinthetriplet groundstateof [Cu2(t-Bupy)4(m-N3)2](ClO4)2(t-Buty=p-tert-butylpyridine)havebeenpublished[11b] andtheauthorsstatethatthespindistributioninthe triplet groundstate in[Cu2(t-Bupy)4(m-N3)2](ClO4)2is domi-nated by a spin delocalization mechanismto which is superimposed a spinpolarization effect within the p-orbitals of the azido group and can the1,1-azidogroupbeconsideredasanalmostuniversal ferromagneticcoupler, oristhe stabilizationof the parallel spinstate only achievedina limitedrange ofbridging angle values?. However, studies conductedoncoupleddimers [12,13]for EEazidobridges seemtoindicatethat, at least for this coordinationmode,1030 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068MOextended-Hu ckel calculations are appropriate. For this reason, we will usethis model when correlating the magnetic properties. Recently, some ab initiocalculations on these systems have been reported [11b,14]. Magneto-structuralcorrelations have beendevelopedbythe authors [1523] andelsewhere [14,2426].Inorder tocompareall magneticresults reportedintheliteratureit is neces-sary to establish just one Hamiltonian to calculate the exchange couplingparameter (J). Inthe literature, the twomost frequentlyusedareH=JijSiSjor H=2JijSiSj. Accordingtothese Hamiltonians, the Jvalue obtainedwhenusing the rst Hamiltonianis twice thanwhenusing the second. However, insome papers J is giveninK, thoughmainly incm1. For these reasons, wedecidedtoadopt theHamiltonianH=JijSiSj(Jincm1) andtochangethereportedvalues inthosecases wherethe 2JHamiltonianwas used. Thedan-ger of adoptingthis assumptionlies inthosecases inwhichtheauthors donotclearlyindicatewhichHamiltoniantheyhaveused.Here twodihedral angles are usedfor magnetostructural correlations: ~ (thedihedral angle betweenthe meanplanes MN1N2N3andN1N2N3M%)andl(thedihedral anglebetweentheplanedenedfor thesixNazidoatomsandtheN1MN3% plane). lisnormallyusedforcomplexeswithdoubleazidobridgesand~forcomplexeswithsingleazidobridges.2.Dinuclearcomplexes2.1. With1,3-azidobridgingligandsThreemodelshavebeenstructurallycharacterized: withonlyoneazidobridge;withtwoazidobridgesandwiththreeazidobridges.1031 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068There are four fully characterized complexes with only one azido bridging ligand,[Ni2(N3)2(Me4-cyclam)2(m-N3)]I (Me4-cyclam=1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane)[26,27] inwhichtwoterminal azidoligandsintranspositionwithregardtothebridge, completetheoctahedral coordinationof theNiIIions,[Ni2(trenpy)2(m-N3)](ClO4)3[28] (trenpy=N,N-bis(2-aminoethyl)-N%-2-(pyridyl-methyl)ethane-1,2-diamine), [Ni2(L)(m-N3)](N3)(ClO4) [29] and [Ni2(L)(H2O)(m-N3)](CF3SO3)2 (L=cryptate host) [29]. The main feature of the rst two complexes,whichisalsopresentintheone-dimensionalsystemswithonlyoneazidobridgingligand in 1,3 coordination mode, is the non-linearity between the two NiIIions andTable1Structural and magnetic parameters for dimers with the [Ni(m1,3-N3)Ni] unit: NiN bond distances (A,),NiNNand~angles(), Jincm1aNiN Reference NiNN ~ J[Ni2(trenpy)2(m-N3)](ClO4)32.133 131.6 180 26.8 [28][Ni2(dmptacn)2(m-N3)](ClO4)3[28] 42.5 17.0 180 142 [26] 2.15 [Ni2(Me4cyclam)2(N3)2(m-N3)]I [29a] 169.9 [Ni2(L)(m-N3)](N3)(ClO4) 1.9982.032 164.7 [Ni2(L)(H2O)(m-N3)](CF3SO3)2165.8 2.037 +11.8 [29b]1.998 157.6aTrenpy=N,N-bis(2-aminoethyl)-N%-2-(pyridylmethyl)ethane-1,2-diamine; dmptacn=1,4-bis(2-pyri-dylmethyl)-1,4,7-triazacyclononane; Me4-cyclam=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradec-ane; L=cryptatehost)1032 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 1.ORTEP plot showing the structure of [{Ni(en)2}2(m-N3)2](PF6)2 (top); Walsh diagram for the fourcombinationsofmagneticorbital uponvariationof landvariationof Z2asafunctionofdihedralanglel(bottom).theazidobridge.Incontrast,thecascadecomplexesformedinthecryptatehost,showtheratherunusual quasi-linearityof theNi N3Ni central unit (Ni NNanglescloseto165).1033 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068According to the authors [29a], this colinearity is due to the geometrical constraintsimposedbythequiterigiddimetalliccryptate. Itisalsofavoredby ppinterac-tions betweenazideandthethreephenyl rings of thecryptate, thephenyl ringsbeing nearly parallel and equidistant to all three aromatic planes. The mainstructural data and the exchange coupled parameter, J, are given in Table 1. It mustbe pointed out that the latest compound is the only example reported withferromagnetic coupling through EE azido bridges as predicted. No magnetic resultsarereportedforthecomplex[Ni2(L)(m-N3)](N3)(ClO4)[29].A single case with only one end-to-end azido bridging ligand and also a pyrazolebridgehasbeenreportedrecentlythoughnomagneticdataaregiven[30].The number of reported dinuclear complexes with two azido bridging ligands hasincreased greatly over the past 10 years. The formula of the core is [Ni2(m-N3)2]2+.Inall thereportedcomplexes but one, thetwoazidoligands arecoplanar. Theexception is [Ni2(L%)2(N3)2(m-N3)2] (L% =1,4,7-trimethyl-1,4,7-triazacyclononane)[24] inwhichthetwoazidobridgingligands cross eachother. Tocompletethecoordinationof the NiIIions there are generallybi-, tri- or tetradentate amineligands. In some cases there are some azido terminal ligands. The counter-anion is,usually, a non-ligating anion such as ClO4, PF6or [B(C6H5)4]. A representativeTable2Structural andmagneticparametersfordimerswith[Ni(m1,3-N3)2Ni] unit: NiNbonddistances(A,);NiNNandlangles(), Jincm1al NiNN NiN J Reference121.1 45.0 [Ni2(en)4(m-N3)2](PF6)24.6 2.181 [21]2.183 119.3[Ni2(1,3-tn)4(m-N3)2](BPh4)22.167 127.7 3.0 114.5 [21]139.0 2.144124.4 6.8 [Ni2(L)2(N3)2(m-N3)2] 90.0 2.167 [33]2.135 138.4[Ni2(L%)2(N3)2(m-N3)2] [24] 36.4b124.5 2.111128.2 2.118[Ni2(tren)2(m-N3)2](BPh4)2[26] 35 20.7 123.3 2.069135.7 2.1952.103 128.5 [Ni2(222-tet)2(m-N3)2](BPh4)222.4 83.6 [39]2.155 129.9[75] 29.1 34.6 2.180 126.7 [Ni2(aep)4(m-N3)2](PF6)22.128 123.82.141 [Ni2(D,L-cth)2(m-N3)2](ClO4)2[76] 141.6 113.0 3.32.098 127.4124.3 [Ni2(D,L-cth)2(m-N3)2](PF6)211.5 2.128 75.1 [76]142.2 2.143a(L=1,5,9-triazacyclododecane; L% =1,4,7-trimethyl-1,4,7-triazacyclononane; tren=2,2%,2-tri-aminotriethylamine; 222-tet =triethylenetetramine; aep=aminoethylpyridine; cth=5,5,7,12,12,14-hexamethyltetraazacyclotetradecane).bThisistheonlycaseinwhichthedoublybridgedmoiety(N3)2isnotplanar.Thedihedralangles,as dened by the authors, are 15.6 and 21.4. This (N3)2 planarity is characteristic of all other dinuclearcomplexes.1034 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 2. ORTEPplotshowingthestructureof[Mn2(bpy)4(m-N3)2](ClO4)2bpy.exampleofthesecomplexesisshowninFig. 1. Themainstructural dataandtheexchangecoupledparameter, J, aregiveninTable2.Basedonthesedataanewmagnetostructuralrelationshiphasbeendevelopedbetween the exchange coupling parameter, J (in cm1), and the structural data [21].ThemainfactordeterminingtheJvalueisthe[Ni (m-N3)2Ni] dihedral anglel.TheresultsofextendedHu ckelmolecularorbitalcalculationsareshowninFig.1as Z2versus l, where Zrepresents each of the gaps created between thesymmetricandantisymmetriccombinationsof theNiIImagneticorbitals(z29z2andx2y29x2y2). According tothe magnetochemical theories [31], JAFisproportional tothis gap(JAF8Z2). Experimental dataagreewiththis model.Thus, the more planar the structure (dihedral angle l close to 0), the moreantiferromagneticisthecoupling. Whenthisdihedral angleincreases, theJvaluedecreases[21]. Inall casesweassumethatthetwoN3ligandsarecoplanar.Toourknowledge,onlyonecaseofMnIIdinuclearcomplexshowingEEazidobridgeshasbeenreported:[Mn2(bpy)4(m-N3)2](ClO4)2bpy[32](Fig.2).Thecrystalstructureshows theMnIIions doublebridgedbytwoazidoligands intheir EEbridging mode, where the bridging angles MnNN and NNMn are 128.6 and131.3, respectively. The [Mn(N3)2Mn]2+unit shows a chair conguration. Theexistence of a non-coordinated bipyridine ligand, which is sited parallel to the bpyligands of the dimer, is a peculiarity of this structure. Preliminary magnetic resultsforthisdinuclearcompoundconrmAFinteractions.In the literature, only one dinuclear complex with a triple 1,3-N3 bridging ligandis reported, [Ni2(1,4,7,-trimethyl-1,4,7-triazacyclononane)2(m-N3)3](ClO4) [24,33].TheNi NNanglesliebetween115.6and119.2(averagevalue=118.1).TheJvalueis 67.9cm1.1035 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)102710682.2. With1,1-azidobridgingligandsAll these complexes, as indicated above, show ferromagnetic coupling. Two kindof systems have been structurally characterized and reported: with two azidobridgesandwiththreeazidobridges(onlytwocomplexes).Thenumberof NiIIdinuclearcomplexesreportedwithtwoEOazidobridgingligands has increased considerably over the past 10 years. The formula of the coreis [Ni2(m1,1-N3)2]2+. All the reportedcomplexes, but two(see Table 3), showa[Ni (N3)2Ni]planarunitwithaninversioncenter.TocompletethecoordinationoftheNiIIionstherearegenerallybi-,tri-ortetradentateamineligands.Insomecases there are some azido terminal ligands. The counter-anion is, usually, anon-ligatinganionsuchasClO4orPF6. Arepresentativestructural exampleofthesecomplexes is showninFig. 3. Themainstructural dataandtheexchangecoupledparameter, J, aregiveninTable3.Table3Structural andmagneticparametersfordimerswith[Ni(m1,1-N3)2Ni] unit: NiNbonddistances(A,)andNiNNi angles(), Jincm1aNi(N3)Ni inv. centre J Refer- NiN NiNNienece[Ni2(en)4(m-N3)2](ClO4)2104.3 Yes 43.7 [49] 2.1442.123[Ni2(terpy)2(N3)2(m-N3)2]2H2O [15a,b] 40.2 Yes 101.3 2.1982.038No [Ni2(terpy)2(N3)(H2O)(m-N3)2]ClO427.2 2.069 [15c] 1032.1722.1142.0602.107 [Ni2(pepci)2(N3)2(m-N3)2]2H2O 102.20 No 72.6 [77]2.163 101.02.1092.1282.092 [Ni2(Me3[12]N3)2(m-N3)2](ClO4)2[78] 103.8 43.9 Yes2.0672.166 [Ni2(232-N4)2(m-N3)2](ClO4)2104.9 Yes 33.8 [78]Yes 46.7 [Ni2(Medpt)2(N3)2(m-N3)2] [35] 2.207 104.0[79] 2.197[80] 34.3 Yes [Ni2(232-tet)2(m-N3)2](PF6)2104.6 2.1772.1832.83 95.1 [Cu1.5Ni0.5(terpy)2(N3)2(m-N3)2] 2H2O Yes [81] 40.21.97a(pepci =N%-(2-pyridin-2-ylethyl)pyridine-2-carbalimide; Me3[12]N3=2,4,4-trimethyl-1,5,9-triazacy-clododec-1-ene; 232-N4=N,N%-bis(2-aminoethyl)-1,3-propanediamine; Medpt=methyl-bis(3-aminoprop-yl)amine).1036 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 3. Representativestructural exampleofthe[Ni2(m1,1-N3)2]2+core.All thesedinuclear complexes haveaNi NNi anglecloseto101104 andare coupled ferromagnetically. J values lie between 20 and 40 cm1approxi-mately. In the ferromagnetic systems, owing to the zero eld splitting (ZFS)manifest at lowtemperatures, the magnitude of Jshouldbe treatedwithcare.Almost all the authors who have investigated this kind of complex use theGinsbergformula[34] whichintroduces theJ, D, gandz%J% (couplingbetweendimers) parameters. With these four parameters the correlation between themcouldbesignicant.Duetotheinversioncenterintheplanar[Ni (N3)2Ni] core, thispart of thestructureis likearhombus. Thecompound[Ni2(Medpt)2(N3)2(m-N3)2] undergoesanasymmetrizationprocess as the temperature falls. This canbe visualizedinthemagneticmeasurements.Fig. 4. MT()and1/M()of[Mn2(terpy)2(N3)2(m-N3)2] 2H2Oasafunctionofthetemperature.Thefull curverepresentsthetheoretical curveforJ=2.43cm1andg=2.0.1037 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068This phase transition creates an hysteresis cycle in the molar susceptibility measure-ments. This hysteresis seems to indicate that the magnetic transition is not reversible,at least during the measurement time. Calorimetric data obtained from some of thesecomplexes also indicate a non-reversible transition. This asymmetrization transitionhasbeenexplainedintermsofasecond-orderJahnTellerdistortion, takingintoaccount thelocal symmetryof thedinuclear[Ni (N3)2Ni] entity(D2h, rhombicsymmetry)beforethearrangement[35].InthecaseoftheMnIIion,thesamedinuclearcorehasbeenobtainedfortwocomplexes: Na[Mn(pzate)(N3)2(H2O)]2H2O(Hpzate=pyrazine-2-carboxilic acid)[36], whichhasthebondangleMnNMnof100.2(1)(magneticdatawerenotreported), and [Mn2(terpy)2(N3)2(m-N3)2]2H2O [37] in which the MnIIions are doublybridged by EON3 ligands, with the tridentate terpy ligand occupying three of thefourequatorialsitesofthecoordinationpolyhedronofeachmetalandaterminalazidogrouptocompletethehexacoordination. Thevalueofthebridgingangleis104.6. The magnetic results for this MnIIcompound are illustrated in Fig. 4. As canbe observed, the MTvalue increases uponcooling, reaching a maximumandremaining stable at lower temperatures. This clearly corroborates intradimer ferro-magneticinteractionthroughEOazidobridges. Themagneticsusceptibilitydatawere analyzed in terms of the dipolar coupling approach for a MnIIdimer, the besttresultgaveasetofparametersJ=2.43cm1andg=2.0.Finally, only two complexes with three azido bridging ligands have so far reported:[Ni2L2(m-N3)3]ClO4 (L=1,4,7-trimethyl-1,4,7-triazacyclononane) [24] and [Ni2L2(m-N3)3]ClO4(L=(N,N%-dimethyl-1,4,7-triazacyclononane)calix[4]arene [38]. The ge-ometry of the triple bridge is similar in both complexes, with an average Ni NNiangle of 85.9 and 86.2, respectively. The NiNi distances are, evidently, shorter thanfordinuclearcomplexeswithonlytwoazidobridgingligands(2.896and2.852A,,respectively). Both complexes show ferromagnetic coupling (J=30.7 and 17.2 cm1,respectively).3.TrinuclearcomplexesOnly two complexes of this kind have been reported in the literature: [Ni3(2,2,2-1038 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 5. ORTEP plot showing the structure of the tetranuclear compound [Ni4(2-oxo-1,3-di-aminopropane)2(2-hidroxo-1,3-diaminopropane)2(m1,1-N3)4](ClO4)2.tet)3(m-N3)3](PF6)3 and the perchlorate analogue [39]. The crystal structure of thesetwo complexes is not known but EXAFS and XANES spectra reveal the occurrenceofazido-bridgedtrinuclearnickel(II)compoundswithNiNi separationsof5.16and5.12A,, respectively. EachNiIIionisplacedinanoctahedral NiN6environ-ment:fournitrogenatomsoftheamineandtwonitrogenatomsofthetwoazidobridges. Theproposedstructureis, thus, anirregulartriangleforbothcomplexes.Inorder tot the magnetic results it was necessarytointroduce twoJvalues:J1,2=72 and 60.3 cm1and J3=36 and 29 cm1, respectively, inagreementwiththeassumed1,3coordinationmodefortheazidobridgingligand.1039 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)102710684.TetranuclearcomplexesThree NiIItetranuclear complexes withEOazidobridging ligandhave beenreported. The rst has the formula [Ni4(2-oxo-1,3-diaminopropane)2(2-hydroxo-1,3-diaminopropane)2(m1,1-N3)4](ClO4)2[40] (Fig. 5). The four NiIIatoms are inadistortedoctahedralenvironmentandarerelatedbyanS4symmetryaxisforminga quasi-perfect square. The oxygen atom of the amine ligand (OHpn and Opn) alsoactsasabridgingligandbetweentwoNiIIions. Magneticmeasurementsindicateferromagnetic coupling with J=21.3 cm1. Magnetization measurements areindicative of an S=4 spin ground state. The second complex reported to date hasacubane-likestructure. Itsformulationis[Ni4(dbm)4(EtOH)4(h1,m3-N3)4] (dbm=dibenzoylmethane)[41,42].EachazidoligandisboundsymmetricallytothreeNiIIionsinanend-onfashion.Magnetic behavior was well reproduced by the one-J model, giving J=11.9 cm1.The third case corresponds to a compound with formula [Ni4(dpkO)4(m-N3)4](dpk=di-2-piridylketone)[43].ItconsistsoftetranuclearunitswheretheNiIIionsare bridged by both m-oxo and m-end-on azido ligands, to give a face-sharedicubane-like coordinationcore withtwomissing vertices (Fig. 6). Preliminarymagnetic measurements for this complex indicate a predominance of ferromagneticinteractions (Fig. 6). The observationof ferromagnetic interactions withinthesecomplexes is consistent with both the previously proposed accidental orthogonalityandspinpolarizationtheories. Inthe rst andthirdcases, the presence of oxo(hydroxo) bridges, allowing two different pathways for the magnetic coupling,preventsanypossiblemagnetostructural correlations.Forthemanganeseiononlyonetetranuclearcompoundhasbeenreported[44],withformula, [Mn2(HL)(N3)2]23MeCN(H2L=1,7,14,20-tetramethyl-2,6,15,19-te-tra-aza[7,7](2,6)pyridiniphane-4,7-diol). It consists of twodinuclear entities withtwooxoandoneEOazidobridges, linkedbymeansofasingleEOazidebridge(Fig. 7). The MnNMn bond angle between dimeric units shows the anomalously1040 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 6. ORTEP plot (top) showing the structure of [Ni4(dpkO)4(m-N3)4] andMT (() andM () forthiscompoundasafunctionofthetemperature(bottom).highvalues of 126.3(2) and128.3(2), whereas theintradimer MnNMnbondanglehasvaluesof84.7(2)and83.1(2). Theglobal magneticbehaviormeasuredbetween3006KshowsantiferromagneticcouplingbutdetailedJvaluesforeachbridgetypewerenotreported.5.One-dimensionaluniformsystemsUniform1Dsystemsareunderstoodtobethosesystemsinwhichallionshavethesamemagneticpathway, i.e. onlyonetypeofazidobridgingligandandonlyone set of structural parameters. In order to t the magnetic data only oneexchangecouplingparameter(J)isnecessary.1041 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 7. Molecular structure of [Mn2(HL)(N3)2]2(H2L=1,7,14,20-tetramethyl-2,6,15,19-tetraaza-[7,7](2,6)pyridiniphane-4,7-diol).5.1. With1,3-azidobridgingligands(AF)Althoughverysimilar, wecandistinguishtwotypesof1Dsystem: transorcis,accordingtothepositionofthetwoadjacentazidobridgingligands.Todate,fortheNiIIion,1042 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068only 1Dsystems with one 1,3-azido bridging ligand have been reported. TocompletethehexacoordinationofeachNiIIion,twobidentateoronetetradentatenitrogen ligands have been used. The main structural data and the magneticbehavior (exchange coupled parameter, J) are given in Table 4 for trans-complexesand in Table 5 for cis-complexes. A representative structural example of trans andcischainsareshowninFig. 8, respectively.Magnetostructural correlations have been made in both kinds of complex, transand cis, with similar results [16,45]. For these correlations extended Hu ckel calcula-tions were performed by means of the CACAO program [46] on a dimeric fragmentmodeledasindicatedinthedrawing.Table4Structural and magnetic parameters for 1-Dtrans-Ni-(mN3)-Ni systems: NiNNand dihedralangles(); Jincm1aReference NiNN ~ J180 100 [82] 120.9 [Ni(tmd)2(m-N3)]n(ClO4)n[Ni(tmd)2(m-N3)]n(PF6)n[83] 126.1 180 55.8b(70.6)49.4 180 [84] 137.2 [Ni(2,2%-mettn)2(m-N3)]n(ClO4)n180 19.4b[83] 136.5 [Ni(2,2%-mettn)2(m-N3)]n(PF6)n(41.1)[Ni(cyclam)2(m-N3)]n(ClO4)n[85] 39.2 166.9 140.7128.2142.4 134.6 [Ni(232-tet)2(m-N3)]n(ClO4)n26.9 [17]124.1135.8 169.3 [Ni(323-tet)2(m-N3)]n(ClO4)n62.7 [17]119.8115.6 175.7 [Ni(macro)2(m-N3)]n(ClO4)n97.8 [86]116.8150.5 A)128.5 41.1 [Ni(cth)2(m-N3)]n(ClO4)nc[87]36.4 130.7 146.2B)131.4a(2,2%-mettn=2,2%-dimethyl-1,3-diaminepropane; 232-tet =N,N%-bis(2-aminoethyl)-1,3-propanedi-amine; 3,2,3-tet =N,N%-bis(amonipropyl)1,3-ethanediamine; macro=2,3-dimethyl-1,4,8,11-tetraazacy-clotetradeca-1,3-diene; cth=meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane).bInthesetwocomplexesthereisaphasetransitionwhenthetemperaturedecreases, whichchangesthe structural parameters and the magnetic results. The results at high temperature, (before thetransition) are shown in parentheses while the results at low temperature (after the transition) are shownwithoutparentheses.cThiscomplexshowstwostructurallydistinct1-Duniformchainsinthecrystal netina2:1ratio.ThesechainsareindicatedasAandB.1043 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Table5Structural and magnetic parameters for 1-D cis-Ni-(mN3)-Ni systems: NiNNi and dihedral angles(); Jincm1a~ NiNN J Reference142.8 18.5 151.8 [45] [Ni(333-tet)2(m-N3)]n(PF6)n151.3125.9 16.8 [88] [Ni(2-methyl)2(m-N3)]n(ClO4)n131.8125.9135.0 146.8 3.2 [88] [Ni(2-methyl)2(m-N3)]n(PF6)n122.6[Ni(bipy)2(m-N3)]n(ClO4)nb133.9 123.7 33.0 [16] 120.1138.9 121.3 126.6122.6 122.6 134.9 22.4 [16] [Ni(bipy)2(m-N3)]n(PF6)nb127.4 120.6 138.0122.4 B1 [75] [Ni(aep)2(m-N3)]n(ClO4)n127.8126.2a(333-tet = N,N -bis(3-aminopropyl)-1,3-propanediamine; 2-methyl = 1,2-diamino-2-methyl-propane; aep=2-(aminoethyl-pyridine)).bThese two chains actually belong to the alternating AF chains because, according to the authors, theasymmetricunitisformedbytwoNi-N3-Nientitiesthatareattachedconsecutivelyinachain.Thetof the magnetic data was made assuming only one value of J, as a uniform chain, because the variationofNi-N-Nanddihedral anglesissmall.Sincebonddistancesaresimilarinall thecomplexesreported, thewiderangeofbondangles (Ni NNanddihedral Ni N3Ni torsion) must playasignicantroleinthesuperexchangeinteractionbetweenthenickel atoms.Inatranschaininwhichthed(z2)orbitalsareorientedinthechaindirection,only the z directionis operative as anexchange pathway, andJAFis solely afunctionof Z2(z2) =[Eb(z2)(s)Eb(z2)(a)]2).Fig. 9representstheMOcalculationsmadewhenvaryingtheNi NNanglebetween 90 and 180 and assuming that the Ni N3Ni (torsion angle) is 180. Forthis gure the maximum coupling is expected for Ni NN angles of 108 while forgreater values the antiferromagnetic interaction must decrease. An accidentallyorthogonal valley, centeredat 164isfound. Thisplot agreeswiththegenerallyaccepted assumption that the main interactions occurs through the p pathway using1044 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068the non-bonding MO of the azido ligand and the | pathway is poorly operative. Theeffect of the dihedral Ni NNNNi angle has also been parametrized between 180and130. Forall Ni NNanglesthemaximumcouplingcanbeexpectedforatorsion value of 180, decreasing gradually thereafter as the torsion angle increases.Themagnitudeofthiseffectislowerthantheeffectofthebondangleandmightbe considered as a secondary factor rening the main factor. As an example, 30 ofvariation in Ni NN angle between 120 and 150 reduces theZ2value by ca. 90%,whereas,forbothangles,30ofvariationintheNi N3NitorsionanglereducestheZ2valuebyca. 35%.On the other hand, calculations involving three nickel atoms and placing the azidobridges in trans or cis arrangement [16,45] show the same general behavior for Z2(in a cis conformation Z2(xy) is not zero), indicating, as expected, that the main factorin the magnitude of the AF interaction are the bond parameters of each bridge andnot how this unit is repeated along one axis. The maximum J values correspond atapproximatelythesameNi NNanglefortransorcisgeometry(108and110,respectively).Intheciscomplexes, thetorsionangleisgreaterthanthat intranscomplexes(Tables 4 and 5) creating, thus, lower AF coupling. Taking into account the helicaltopology of these cis systems, there is further factor that needs to be considered, thecompactness of this helix. This is due to the sum of all the small differences in anglesand distances that are, however, difcult to visualize independently: the more compactthehelicoidal structure, thegreatertheantiferromagneticcoupling.Finally,theseuniformone-dimensionalsystemsobtainedfromNiII(S=1)havebeen studied extensively by the physicist as they provide good experimental examplesFig. 8. Representativestructural exampleofatransuniformchain, Aandacisuniformchain, B.1045 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 9. Plotof Z2infunctionof i1andi2fora[NiNi] monobridgedEEazidosystem.forinvestigatingtheso-calledHaldanesgap(aquanticgapcreatedbetweenthegroundstate(S=0) andtherst excitedstate, whentheSvalueisinteger[47]).Incontrast withnickel, onlyonehomogeneouschainwithadoubleEEazidebridge has been reported for the manganese ion [48]. This has the formula[Mn(pyOH)2(N3)2]n(pyOH=2-hydroxypyridine). In this case the pyOHligandcoordinatesthemanganeseatominmonodentateformthroughtheoxygenatom,giving a trans-system(Fig. 10). The main bond parameters related with thesuperexchangepathwayareMnN2.245(2) 2.250(2)A,, bondangles: MnNNFig. 10. ORTEPplot showingthe structure of the double EEazidobridging1Dcompound[Mn(-pyOH)(N3)2]n(pyOH=2-hydroxypyridine).1046 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 11. Schematicrepresentationofthedouble1,1-azidobridging1Dcompounds.122.3(1) 123.8(1), lparameter=38.0. Thecouplingwas moderatelyantiferro-magnetic with J=7.0 cm1. The factors thought to inuence the superexchangewerestudiedinasimilarfashionastothatwithnickel, andtheresultswerealsofoundtobesimilar:themaximumAFcouplingoccursforplanarMn(N3)2Mnrings, (lparameter=0), whilecouplingdecreasesasthelparameterincreases.5.2. With1,1-azidobridgingligands(F)FortheNiIIion,onlytwocomplexesofthistypehavebeenreportedfully[49].Thegeneral formulais[Ni(L)(m-N3)2]n(L=ethylenediamine, 1,3-diaminopropaneand2,2%-dimethyl-1,3-diaminopropane). Fig. 11is aschematic representationofthesechains.InTable6themainstructuralandmagneticparametersarereported.Unfortu-nately,thecrystalsforthetwostructurespresentedpoorcrystallinitypreventingagoodrenement. Thus, thecrystal data(Table6)mustbetreatedwithcare. Thethree complexes are ferromagnetic, as expected given the coordination mode of theTable6Structural andmagneticparametersfor1-DNi-(m1,1-N3)Ni systemsaNiNNi JbD ReferenceUnitA UnitB[Ni(en)(N3)2]n[49] 6.3 35.6 100.9 103.123.8 100.9 0 95.2108.1 35.2 4.4 [49] [Ni(tn)(N3)2]n100.1105.0 101.4 39.8 029.4 [Ni(2,2%-mettn)(N3)2]n[49] 6.7 33.8 0aNiNNi angles(); Jincm1. (en=ethylenediamine; tn=1,3-diaminopropane; 2,2%-mettn=2,2%-dimethyl-1,3-diaminopropane).bTherstJvalueisfoundbyassuming D "0; thesecondonebyassuming D =0.1047 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 12. Schematicrepresentationof thetransuniformchain[Ni(333-tet)(m-N3)]ClO4(333-tet =N,N%-bis(3-aminopropyl)-1,3-propanediamine) (A) showing the different set of structural parameters (B)showingthetorsionanglebetweenneighboringazidogroups.azidobridgingligand. Preliminaryresults withhighelds (1.01.5T) indicateaverystrongdependenceof theexternal eldinthelowtemperatureregion. Suchbehaviormaybeindicativeofmetamagneticsystems.Forthisreasonthesuscepti-bility curves were drawn and tted at very low elds. The magnetic parameters wereobtained assuming D=0 or including the D parameter in the de Neef formula [50],which allows this kind of uniform chains to be tted for S=1. For this reason therearetwoJvaluesinTable6.With the same metal azide squeleton the manganese derivative [Mn(2-bzpy)(N3)2]n chain has been characterized in which 2-bzpy is 2-benzoylpyridine [51].The bridges are not equivalent but show little deviation from the mean MnNMnvalueof 100. Duetothelargevolumeof the2-bzpyligandthechainsarewellisolated with an interchain MnMn distance of 9.93 A,, which minimizes theinterchaincoupling. Asexpected, theintrachaincouplingwasfoundtobeferro-magnetic, showingaTvalueof32.8emuKmol1at2K.6.One-dimensionalalternatingsystems6.1. Withonly1,3-N3bridgesTwotypes of this complexhavebeenreportedtodate: (a) Withtwodifferentgeometries in adjacent 1,3-N3azido bridges. This is the case of [Ni(333-tet)(m-N3)]ClO4(333-tet =N,N%-bis(3-aminopropyl)-1,3-propanediamine) [45]. This one-dimensional system can be schematized asNi NaNbNaNi NcNdNcNi NaNbNaNi . Thus, the most interesting feature of this compound is theexistence of inversion centers at the central nitrogen of two consecutive yet differentazidogroups.ThetwoNi NaandNi Ncbonddistancesarequitedistinct,2.0771048 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068and2.204A,, respectively. Thetwobondangles, Ni NaNbandNi NcNd, are142.4and123.6, respectively, thelargeranglecorrespondingtotheshorterbonddistance. ThisfeatureisshowninFig. 12. Eachfragment of theveNi N3Niatomsiscoplanar, andconsequentlythetorsionangleis180. Thetorsionanglebetween neighboring azido groups is 93.4, as shown in Fig.12 B. An equation fortheanalysisof magneticsusceptibilitydataof alternatingchainswithlocal S=1hasbeenproposedbyBorrasetal.[52]forcalculationsonacyclicsystemoffourspin pairs that gives a good approximation up to kT/J1=0.4. By using thisexpression, the value of the couplingparameters has beenoptimizeduptothevaluesJ=80.7cm1,J% =37.4cm1(alternatingparameterJ%/J=h=0.46)andg=2.4. It is interesting topoint out that whenusing the equationfor auniformchain, theJvalueobtainedis veryclosetothemeanof thecalculatedvalues for analternatingsystem(J=62.1cm1). Accordingtothemagneto-structural correlationsgivenintheprevioussectionforuniformchains, twoverydifferentvaluesofJareexpectedforthetwodifferentbridgesofthisalternatingchainbecausetheNi NNanglesareverydifferent(142and123, respectively).ThegreaterexperimentalJvalue(J=80.7cm1)canbeassignedtothebridgeof 123 (closer to 108110 for which the maximum susceptibility is expected) andthesmallervalue(J=37.4cm1)tothegreaterangle(142.4).Thealternatingparameter h is 0.46 and the theoretical ratio between the calculated gaps when usingthe extended-Hu ckel OMmodel is 0.52, showing excellent agreement withtheexperimental value [45]. This compound has been extensively studied as a model ofthegaplesspointoftheAfeckHaldaneconjecture[53].Inthecaseof manganese, onesingularcompoundwithalternatingdoubleEEazido bridge has also been reported: the [Mn(3-Etpy)2 (N3)2]n compound (3-Etpy=3-ethylpyridine) [48], which shows trans coordination and two different sets of bondparameters in each alternating Mn(N3)2Mn unit with a different degree of chairdistortion withl parameters of 6.9 and 11.6 (Fig. 13). In good agreement with theabove model, the coupling was found to be antiferromagnetic and closely related tothelparameter, withJ=13.8/ 11.7cm1, respectively. (b)Withconsecuti6esingleanddouble 1,3-azidobridgingligands. This is the case of [(Ni2(dpt)2(m-N3)(m-N3)2)]n(ClO4)n(dpt =bis(3-aminopropyl)amine) [23] (Fig. 14). Thestructurecon-sists of a 1-DNi (N3)2Ni N3Ni system. The part with a single azido bridgeislikeall systemsdescribedforuniformchains: theNi NNangleis119.2andthe torsion angle Ni N3Ni is 0(180). The most interesting feature of thisstructure is the relative positionof the twoazidobridges inthe Ni (N3)2Nifragment.ThestructureoftheNi (N3)2Nifragmentderivesfromthesymmetricrotationofthecoordinationpolyhedraofthenickel atomsaroundthex-axis, asshown in Fig. 15 B. As a result of this movement, the nickel atoms and the centralnitrogen atoms of the azido groups are always maintained in the xy plane, whereastheN(azido)atomslinkedtothenickel atomsleavetheoriginal plane(distortiontypez),z being in this case the angle dened for the N*N axis and the xy plane.For this complex, the corresponding meanz angle is roughly 20. Extended-Hu ckelMO calculations have been performed on a dinuclear fragment modeled as in Fig.15B. Thecalculationsweremadebyvaryingthe zangle, maintainingall other1049 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068parametersasconstants. Thecorresponding Z2plotversuszisgiveninFig. 15C together with the variation for thel distortion (Fig. 15 A), in order to comparebothkindsof distortion. Z2hasamaximumfor z=0asexpected, reachingavalue of 0 when z=32, and subsequently increases slowly. This behavior isdifferent fromthat foundfortheldistortion, forwhich Z2decreasescontinu-ouslyintheinterval060.TheJvaluesfoundbyusingtheBorrasequation[52]are: J=84.6 cm1and J% =41.4 cm1(h=0.49). According to our previousFig. 13. ORTEP plot (top) showing the structure of the compound[Mn(3-Etpy)(N3)2]n(3-Etpy=3-ethylpyridine)andMT()and xM()ofthiscompoundasafunctionofthetemperature(bottom).1050 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 14. Structure of the 1Dsystemwithconsecutive single anddouble 1,3-azidobridging ligands[Ni2(dpt)2(m-N3)(m-N3)2]n(ClO4)n.discussion, these twoJvalues canbe assignedunequivocallytothe single anddoubleazidobridges,respectively.TheNi NNangleof119.2(singlebridge)isoneof thelowest reportedtodate, thustheJvaluemust beoneof thegreatest( 84.6cm1).6.2. With1,3-N3and1,1-N3bridgesWhen in the synthetic procedure the Ni/diamine/azide ratio is 1/1/2 not only werenewferromagnetic chains obtainedbut, depending onthe diamine usedinthesynthesis, newferroantiferromagneticspecieswerealsosynthesized. Thealterna-tioncanbeverysimple, FAF(Fig. 16A), ormuchmorecomplicatedlike, forexample,FFFAF(Fig.16B).Inallthesecases,themagneticdatahavebeeninterpreted and tted by numerical extrapolation of S=1 rings of nite length [54].ForthesecalculationsanisotropicHeisenbergsystemwithquanticspinS=1hasbeen assumed. The main structural and magnetic parameters for this kind ofone-dimensional alternatingchainare showninTable 7. Fromthis Table it ispossible to realize that the AF coupling parameter J agrees with the previous resultsdescribed above: the higher thel angle the lower the J value. The J values for theF part also agree with those reported for dinuclear complexes. In all cases the angleiscloseto100.Thesamekindof systemhasalsobeenobtainedforMnIIioninthecomplex[Mn(bpy)(m1,1-N3)(m1,3-N3)]n[55] (Fig. 17). Theazidoligandsarearrangedcisand1051 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 15. Schematic representation for the two types of distortion found for the planar [Ni2(m1,3-N3)2]2+core(A, d-type; B, r-type); plotof Z2versuszorlfora[Ni2(m1,3-N3)2]2+core(C).1052J.Ribasetal./CoordinationChemistryRe6iews193195(1999)10271068Fig. 16. Structureof1DsystemswithconsecutiveFandAFazidobridgingligands(A)andwiththeFFFAFazidobridgingset(B).1053 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068are perpendicular to each other. The MnNMn% angle in the EO bridge is 101.0.For theEEbridges theMnNNangles are131.1(5) and127.3(5)for achairconformation of the Mn(N3)2Mn unit and a torsion angle of 41.9. Due to thesestructural features, alternating ferromagnetic (through EObridges) and antiferromag-neticinteractions(throughEEbridges)shouldbeexpected.Thethermalvariationof bothM andMT values, shown in Fig. 17, can only be explained assuming twoalternatingexchange constants: apositive J1for EOandanegative J2for EEpathways. However, noformula was available inthe literature for anS=5/2ferroantiferromagnetic alternating chain and for this reason a model was developed[55b].Thebestagreementobtainedbyleastsquaresrenementcorrespondstothefollowingsetofparameters: J1=9.58cm1; J2=11.80cm1, g=2.01. Anewtopology doubleEOandsingleEEalternated has recentlybeenreportedfor[Ni2(Medien)(m1,1-N3)2(m1,3-N3)]n(ClO4)n [56] (Fig. 18). The structural parameters areunchangedforthetwocoordinationmodesand,ascanbeexpected,themagneticbehavior corresponds to a FAF alternating system. The data have been tted withan expression built on the basis of the increasing ring systems calculations [65]. Themainstructural andmagneticparametersareshowninTable7.Finally, a unique case with one end-to-end and three end-on bridges in alternationhas been reported. It is the complex [Ni2(tmeda)2(m1,1-N3)3(m1,3-N3)]n(tmeda=N,N,N%,N%-tetramethylethylenediamine) [57] (Fig. 19). The most important featuresoccurintheNi NNi angles.The average value in the EO bridge is 84.2, which is one of the lowest values forany metal ion with azido bridging ligand in EO mode. In the EE bridge the Ni NNTable7Structural andmagneticparametersforFAF1-Dsystemsal NiNNi NiN-N Reference Type J, J%,J ~FAF 118.2 101.5 35.2 180 26 [Ni(bipy)(m-N3)2]n[89]129.9 2.6[90] 17 180 :0 [Ni(NN-dmen)(m-N3)2]n98.2 121.1 FAF139.4 156 [54b]FAF 125.6 98.7 [Ni(aep)(m-N3)2]nb b5 [90][54b] 28 124.4121.6117.9FAF 138.1 100.8 [Ni(Medien)(m-N3)]ClO4 162.8 34.7 [56]125.7 98.9 38.2120 [Ni(NN%-dmen)(m-N3)2]n180 8.4 100.5 130.9 FFFAF [54a]132.9 101.2 20102.9 37aNiNNi, NiN3Ni land~angles (), Jincm1. (N,N-dmen=N,N-dimethylethylenediamine;aep=2-aminoethylpyridine, N,N%-dmen=N,N%-dimethylethylenediamine; Medien=Methyldiethylene-triamine).bThisNi(N3)2Niisfullyasymmetric.Fournitrogen(azido)atomsandoneNiformaplanewhilethe other twonitrogen(azido) atoms andthe other Ni are clearlyseparatedfromthis plane. Thedistancestothemeanplaneare1.32A, (Ni)and1.62A, (N-azido).1054 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig.17.Structureof[Mn(bpy)(m1,1-N3)(m1,3-N3)2]n(top)andMT()andM()forthiscompoundasafunctionofthetemperature(bottom).angleis138withaNi N3Ni dihedral angleof180. ThedatahavebeenttedwiththeBorrasformula[52]foranantiferromagneticalternatingS=1chainandwith the standard formula for a dinuclear NiIIunit (i.e. assuming that JEO value, iszero). The parameters of the best t and their reliability factors (R) are J1=31.8cm1, J2=6.9 cm1, g=2.35 and R=1.5104for the alternating chain andJ=29.3 cm1, g=2.26 and R=6.6103for a dinuclear unit. The followingFig. 18. Structureofthecompound[Ni2(Medien)(m1,1-N3)2(m1,3-N3)]n(ClO4)n.1055 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 19. OPTEP plot showing the structure of the triply EO and single EE azide bridging 1D compound[Ni(tmeda)(m-N3)2]n.values were determinedfor the maximumsusceptibility: 0.01141(experimental),0.01160 (alternating chain), and 0.01192 cm3mol1(dinuclear unit). In both casesthe experimental value in this zone is lower than the theoretical, and the differenceis even greater for the dinuclear model. This suggests AF coupling is present in bothpartsofthechain(J1andJ2negative).6.3. OtheralternatingchainsAn example of a different (and more evident) strategy to obtain alternating chainsis the complex [(Ni2(dpt)2(m-ox)(m-N3))n](PF6)n in which two different bridging unitsalternate: oxalato and azido, respectively [20] (Fig. 20). The perchlorate analogue isalsoreportedbut without crystal structure. Theoxalatogroupiscoordinatedinbis-bidentate formas occurs habitually, and the azido ligand is coordinatedin end-to-end mode. Two kind of non-equivalent nickel atoms are presentFig. 20. Structureof thealternatingchaininwhichtwodifferent bridgingunits(oxalatoandazido)alternate: [Ni2(dpt)2(m-ox)(m-N3)]n(PF6)n.1056 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068in the chain, which show different bond parameters and different coordination modesoftheamminateligand.Themostremarkablefeaturesoftheazidobridgearetheasymmetricend-to-endcoordination, withNi NN=120.1and119.41, respec-tively, and the unusually high value (102.1) for the torsion angle of Ni N3Ni entity.Applying the Borras equation [52] for an alternating 1-D chain with S=1 the J valuesobtainedforthehexauorophosphatecomplexare: J1=27.4cm1, J2=2.7cm1, g=2.19 (h=0.1) (those for the perchlorate analogue are very similar). Takingas reference the bibliographic J values for Ni oxNi and Ni N3Ni systemsreportedtodate, assignationofthecouplingparametersisimmediate: forall thenickel oxalato dinuclear systems, the J parameters have been found in the range 30to 39 cm1. This practically constant J value is a consequence of the low exibilityof the oxalato ligand, roughly planar in all cases, which shows no signicantdifferencesinthebondparametersinthebridgingregion.ThelowvalueforJ2isdue, undoubtedly, to the very high torsion angle of the Ni N3Ni entity (102.1).7.Two-dimensionalsystemsWhile the discrete molecules or 1-D systems with azido bridge have been found,so far, to be mainly nickel derivatives, the high dimensional compounds are clearlydominatedbythe manganese(II) compounds. Inadditiontotheir structure, themagnetic properties of this kindof compoundare especially interesting at lowtemperatures. Indeed it is not rare to nd ordered systems with the properties of weakmolecularmagnets.7.1. Withonlyazidoasbridgingligand7.1.1. WithsingleEEbridgesForNiIIthesimplestcasereportedintheliteratureisthesaltCs2[Ni(N3)4]H2O[58]. The anionic part consists ina2-Dstructure formedbyNi(N3)6octahedraconnected via four azido groups placed in a distorted plane. No magnetic data havebeenreported.1057 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 21. Structureof thetwo-dimensional compound[Mn(4-acpy)2(N3)2]n(4-acpy=4-acethylpyridine)andMofthiscompoundasafunctionofthetemperatureandeld.For the manganese ion two compounds with the general formula [Mn(R-py)2(N3)2]ninwhichR-pyare pyridine derivatives have beencharacterized. For4-acpy(4-acetylpyridine) [59] or Minc(methylisonicotinate) [60], thecompoundsconsist of manganese atoms with the two pyridinic ligands coordinated in trans andfour end-to-end azido bridges, which link to the four neighboring manganese1058 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068atoms, giving a quadratic layer (Fig. 21). The two compounds showsimilarstructural parameters in the bridging region: MnNN bond angles of 129.0/151.6and128.3/149.7,respectively.TheMnN3Mntorsionanglesare98.1and94.6forthe4-acpyandMinccompounds, respectively. Asmaybeexpectedfromthetheoretical calculations, the coupling was antiferromagnetic for the two com-pounds, 3.83 cm1(4-acpy) and 2.24 cm1(Minc). Owing to cantingphenomena,the[Mn(4-acpy)2(N3)2]ncompoundshowslongrangeorderbelowtheTc=28Kandspontaneous magnetizationandhysteresis loopmeasuredat 2K[61].7.1.2. EEandEObridgesWith NiIIand the 2,2%-dimethyl-propanediamine ligand (2,2%-mettn), a newtwo-dimensional systemwith ferroantiferromagnetic alternation has been ob-tained: [Ni(2,2-mettn)(m-N3)]n[62,63]. InthiscomplexeachbridgingazidoligandcoordinatestwoNiIIionsinanEOmodebut, atthesametime, thissameazidoligand coordinates the neighboring NiIIion in an EE coordination mode (Fig. 22).This linkage is extended through a layer and the layers are linked by hydrogen andvan der Waals forces. The main interest of this systemlies in its magneticproperties: it shows a canting phenomenon at low temperature. The cantingbehaviorcanbedenedasantiparallel spinscantedwithasmall angle. Thetotalresultofthesesmall vectorsisnotzerowhenthecantinganglevariesfromzero.This canting gives the complex the characteristics of a molecular magnet (hysteresisloop, for example). It is, on the other hand, the molecular complex that shows thiscantingphenomenonatthehighesttemperature(closeto50K).Fig. 22. Structureofthetwo-dimensional compound[Ni(2,2-mettn)(m-N3)]n(2,2-mettn=2,2-dimethyl-propane-1,3-diamine).1059 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 23. General structureofthetwo-dimensional compounds[Ni(amine)(N3)2]n(amine=N-isopropy-lethylenediamine, N,N-diethylethylenediamineandN,N-diethyl-N%-methylethylenediamine).For N-isopropylethylenediamine, N,N-diethylethylenediamineandN,N-diethyl-N%-methylethylenediamine three new two-dimensional complexes of general formula[Ni(amine)(m-N3)2]n have recently been reported [64]. The structural arrangement isthesameineachcase, consistingof dinuclear nickel units bridgedbymeans ofdouble EO azido bridges, linked to the four equivalent units by means of four singleEEazidobridges(Fig. 23).Todescribethemagneticdataasimpliedpatternbasedonasphereformofeight local S=1 centers was designed using the full-diagonalization matrix method[65].ThetwoJvaluesareferromagneticforthethreecomplexes.MainstructuralandmagneticparametersaregatheredinTable8.The same structure was foundpreviously infour MnIIcompounds withthegeneralformula[Mn(R-py)2(N3)2]ninwhichR-pyisEtnic(ethylisonicotinate)[66],4-CNpy (4-cyanopyridine), 3-acpy (3-acetylpyridine) [61] and Enic (ethylnicotinate)[67]. Bridgingbondparametersaresimilar, withtheEOMnNMnbondangleTable8Structural andmagneticparametersfor2-DNiIIferromagneticcompoundsaJ NiNN Reference NiNNi ~99.3 116.8 27.3 64 [Ni(N-ipen)(m-N3)2]n125.91.3 132.664 30.2 112.8 [Ni(N,N-deen)(m-N3)2]n100.9 132.50.8 134.8137.5 100.5 [Ni(N,N-de-N%-men)(m-N3)2]n110.4 64 26.1145.0 9.4aNiNNi angles(); NiNNangles(); NiN3Ni ~angles(); Jincm1(N-ipen=N-isopropyl-ethylenediamine; N,N-deen=N,N-diethylethylenediamine; N,N-de-N%-men=N,N-diethyl-N%-methyl-ethylenediamine).1060 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 24. Typical propertiesderivedfromthecantingphenomenonin[MnL2(m-N3)2]n.between100.5and104.1andMnN3Mntorsionangles(~)comprisedbetween124.7and114.7. Jparameterswerenotcalculatedbecausetheanalytical expres-sions for alternating 2-D systems were not available, but as expected, the magneticbehavior corresponds to a ferroantiferromagnetic system. Magnetic data for Eniccompound were not reported, but for the other three compounds long range orderwasfoundat16K(Etnicand3-acpy)and28K(4-CNpy).Thetypicalpropertiesderived from the canting phenomenon including hysteresis loop, broadening of theEPRsignal whenTisclosetotheTcwerecharacterized, [61] (Fig. 24).7.2. CompoundswithazideandasecondbridgingligandAlternating2-Dsystemsof manganesehavebeenobtainedfollowingasimilarstrategy by using a secondbridging group: the rst of these withcarboxylatebridges, withformula[Mn(py2c)(N3)(H2O)]n, Hpy2c=pyridine 2-carboxylic acid[68], in which the oxygen atoms from the carboxylate group act as a bridge betweentwomanganeseatomsbymeansof oneof theoxygenatoms, givingadinuclear1061 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068entityMn2O2(MnOMn=105.9). Eachunitislinkedtofoursimilarunitsbymeans of four end-to-endazidobridges withnormal MnNNbondangles of139.5 and 127.4. Fromthe bond parameters in the bridging region and thecoordination mode of the azido groups, antiferromagnetic behavior should beexpected, butthemagneticdatawerenotreported.Afteravarietyof experiments, substitutionof thebipyridineligandbythat ofthe bipyrimidine (bipym) in the alternating ferroantiferromagnetic 1-Dcom-pound[Mn(bpy)(m1,1-N3)(m1,3-N3)] [55] ledtothe formationof the 2-Dcomplex[Mn2(m1,1-N3)4(m-bipym)]n[69]. The crystal structure of this compound showschainsof MnIIionsconnectedbydouble1,1-N3azidobridges. Thesechainsarealternativelyconnectedbybis-bidentatebipyrimidineligands(Fig. 25). Thewholestructure canalsobe describedas ahoneycombsheet, consistingof aninnitehexagonal arrayof MnIIions bridgedbybis-bidentatebipyrimidineligands anddoubleend-onazidobridges. This 2-Dpolymer extends inthexyplanebytherepetitionofalmostcircularrings(Fig. 25).The magnetic measurements for this 2-Dindicates ferromagnetic interactionsinthehightemperaturerange, followedbyantiferromagneticinteractions corre-spondingtothelowtemperature. Unfortunately, thelackof atheoretical modelforthismagneticplanehasprecludedadetailedanalysisofitsmagneticbehaviortodate.8.Three-dimensionalsystems3-Dcompounds represent achallengefor researchers intheeldof magneticmolecular systems. This is because of the needbothtounderstandtheir mag-netostructural correlations andtoobtain3-Dantiferro- or ferromagnets withpossibletechnical applications. 3-Dsystems withazidobridginggroupcouldbeFig. 25. Structureofthehoneycomb 2-Dcomplex[Mn(m1,1-N3)(m-bipym)]n.1062 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 26. Plot ofMT () andM () of [Mn(py)2(m1,3-N3)2]n as a function of the temperature showingthehighTcof40Kforthelongrangeorder.obtained by either using metal:monodentate ligands in a 1:1 ratio, connecting2-Dcompounds throughtrans-bidentate ligands or by preparationwithout theuseofancillaryblockingligands.One 3-Dcompound, [Mn(py)2(m1,3-N3)2]nhas been obtained using this lastapproach[70]. Thecompoundshowsatransarrangementofthepyridineligandsandfour EEbridges tothe neighboringmanganese atoms inasimilar waytothe 2-Dcompounds with R=4-acpy or Minc. In these 2-Dcompounds thepyridine-like ligands are placed above and below the two-dimensional sheet,whereasinthe3-Darrangement thepyligandsareplacedalongchannels. Bondangles lie inthe normal range, MnNN=134.1and145.4, andmagneticallyshowthe expectedantiferromagnetic coupling, withJ=1.35cm1(Fig. 26).ThehighTcof 40Kisnoticeableforthelongrangeorderwiththepresenceofacantingphenomenon[61].Asecond 3-Dcompound with only azide bridges was [N(CH3)4]n[Mn(m1,3-N3)3]n[71]. Theproportionof threeazidoligands per divalent ionwas achievedbyusinganappropriatecation. Thecrystal structureof thecompoundisshowninFig. 27. This structuremaybedescribedas adistortedperovskitewheretheMnIIis shiftedfromtheoriginof theunit cell byca. 0.25A, alongtheb-axis.Theazidogroupsact asend-to-endbridgingligandsbetweenmetalstoformthe3-Dnetwork. Eachmanganeseionshows aslightlydistortedoctahedral coordi-nationsphere. The [N(CH3)4]+cations are locatedwithinthe holes formedbythe MnII-azido sublattice (Fig. 27). The m1,3-N3bridges formtwo asymmetricangleswiththemetal: MnNNaround165andNNMn% around135.The magnetic measurements for this complexclearlyindicate the existence ofAFinteractions (Fig. 27). The magnetic behavior canbe explainedassuminga1063 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068regular 3-Dnetworkas indicatedbythecrystal structure. Thebest least-squaretoftheexperimental datatotheexpressionofthemagneticsusceptibilityforasimple cubic Heisenberg antiferromagnetic system [72] is obtained with theparametersJ=1.21cm1andg=2.01.Anewexample of 3-DMnIIazido compound is a polymorph of the 2-D[Mn2(m1,1-N3)4(m-bipym)]nsystem [69], with the formula [Mn2N3(m1,1-N3)(m1,3-N3)(m-bipym)]n, [69b]. The crystal structure consists of chains of manganeseatomsalternatelybridgedbybipymandtwoend-onazidogroups(Fig. 28). Thechains are connectedbymeans of end-to-endazidogroups invicinal positions,whichensureslinkagesintwodifferent directions. ItsmagneticpropertiesareinFig. 27. Structure of the three-dimensional compound [N(CH3)4]n[Mn(m1,3-N3)3]n (top) andMT () ofthiscompoundasafunctionofthetemperature.1064 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068Fig. 28. Structureofthethree-dimensional compound[Mn2N3(m1,1-N3)(m1,3-N3)(m-bipym)]n.Fig. 29. Structure of the three-dimensional compound [Mn(m4,4%-bpy)(m1,3N3)2]n complex showing thehelical chains of MnIIions of two different types (top) andMT of this compound as a function of thetemperatureatdifferentmagneticelds.1065 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068accordwithpredominantantiferromagneticinteractions,whicharepropagatedbythe bipymand the end-to-end azido bridges and which are larger than theferromagneticinteractionscorrespondingtotheend-onazidobridges.Another interesting example of 3-DMnII-azido systems is the [Mn(m-4-4%-bpy)(m1,3-N3)2] complex [73]. The crystal structure (Fig. 29) shows helical chains ofMnIIions of two different types. The chains of the rst type are connected by singleEE azido bridges ( MnN3MnN3), while the second type exhibit two consec-utiveEEazidobridges, followedby4,4%-bipyridinebridges ( MnN3MnN3Mn4,4%-bpy). The whole structure has the form of a complex 3-D network as aresult of the propagation of these different helicals. The MnIIion exhibits a slightlydistortedoctahedral environment formedby six nitrogenatoms, four fromtheazidoandtwofromthe4,4%-bpy, respectively. TheMnNdistances rangefrom2.185(6) to2.301(4) A, (bpy). TheEEazidobridgesformtwoasymmetricangleswiththemetal, withvaluesof153.0(5)and121.9(4).The magnetic behavior of this complex is characteristic of a systemwithantiferromagnetic interactions in the high temperature region, up to 50 K, where amagnetic transition to a ferromagnetic state occurs. The temperature dependence ofthe molar magnetization was investigated within a magnetic eld of 50 Gauss. Theeld-cooled magnetization showed an abrupt break at Tc=45 K which is charac-teristicofamagneticallyorderedstateexistingbelow45K[74].The eld dependence behavior of the magnetization at 5 K can be assigned to anantiferromagnetic system with a weak ferromagnetic component. At magnetic eldslowerthan2500Gauss, magnetichysteresisisobserved. Thiskindofbehaviorisprobablyduetoacantingphenomenon.AcknowledgementsTheauthorsthanktheSpanishgovernment(GrantPB96/0163)andtheBasquegovernment(GrantPI96/39)forthenancial support.References[1] R.D. Willet, D. Gatteschi, O. Kahn (Eds.), MagnetoStructural Correlations in Exchange CoupledSystems, NATOASISeries, Reidel, Dordrecht, 1985.[2] O. Kahn, MolecularMagnetism, VCHPublishers, Weinheim, 1993.[3] M.M. Turnbull, T. Sugimoto, L.K. Thompson (Eds.), Molecular-Based Magnetic Materials:Theory, TechniquesandApplications. ACSSymposiumSeries, n.644, ACS, Washington, 1996.[4] A. Caneschi, D. Gatteschi, L. Pardi, R. Sessoli, Clusters, Chains andLayeredMolecules: theChemists Way to Magnetic Materials, in: A.F. Williams (Ed.), Perspectives in CoordinationChemistry, VCH, Weinheim, 1992.[5] E.Coronado,P.Delhaes,D.Gatteschi,J.S.Miller(Eds.),MolecularMagnetism:fromMolecularAssembliestoDevices, NATOASISeries, Vol.321, Kluwer, Dordrecht, 1995.[6] D.W. Bruce, D. OHare(Eds.), InorganicMaterials, Wiley, 1992.[7] P. Delhaes, M. Drillon(Eds.), OrganicandInorganicLow-Dimensional Materials, NATOASISeries, vol. 168, PlenumPress, NY, 1987.1066 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068[8] D.Gatteschi,O.Kahn,J.S.Miller,F.Palacio(Eds.),MagneticMolecularMaterials.NATOASISeries, Vol.198. Kluwer, Dordrecht, 1993.[9] C.J.OConnor(Ed.),ResearchFrontiersinMagnetochemistry.WorldScientic,Singapore,1993.[10] J. Commarmond, P. Plumere, J.M. Lehn, Y. Agnus, R. Louis, R. Weiss, O. Kahn, I. Morgensten-Badarau, J. Am. Chem. Soc. 104(1982)6330.[11] (a) M.F. Charlot, O. Kahn, M. Chaillet, C. Larrieu, J. Am. Chem. Soc. 108 (1986) 2574. (b) M.A.Aebersold, B. Gillon, O. Platevin, L. Pardi, O. Kahn, P. Bergerat, I. vonSeggern, F. Tuczek, L.Ohrstrom, A. Grand, E. Levie`vre-Berna, J. Am. Chem. Soc. 120(1998)5238.[12] A. Bencini, C.A. Ghilardo, S. Midollini, A. Orlandini, Inorg. Chem. 28(1989)1958.[13] O. Kahn, T. Mallah, J. Gouteron, S. Jeannin, Y. Jeannin, J. Chem. Soc. Dalton Trans. (1989) 1117.[14] (a)O.Castell,R.Caballol,V.M.Garc a,K.Handrick,Inorg.Chem.35(1996)1609.(b)E.Ruiz,J. Cano, S. Alvarez, P. Alemany, J. Am. Chem. Soc. 120(1998)11122[15] (a) M.I. Arriortua, R. Cortes, L. Lezama, T. Rojo, X. Solans, M. Font-Bard a, Inorg. Chim. Acta174(1990)263.(b)T.Rojo,L.Lezama,R.Cortes,J.L.Mesa,M.I.Arriortua,J.Magn.Mat.83(1990)519. (c)R. Cortes, L. Lezama, T. Rojo, M. KarmeleUrtiaga, M. Isabel Arriortua, IEEETrans. onMagn. 30(1994)4728[16] R. Cortes, K. Urtiaga, L. Lezama, J.L. Pizarro, A. Gon i, M.I. Arriortua, T. Rojo, Inorg. Chem. 33(1994)4009.[17] A. Escuer, R. Vicente, J. Ribas, M.S. El Fallah, X. Solans, M. Font-Bard a, Inorg. Chem. 32 (1993)3727.[18] A. Escuer, R. Vicente, J. Ribas, M.S. El Fallah, X. Solans, M. Font-Bard a, Adv. Mater. Res. 1(1994)581.[19] A. Escuer, R. Vicente, M.S. El Fallah, X. Solans, M. Font-Bard a, J. Chem. Soc. DaltonTrans.(1996)1013.[20] A. Escuer, R. Vicente, X. Solans, M. Font-Bard a, Inorg. Chem. 33(1994)6007.[21] J. Ribas, M. Monfort, C. Diaz, C. Bastos, X. Solans, Inorg. Chem. 32(1993)3557.[22] J. Ribas, M. Monfort, C. Diaz, A. Escuer, R. Vicente, M.S. El Fallah, X. Solans, M. Font-Bard a,Adv. Mater. Res. 1(1994)573.[23] (a) R. Vicente, A. Escuer, J. Ribas, X. Solans, Inorg. Chem. 31(1992) 1726. (b) R. Vicente, A.Escuer, Polyhedron14(1995)2133.[24] P. Chaudhuri, T. Weyhermu ller, E. Bill, K. Wieghardt, Inorg. Chim. Acta252(1996)195.[25] D.M. Duggan, D.N. Hendrickson, Inorg. Chem. 13(1974)2929.[26] C.G. Pierpont, D.N. Hendrickson, D.M. Duggan, F. Wagner, E.K. Bareeld, Inorg. Chem. 14(1975)604.[27] F.Wagner,M.T.Mocella,M.J.J.DAniello,A.H.J.Wang,E.K.Bareeld,J.Am.Chem.Soc.96(1974)2625.[28] G.A. McLachlan, G.D. Fallon, R.L. Martin, B. Moubaraki, K.S. Murray, L. Spiccia, Inorg. Chem.33(1994)4663.[29] (a) L. Fabrizzi, P. Pallavicini, L. Parodi, A. Perotti, N. Sardone, A. Taglietti, Inorg. Chim. Acta 244(1996) 7. (b) A. Escuer, C.J. Harding, Y. Dussart, J. Nelson, V. McKee, R. Vicente, J. Chem. Soc.DaltonTrans. (1999)223.[30] H.Weller,L.Siegfried,M.Neuburger,M.Zenhder,T.A.Kaden,HelveticaChim.Acta80(1997)2315.[31] P.J. Hay, J.C. Thibeault, R. Hoffmann, J. Am. Chem. Soc. 97(1975)4884.[32] R. Cortes, K. Urtiaga, L. Lezama, M.L. Rodr guez, M.I. Arriortua, T. Rojo, Inorg. Chem.,submittedforpublication[33] P. Chaudhuri, M. Guttmann, D. Ventur, K. Wieghardt, B. Nuber, J. Weiss, J. Chem. Soc. Chem.Commun. (1985)1618.[34] A.P. Ginsberg, R.L. Martin, R.W. Brookes, R.C. Sherwood, Inorg. Chem. 11(1972)2884.[35] A. Escuer, R. Vicente, J. Ribas, X. Solans, Inorg. Chem. 34(1995)1793.[36] M.A.S. Goher, F.A. Mautner, A. Popitsch, Polyhedron12(1993)2557.[37] R. Cortes, J.L. Pizarro, L. Lezama, M.I. Arriortua, T. Rojo, Inorg. Chem. 33(1994)2697.[38] P.D. Beer, M.G.B. Drew, P.B. Leeson, K. Lyssenko, M.I. Ogden, J. Chem. Soc. Chem. Commun.(1995)929.1067 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068[39] A. Escuer, I. Castro, F.A. Mautner, M.S. El Fallah, R. Vicente, Inorg. Chem. 36(1997)4633.[40] J. Ribas, M. Monfort, R. Costa, X. Solans, Inorg. Chem. 32(1993)695.[41] M.A. Halcrow, J.C. Huffman, G. Christou, Angew. Chem. Int. Ed. Engl. 34(1995)889.[42] M.A. Halcrow, J.-S. Sun, J.C. Huffman, G. Christou, Inorg. Chem. 34(1995)4167.[43] K. Urtiaga, Z.E. Serna, G. Barandika, L. Lezama, M.I. Arriortua, R. Cortes, ProceedingsoftheXXXIIIICCC, p. 421.[44] S. Brooker, V. McKee, J. Chem. Soc. Chem. Commun. (1989)619.[45] A. Escuer, R. Vicente, J. Ribas, M.S. El Fallah, X. Solans, M. Font-Bard a, Inorg. Chem. 33 (1994)1842.[46] C. Mealli, D.M. Proserpio, J. Chem. Educ. 67(1990)3399.[47] (a)L.K.Chou,D.R.Talham,W.W.Kim,P.J.C.SignoreandM.W.Meisel,PhysicaB194(1994)313. (b) T. Takeuchi, H. Hori, M. Date, T. Yosida, K. Katsumata, J.P. Renard, V. Gadet, M.Verdaguer, J. Magn. Magn. Mat. 104 (1992) 813. (c) T. Takeuchi, T. Yosida, K. Inoue, M.Yamashita, T. Kumada, K. Kindo, S. Merah, M. Verdaguer, J.P. Renard, J. Magn. Magn. Mat.140(1995)1633.[48] A. Escuer, R. Vicente, M.A.S. Goher, F.A. Mautner, Inorg. Chem. 37(1998)782.[49] J. Ribas, M. Monfort, C. Diaz, C. Bastos, X. Solans, Inorg. Chem. 33(1994)484.[50] T. deNeef, PhDThesis, Eindhoven, 1975.[51] A. Escuer, R. Vicente, F.A. Mautner, M.A.S. Goher, Proceedingsof theXXXIIIICCC, p.356.[52] J.J. Borras-Almenar, E. Coronado, J. Curely, R. Georges, Inorg. Chem. 34(1995)2699.[53] (a)M. Hagiwara, Y. Narumi, K. Kindo, M. Kohno, H. Nakano, R, Sato, M. Takahashi, Phys.Rev. Lett. 80(1998)1312. (b)M. Kohno, M. Takahashi, M. Hagiwara, Phys. Rev. B, 57(1998)1046.[54] (a)J.Ribas,M.Monfort,I.Resino,X.Solans,P.Rabu,F.Maingot,M.Drillon,Angew.Chem.Int. Ed. Engl. 35(1996)2520. (b)F. Esposito, G. Kamieniarz, Phys. Rev. B57(1998)7431.[55] (a) R. Cortes, L. Lezama, J.L. Pizarro, M.I. Arriortua, X. Solans, T. Rojo, Angew. Chem. Int. Ed.Eng.33(1994)2488.(b)R.Cortes,M.Drillon,X.Solans,L.Lezama,T.Rojo,Inorg.Chem.36(1997)677.[56] A. Escuer, R. Vicente, M.S. El Fallah, S.B. Kumar, F.A. Mautner, D. Gatteschi, J. Chem. Soc.DaltonTrans. (1998)3905.[57] J. Ribas, M. Monfort, B. Kumar-Gosh, X. Solans, Angew. Chem. Int. Ed. Engl. 33(1994)2087.[58] H.E. Maier, H. Krischner, H. Paulus, Zeitschriftfu rKristallographie157(1981)277.[59] A. Escuer, R. Vicente, M.A.S. Goher, F.A. Mautner, Inorg. Chem. 34(1995)5707.[60] A. Escuer, R. Vicente, M.A.S. Goher, F.A. Mautner, J. Chem. Soc. DaltonTrans. (1997) 4431.[61] A. Escuer, R. Vicente, M.A.S. Goher, F.A. Mautner, Inorg. Chem. 36(1997)3440.[62] M. Monfort, J. Ribas, X. Solans, J. Chem. Soc. Chem. Commun. (1993)350.[63] J. Ribas, M. Monfort, X. Solans, M. Drillon, Inorg. Chem. 33(1994)742.[64] M. Monfort, J. Ribas, I. Resino, M.S. El Fallah, H. Stoeckly-Evans, ProceedingsoftheXXXIIIICCC, p.386.[65] CLUMAGProgram, D. Gatteschi, L. Pardi, Gazz. Chim. Ital 123(1993)231.[66] A. Escuer, R. Vicente, M.A.S. Goher, F.A. Mautner, Inorg. Chem. 35(1996)6386.[67] M.A.S. Goher, N.A. Al-Salem, F.A. Mautner, J. Coord. Chem. 44(1998)119.[68] M.A.S. Goher, M.A.M. Abu-Youssef, F.A. Mautner, A. Popitsch, Polyhedron11(1992)2137.[69] (a) G. De Munno, M. Julve, G. Viau, F. Lloret, J. Faus, D. Viterbo, Angew. Chem. Int. Ed. Engl.35 (1996) 1807. (b) R. Cortes, L. Lezama, J.L. Pizarro, M.I. Arriortua, T. Rojo, Angew. Chem. Int.Ed. Engl. 35 (1996) 1810. (c) R. Cortes, M.K. Urtiaga, L. Lezama, J.L. Pizarro, M.I. Arriortua, T.Rojo, Inorg. Chem. 36(1997)5016.[70] M.A.S. Goher, F.A. Mautner, Croat. Chim. Acta63(1990)559.[71] F.A. Mautner, R. Cortes, L. Lezama, T. Rojo, Angew. Chem. Int. Ed. Engl. 35(1996)78.[72] G.S. Rushbrook, P.J. Wood, Mol. Phys. 1(1958)257.[73] (a) R. Cortes, S. Mart n, L. Lezama, J.L. Pizarro, M.I. Arriortua, T. Rojo, Inorg. Chem., submittedfor publication. (b) H.Y. Shen, D.Z. Liao, Z.H. Jiang, S.P. Yan, B.W. Sun, G.L. Wang, X.K. Yao,H.G. Wang, Chem. Lett. (1998)469.1068 J. Ribasetal. / CoordinationChemistryRe6iews193195(1999)10271068[74] R. Cortes, S. Mart n, L. Lezama, G. Barandika, T. Rojo, J.L. Pizarro, Proceedings of the XXXIIIICCC, p. 352.[75] J. Ribas, M. Monfort, B. Kumar-Gosh, R. Cortes, X. Solans, M. Font-Bard a, Inorg. Chem. 35(1996)864.[76] A.Escuer,R.Vicente,M.S.ElFallah,X.Solans,M.Font-Bard a,Inorg.Chim.Acta278(1998)43.[77] R. Cortes, J.I.R. Larramendi, L. Lezama, T. Rojo, K. Urtiaga, M.I. Arriortua, J. Chem. Soc.DaltonTrans. (1992)2723.[78] R. Vicente, A. Escuer, J. Ribas, M.S. El Fallah, X. Solans, M. Font-Bard a, Inorg. Chem. 32 (1993)1920.[79] A. Escuer, R. Vicente, J. Ribas, J. Magn. Magn. Mater. 110(1992)181.[80] A.Escuer,R.Vicente,M.S.ElFallah,X.Solans,M.Font-Bard a,Inorg.Chim.Acta247(1996)85.[81] R. Cortes, L. Lezama, J.I.R. Larramendi, M. Insausti, J.V. Folgado, G. Madariaga, T. Rojo, J.Chem. Soc. DaltonTrans. (1994)2573.[82] X. Solans, C.Diaz, M.Monfort, J.Ribas, XV International Crystallographic Meeting. Burdeos, 1990[83] M. Monfort, J. Ribas, X. Solans, M. Font-Bard a, Inorg. Chem. 35(1996)7633.[84] C. Bastos, C. Diaz, M. Monfort, J. Ribas, X. Solans, Abstracts 29thICCC, Lausanne, 1992, p. 644.[85] A. Escuer, R. Vicente, J. Ribas, M.S. El Fallah, X. Solans, Inorg. Chem. 32(1993)1033.[86] R. Vicente, A. Escuer, J. Ribas, M.S. El Fallah, X. Solans, M. Font-Bard a, Inorg. Chem. 34 (1995)1278.[87] A. Escuer, R. Vicente, M.S. El Fallah, J. Ribas, X. Solans, M. Font-Bard a, J. Chem. Soc. DaltonTrans. (1993)2975.[88] J. Ribas, M. Monfort, C. Diaz, C. Bastos, C. Mer, X. Solans, Inorg. Chem. 34(1995)4986.[89] (a) G. Viau, M.G. Lombardi, G. De Munno, M. Julve, F. Lloret, J. Faus, A. Caneschi, J.M.Clemente-Juan, J. Chem. Soc. Chem. Commun. (1997) 1195. (b) J.J. Borras-Almenar, J.M.Clemente-Juan, E. Coronado, F. Lloret, Chem. Phys. Lett. 275(1997)79[90] J. Ribas, M. Monfort, B. Kumar-Gosh, X. Solans, M. Font-Bard a, J. Chem. Soc. Chem. Commun.(1995)2375...