Complexes of Carbon Monoxide and Its Relatives: An Organometallic Family Celebrates Its Birthday

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  • International Edition in English

    Complexes of Carbon Monoxide and Its Relatives: An Organometallic Family Celebrates Its Birthday **

    By Helmut Werner*

    Dedicated to Professor Max Schmidt on the occasion of his 65th birthday

    Nickel tetracarbonyl was discovered one hundred years ago. Its hundredth birthday deserves to be celebrated not only by metal carbonyl chemists. The pioneering work of Mond, Langer, and Quincke has led to developments which, particularly in the last 30 years, have had consequences in many areas outside that of metal carbonyl chemistry. The parent ligand of the family, CO, has been joined by a series of relatives which are isoelectronic with it and which in some cases are even more effective as K acceptors. These are in the main extremely reactive molecules such as CS, CNH, and C=CH,, which, though very short-lived in the free state, form very stable complexes with transition metals. This article documents the family relation- ships by structure and reactivity comparisons; attention is also drawn to the synthetic potential of the metal carbonyl analogues, which is still virtually untapped. The final section ventures a look towards the next 100 years, which promise to be just as exciting as the past century.

    Volume 29 . Number 10 October 1990

    Pages 1077 - 1 176

    1. Introduction

    An organometailic family celebrates its hundredth birth- day this year. In August 1890 an article by Ludwig Mond, Carl Lunger, and Friedrich Quincke appeared in the Journal of the Chemical Society with the title Action of Carbon Monoxide on Nickel; in this paper the authors described the synthesis of nickel tetracarbonyl.] Although this com- pound, isolated as a colorless, clear, extremely toxic liquid, was not the first metal compound containing carbonyl li- gands- -the first, [Pt(CO),Cl,], had already been synthesized by Schiitzenberger in 1868 [-its remarkable properties and its industrial applicability to the preparation of ultrapure nickel were immediately recognized by Mond. This serendip- itous discovery 13] started a development which reached its

    ~~

    [*] Prof. Dr. H. Werner lnstitut fur Anorganische Chemie der Universitat Am Hubland, D-8700 Wiirzburg (FRG)

    [**I An extended version of a plenary lecture presented at the IVth European Symposium on Inorganic Chemistry, 12-15 September 1988, Freiburg (FRG).

    first peak in the work of Walter HieberC4 and which, during the last 40 years, has played a very important role in the often-cited renaissance of inorganic chemistry.

    This review will not focus, however, on the chemistry of metal carbonyls, which remains just as fascinating and as topical as it has always been (see Fig. 1). Nickel tetracar- bony1 and other stable binary carbonyls, some of them al- ready described by Ludwig M ~ n d , [ ~ ] have been joined in re- cent years by very labile carbonyls such as [Cu,(CO),], [Ag,(CO),], [Pt(CO),], and [Ti(CO),].[] Thanks in great part to the pioneering work by Chini, Longoni, Dahl et a1.I6] and Lewis et al.,17] carbonylmetal clusters have achieved a degree of importance undreamed of twenty years ago, in particular as cage frameworks for both smaller and larger metal and nonmetal atoms.*] Moreover, carbonyl metalates have found an important place in synthetic chemistry-not only because of the importance of Collmans reagent, Na,[Fe(CO),] .[I There are still unmarked areas on the metal carbonyl map, however, as shown, for example, by the fact that the synthesis of salts of the hexacarbonyltitanate dian- ion, [Ti(C0),lze, the first stable binary carbonyl compound

    Angrw Chem. In!. Ed. Engl. 29 (1990) 1077-1089 0 VCH VerlagsgesellschuJi mbH, 0-6940 Wernheim, 1990 0570-0833190/10t0-1077 S 3 . 5 0 i .2510 1077

  • ecule to one or several metal atoms can be either end-on or via p2 or p3 bridges (Fig. 2). This is also true for the relatives.

    The family of CO-like molecules has two separate branch- es: the first includes the molecules formed from CO by sub- stitution of the carbon atom by a homologous element of the fourth main group or by an isoelectronic ion (Bo, N@). The second contains the molecules derived from CO by replacing

    terminal u2 bridge p3 bridge I ICO),l IFe,lCOl,l IRh6(C01,61

    RLJ

    0

    oc; co Pentamer Fig. 2 Coordination possibilities for CO.

    Fig. 1 . [Ni(CO),] and other, much later synthesized metal carbonyls

    of titanium, was described by Ellis only in 1988, that is, almost 100 years after the discovery of nickel tetracar- bonyl.]

    The metal carbonyls have meanwhile been joined by rel- atives, which are the subject of this article. Remarkably, their history is almost as long as that of their ancestor. Soon after the publication of Monds results, Sabatier et al. in Toulouse attempted to bond NO, NO,, C,H,, and C,H, to nickel and similar metals; although they were unsuccess- ful, their attempts did in fact lead to the discovery that nickel could be used as a catalyst in hydrogenation.{] There was then a gap of 75 years before Wilke et al. were successful in preparing [Ni(C,H4)J ,[I2] a compound similar to [Ni(CO),] although structurally different. It was then used in the syn- thesis of further nickel(o) complexes, some of which are cat- alytically extremely active. 31

    Here, however, ethylene will not be regarded as a relative of carbon monoxide. Although there are indeed close rela- tionships between metal carbonyls such as [Ni(CO),] and compounds such as [Ni(C,H,)3],[21 [Ni(PC13),],141 [Ni(P- Ph3),],[ and K,[Ni(C E CH)4],[61 only those molecules or fragments that are valence-isoelectronic with CO will be con- sidered relatives. Such ligands resemble CO not only formal- ly but also in their coordination properties and thus form a family with it. For example, the bonding of the parent mol-

    the oxygen atom by S, Se, or Te or by an isoelectronic frag- ment such as NH or CH,. The corresponding classes of compounds will not be discussed in detail; instead, this arti- cle will make clear the relationships within the family as well as the particular features of the various members of the family.

    2. The Relatives of the Type [XEO]: Known and Unknown Examples

    The closest relative of CO, at least from the point of view of the preparative chemist dealing with this type of molecule, is surely SiO (Scheme 1). This molecule is formed when SiO, is heated with Si under vacuum at 1250 C and can be detect- ed both in the gas phase and at low temperatures in a suitable matrix. The Si-0 bond length of 1.51 8, agrees well with the theoretical value of 1.50 8, and shows that SiO is isosteric with CO.[]

    pioq m/ PF] salts stable

    unknown p Z T salts short-lived 5-0 1.51 8,

    N-o 1.06a

    Scheme 1.

    Helmut Werner was born in 1934 in Miihlhausen (Thiiringen). He studied chemistry at Jena, where he received his Diplom under Franz Hein, and at the Technische Hochschule in Munich, where he received his doctorate under E. 0. Fischer. After postdoctoral research under J: H. Richards at the California Institute of Technology in Pasadena (1962- 1963), he completed his Habilitation at the end of t966 on kinetic investigations of substitution reactions of organometallic complexes. In 1968, he began work in inorganic chemistry at the Universitat Zurich, where he became a fullprofessor in 1970. At the end of 1975, he moved 10 the Universitat Wiirzburg. Helmut Werner has received numerous prizes and honors, including the Alfred-Stock- Gedachtnispreis of the German Chemical Society: since 1988, he has been a full member of the Deutsche Akademie der Naturforscher Leopoldina.

    1078 Angew. Chem. Inl. Ed. Engl. 29 (1990) 1077-1089

  • [Cr( NO141 [ M n ( C O ) ( N 0 I 3 ] [Fe(C0)2(NO)p] [Co(COf3NO] [Ni t C 0 l 4 ]

    0

    N 1.17 I

    1.76 I

    0

    Fig. 3. The [M(CO),(NO),. .] family (n = 0-4)

    0

    N 1.17 I 1.69 I

    1.88 ,Fe, 1.15

    \ c l o 0 I N C

    \ 0

    Metal complexes with SiO as a ligand are still unknown. Although it has been possible during the last 30 years, start- ing with the synthesis of the first metal cyclobutadiene com- plexes by Criegee et a1.[181 and Hiibel et al.,91 to bind many molecules that are extremely short-lived in the free state (nonexistent molecules) within the coordination sphere of a metal and thus to investigate them, corresponding at- tempts to isolate or detect complexes of the type [M(SiO)L,] have so far been unsuccessful. Even matrix studies have pro- vided no evidence for their existence.lZo1

    The situation with regard to the anion BOe is similar. The protonated molecule HBO has been detected experimental- I Y ; [ ~ I however, salts derived from it such as MBO (M = Na, K, etc.) still do not exist. Rosmus[] recently showed from SCF calculations that the potential energy functions of KBO and KCN are very similar, so that the molecule would prob- ably be quasi-linear. This structural prediction also applies to methyl boroxide, which is formally derived from BOe by methylation; this molecule was first prepared as an interme- diate by Paetzoldet al.Iz3] and was characterized very recent- ly by its PE spectrum.[z41 In view of the vast scope of the chemistry of metal carbene complexes [M( = CXY)L,] it seems remarkable that only very little is known about com- parable (isoelectronic or isosteric) compounds of the type [M(= BXY)L,].251

    The picture becomes completely different on moving to the right of CO, that is, to NO@. Not only are many salts of this cation known (the N-0 distance of 1.06 8, corresponds exactly to that of a triple bond, as in CO) but a great deal is also known about its coordination properties.[z61 In fact, the first nitrosyl complex, the dication [Fe(NO)(H,O),]@, was discovered in 1790 by Priestley! Although carbonyl and ni- trosyl metal compounds have many similarities, very little is known about species of the type [M(NO),] or about the corresponding cations or anions. Only [Cr(NO),] has been characterized: Herberhold and Razavi obtained this complex as a brown, low-melting solid by photolysis of [Cr(CO),] in an NO atmosphere.[] The species [Co(NO),] has also been reported ;[*I however, clear evidence for the existence of this compound is still wanting. Binary metal nitrosyl clusters are still an unknown class of substances; they would be of gener- al interest in connection with the bonding of NO at metal surfaces.

    Mixed carbonyl-nitrosyl metal complexes are better known. As early as 1922, Robert Mond (the son of Ludwig Mond) et al. described the compound [CO(CO),(NO)];~~ Hieber et al. and later Lewis et al. prepared the isoelectronic molecules [Fe(CO),(NO)2][301 and [Mn(CO)(NO),] The formal analogy between such compounds, which was later

    0 1.14 1 ,. L 1.84 I

    \ 0

    confirmed on a structural basis (Fig. 3), prompted See1 in 1942 to propose the nitrosyl shift relationship,[3z1 which is still of importance today as a heuristic principle. According to this postulate, it is possible to replace Ni in [Ni(CO),] by Co when one of the CO ligands (two-electron donor) is simultaneously replaced by the three-electron donor NO. The metal atoms in the ML, series (Fig. 3) thus act as pseudonickel atoms. Only one analogue of iron penta- carbonyl, the manganese complex [Mn(CO),(NO)] ,[331 has been described; the remaining members of the ML, series,

    remain to be discovered. NO0 is a stronger 71 acceptor than CO, as can be seen both

    from the stretching vibration frequencies in the IR spectra and from the bond distances determined by Hedberg and co -worke r~[~~ (see Fig. 3). In particular, the differences in the Fe-C and Fe-N bond distances in the compound [Fe(CO),(NO),] confirm that the nitrosyl ligands are more strongly bound; this is also documented by the easier ex- change of CO (e.g., for PR,) in substitution reactions of carbonyl-nitrosyl metal complexes.

    Although, as already mentioned, binary metal nitrosyl clusters do not yet exist, bi- and polynuclear compounds with NO as a ligand are already known;[26] they show the same possibilities for coordination of NO (or, to be more precise, NOe) as for CO in the corresponding metal car- bony1 complexes (see Fig. 4). When cyclopentadienyl is used

    [cr(co>,(No),l, IV(CO),(NO),l, and [Ti(CO)(NO),I still

    terminal p2 bridge ,u3 bridge

    Fig. 4. Coordination possibilities for NO.

    as the coligand, an end-on (linear) M-N-O arrangement is found in [CpNi(NO)] /351 whereas p, and F, bridges exist in

    NO)] /371 respectively. The structural relationship with CO is extremely obvious here.

    [Cp,(No)2Cr,(~-No)Z11361 and [Cp3Mn3(12-N0)3(p3-

    Angew. Chern. Int. Ed. Engl. 29 l1990) 1077-1089 1079

  • Until a few years ago almost nothing was known about analogies in reaction behavior between CO and NO in the coordination sphere of a metal. In particular, it was unclear whether one of the most typical reactions of coordinated CO, insertion into a M-C bond in the presence of an alkyl or aryl ligand (which is also the key step in important catalytic processes such as the 0x0 process or the Monsanto acetic acid is also possible for NO. Bergman et al.[391 were the first to show that an alkyl(nitrosy1)metal complex, formed in the same way as an alkyl(carbony1) compound by alkylation of the corresponding anion, reacts with PPh, to give a nitrosoalkane derivative via N-C bond formation (Fig. 5). The coordination number of the metal remains un-

    Fig. 5. NO insertion. R = alkyl

    changed. In a very similar manner the same group has syn- thesized and structurally characterized the complex [C,Me,Ru(N(O)Et)(Ph)PMe,Ph] starting from PMe,Ph and [C,Me,Ru(NO)(Et)Ph] .1401 On the basis of mechanistic studies,[39b1 there is no doubt that the NO insertion is in fact an alkyl migration to the nitrosyl ligand, as is the case for the CO insertion.[41] Legzdins et al.[421 and VolZhardt et al.[431 have shown very recently that this process occurs not only with neutral donors such as tertiary phosphanes but also with NO@ itself.

    3. The CO Homologues CS, CSe, and CTe: Unstable Molecules with a Rich Coordination Chemistry

    Whereas CO and CO, are both thermodynamically stable, for the homologous sulfur compounds this is only true for CS,. The molecule CS, which can be formed from CS, by photolysis, thermolysis, or cold electrical discharge, is stable only below - 160 C (Scheme 2) and polymerizes very rapid-

    stable salts salts unknown -1

    stable

  • homologous to [CpMn(CO),], and also to isolate the inter- mediate [CpMn(CO),(q2-CS,)] .Is2 It reacts with PPh, to give SPPh, and [CpMn(CO),(CS)]; presumably, the phos- phane attacks the sulfur atom bound to the metal and not the exocyclic sulfur atom. The same principle has been applied to prepare further thiocarbonyl and selenocarbonyl com- plexes (the latter starting from CSe,),[501 whereby the isola- tion of [CpMn(CO)(CS),] as the first compound with two CS ligands deserves particular

    r 1

    PPh3 CI \ Rh/pph3

    -SPPhs / - - \cs Ph3P

    c2 p.

    I P R 3

    - S P R j oc I cs co

    Fig 7 Synthesis of metal thiocarbonyl complexes from CS,.

    The second likely source for thiocarbonyl ligands is thiophosgene. As shown in Figure8, its...

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