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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 relationships 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 birthday 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 compound, isolated as a colorless, clear, extremely toxic liquid, was not the first metal compound containing carbonyl ligands- -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 serendipitous 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 already described by Ludwig M ~ n d , [ ~ ] have been joined in recent 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 Collman’s 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 substitution 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 tetracarbonyl.“’]

The metal carbonyls have meanwhile been joined by “relatives”, which are the subject of this article. Remarkably, their history is almost as long as that of their “ancestor”. Soon after the publication of Mond‘s results,“’ Sabatier et al. in Toulouse attempted to bond NO, NO,, C,H,, and C,H, to nickel and similar metals; although they were unsuccessful, 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 synthesis 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 relationships between metal carbonyls such as [Ni(CO),] and compounds such as [Ni(C,H,)3],[’21 [Ni(PC13),],1’41 [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 formally 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 fragment such as NH or CH,. The corresponding classes of compounds will not be discussed in detail; instead, this article 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 detected 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, starting with the synthesis of the first metal cyclobutadiene complexes 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 attempts 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 probably 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 intermediate by Paetzoldet al.Iz3] and was characterized very recently 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 comparable (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 nitrosyl 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 general 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 pentacarbonyl, 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 synthesized 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 <-16O’C

Scheme 2.

ly at higher temperatures ;[441 this process can occur in an explosive manner. In spite of its extreme reactivity, the spectroscopic data for CS are known very precisely, and clear evidence on the mechanism of its formation from CS, is available.r44c1 Much less is known about CSe,[451 and practically nothing about CTe; the latter has so far resisted all attempts to detect it, so that it still belongs to the group of “nonexistent” diatomic molecules.

The extreme tendency of CS to polymerize makes it under- standable that so far almost all attempts to synthesize thiocarbonyl metal complexes fromfree CS and a metal or metal compound have been unsuccessful. Only the cocondensation of CS with nickel atoms in an argon matrix at 10 K gives a product whose IR spectrum indicates it to be [Ni(CS),] . [ 4 6 1

In contrast to [Ni(CO),], it is unstable at room temperature, decomposing to give a black solid which is difficult to char- acterize.

In the case of iron and chromium, whose carbonyl complexes [Fe(CO),] and [Cr(CO),] are much more stable than mi(CO)4J, the corresponding binary metal thiocarbonyls [M(CS),] are still unknown. Mixed carbonyl-thiocarbonyl complexes are however known (Fig. 6). In the 1970s Angelici

PPh3

Fig. 6. Metal carbonyls and mixed CO-CE metal complexes.

et al. and Butler et al. synthesized the [Cr(CO),] analogues [Cr(CO),(CS)] and [Cr(CO),(CSe)],r4’1 and somewhat later Petz reported the preparation of the [Fe(CO),] analogue [Fe(CO),(CS)] .[481 These complexes, which are stable under normal conditions, are volatile and can even be handled in air; their other properties are also very similar to those of the parent compounds [M(CO),].

The characterization of a complete series of the general composition [M(CE)L,] with E = 0, S, Se, and Te was carried out by Roper and co-workers, who obtained the complexes [OsCl,(CE)(CO)(PPh,),] by reaction of [OsCl,(CCI,)- (CO)(PPh,),] with EH@ .f491 Exact structural data are available for all the members of the series; [OsCl,(CTe)(CO)- (PPh,),J was the first metal tellurocarbonyl compound to be described.

Since free CS, CSe, and CTe are not available as starting materials, it is necessary to find suitable precursors for the synthesis of complexes with carbon monochalcogenides as ligands. As is shown in Figures 7-9 for CS, three routes for the preparation of compounds of the type [M(CS)L,] have so far proved generally applicable.[501 In each case, the thiocar- bony1 ligand is generated at the metal and remains without exception strongly bonded to it.

The oldest and with hindsight most obvious synthetic method starts from CS,; its discovery is, so to speak, a “by- product” of the trailblazing work of the Wilkinson school on the catalytic abilities of [RhCl(PPh,),] . When studying the behavior of this compound in various solvents, Baird and Wilkinson noticed that it reacted with CS, to give the square- planar complex trans-[RhCl(CS)(PPh,),] via an unstable intermediate with a presumably a-bonded and a 0-bonded CS, ligand.f5’] In a similar manner (see Fig. 7) Butler and co- workers were able several years later to obtain the manganese thiocarbonyl compound [CpMn(CO),(CS)] , which is

1080 Angew. Chem. Int. Ed. Engl. 29 (1990) 1077-1089

homologous to [CpMn(CO),], and also to isolate the intermediate [CpMn(CO),(q2-CS,)] .Is2’ It reacts with PPh, to give SPPh, and [CpMn(CO),(CS)]; presumably, the phosphane 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 complexes (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 reaction with [Fe(C0),lze and [M,(C0),o]2G (M = Cr, Mo, W) gives the complexes [Fe(CO),(CS)] and [M(CO),(CS)], although the yields are not particularly Another synthesis of [Cr(CO),(CS)] starts from the metal arene compounds [(C,H,R)Cr(CO),(CS)] (R = H, CO,Me), which can be formed from [(C,H,R)Cr(CO),(thf)] and CS, ; the aromatic ring ligand is then displaced by C0.147b1

SCCI, reacts with square-planar rhodium(1) and iridium(1) complexes such as [RhCI(PPh,),] and [IrCI(N,)(PPh,),] via oxidative addition to give the corresponding thiocarbonyl- rhodium(rr1) and -iridium(IrI) compounds.[531 The presumed intermediate contains the unit M[C( = S)Cl](Cl), which forms the stable final product via a 1,2 shift of chlorine from carbon to the metal. Treatment of [Ru,(CO),,] with thiophosgene followed by fragmentation of the cluster results in a similar reaction, leading to [RuCl,(CS)(CO),]

A third method for the synthesis of CS complexes starts from heteroallenes such as CSSe (Fig. 9). h t h e course of our initial work on the coordination chemistry of CS,, we found that the compounds [CpM(PMe,)(q2-CS,)] (M = Co, Rh) are formed readily and in good yields starting from [Cp- Co(PMe,),] and [CpRh(PMe,)(C,H,)]; however, the former do not undergo sulfur abstraction with PR, .ls5] At the same time we observed that the reaction of [CpCo(PMe,),] with COS gives the carbonyl complex [CpCo(PMe,)(CO)], so that we expected that reaction with CSSe would form the thiocarbonyl compound [CpCo(PMe,)(CS)] . This is in fact the case, although, depending on the reaction conditions, its formation may be accompanied by that of the corresponding thiocarbonylselenide complex [CpCo(PMe,)(q 2-CSSe)] The latter reacts quantitatively with PPh, to give [CpCo- (PMe,)(CS)]. In the case of rhodium, the conversion of [Rh](C,H,) to [Rh](q2-CSSe) and [Rh](CS), where [Rh] =

[CpRh(PMe,)], can be readily carried out stepwise; here, as for the case where cobalt is the central atom, the reaction of [Rh](q’-CSSe) with PPh, leads solely to [Rh](CS) and not to [Rh](CSe).r’61

Me,P T I L

I ( M =‘Pd, P I ]

M-Pt , PR3

( L l M - C S ‘ c s 1 E = S, Se )

Fig. 9. Synthesis of metal thiocarbonyl complexes from heteroallenes.

A fragmentation of coordinated CS, to give CS and S or of CSSe to give CS and Se can be caused not only by PR, but also by nucleophilic metal complexes. In earlier studies we had already observed that the cobalt compound [CpCo- (PMe,)(q2-CS,)] reacts with the metal base [CpCo(PMe,),] to give the trinuclear cluster [(CpCo),(p,-CS)(p,-S)] in almost quantitative ~ i e l d . 1 ~ ~ 1 A similar metal-initiated cleav- age of CS, or CSSe takes place when either [Pt(PPh,),] or [Pt(C,H,)(PPh,),] is allowed to react with the chelate phosphane complexes [(L-L)M(qz-CSE)] (M = Pd, Pt; E = S,

Angm Chem hi . Ed. Engf 29 (1990) 1077- I069 1081

Se; L-L = Ph,PC,H,PPh,, o-C,H,(CH,PPh,),).[581 The products (Fig. 9) are the binuclear sulfido- or selenido- bridged thiocarbonyl compounds [(L-L)M(p-E)Pt(CS)- (PPh,)]; it seems likely that the complexes [(L-L)M (p-CSE)Pt(PPh,),] are formed as intermediates. In the case where L-L = o-C,H,(CH,PPh,),, M = Pd or Pt, and E = S or Se, products with this composition can be Surprisingly, the addition of the ligands Ph,PC,H,PPh, or o-C,H,(CH,PPh,), to the complexes [(L-L)M(p-E)Pt(CS)- (PPh,)] leads not only to the displacement of PPh, but also to a recombination of the fragments CS and E to give CSE, so that the final product is an unsymmetrical binuclear complex of the type [(L-L)M(p-CSE)Pt(L-L)] with two chelating phosphane ligands. There are no other known examples of such a process (fragmentation of a triatomic molecule and its re-formation under the influence of the same metal). The thermolysis of [(PPh,),Pt(p-CS,)Pt(PPh,),] and [(L-L)Pt(p-CS,)Pt(PPh,),] (L-L = o-C,H,(CH,PPh,),) leads to the formation of the compounds [Pt(CS)(PPh,),] and [Pt(CS)(L-L)]; these are the first mononuclear thiocarbonyl complexes of plat in urn(^).[^^^

There are only small differences between the modes of coordination adopted by CS and CO. As shown in Figure 10,

terminal p2 bridge p3 bridge

0

Fig. 10. Coordination possibilities for CS

the thiocarbonyl ligand can be coordinated both end-on or in a bridging manner (between two or three metal atoms); it should be noted that, when CO and CS are both present in the same molecule (as, for example, in [Cp,Fe(CO),- (CS),][601), the formation of CS rather than CO bridges is observed.

CS is a stronger n acceptor than CO; this is shown not only by MO calculations[611 but also by the structural data of mixed carbonyl thiocarbonyl metal complexes. For example, the Cr-CS distance in [(C,H,CO,Me)Cr(CO),(CS)] is shorter by 0.05 8, than the Cr-CO distance (mean value)[621, and a similar difference is observed in the cation [Ir- (C0),(CS)(PPh,),le (as the PF, salt).[631 This behavior indicates a stronger synergistic interaction between the metal and the thiocarbonyl ligand. Interestingly, the sulfur atom is clearly nucleophilic in all coordination modes of CS; it can not only undergo alkylation but also add 14-electron fragments such as [Cr(CO),] or [CpMn(CO),]. The resulting complexes have a bent arrangement for (M),C-S-M’, the exact structure of which has been determined for both n = 1 (e.g., in [(C,H,Me)Cr(CO),CSCr(CO),][641) and n = 2 and 3 (e.g., in [Cp(PMe,)Co(p-CO){p-CSMn(CO),Cp)Mn(CO)-

and [(CpCo),(p,-S){ p,-CSCr(CO),)], respectively) [571 by X-ray structural analyses. Thus, CS is considerably superior to the parent molecule CO with respect to this addi- tional bonding type and offers a synthetic potential for het- erometal multinuclear complexes which has so far hardly been used.

4. Isocyanides as Ligands: CO Analogues with Advantages and Disadvantages

The molecule CNH is strictly isoelectronic with CO; like CS, it is extremely labile in the free state, isomerizing very rapidly to HCN.[661 However, it can be readily generated in the coordination sphere of a metal, where it is stable (a further analogy to CS). The formation of a CNH ligand can be carried out most simply by protonation of anionic cyano complexes; for example, treatment of [M(C0),(CN)le with HCI or [(C,H,R)Mn(CO),(CN)]@ with H,PO,: in this manner, the neutral compounds [M(CO),(CNH)] (M = Cr, Mo, W)[671 and [(C,H,R)Mn(CO),(CNH)] (R = H, Me)[,’] are formed. Both the spectroscopic properties and the behavior of the complexes [M(CO),(CNH)] (M = Cr, W) leave no doubt that the M-CNH bond is considerably stronger than the M-NCH bond in the corresponding isomers [M(CO),(NCH)] , which are formed by allowing [M(CO),(thf)] to react with HCN.[67b1

Scheme 3

Alkyl and aryl isocyanides, which can be prepared in various ways, are much more stable than the parent compound CNH.169,701 They are generally stronger D donors than CO; as a consequence, numerous binary (“homoleptic”) cationic complexes can be isolated as the corresponding salts (see Table 1): examples are [M(CNR),]’@ (M = Cr, Mo, W), [Fe(CNR),Ize, [Ni(CNR),IZ@, [Ag(CNR),]@, and [CO,(CNR>,,,]“~.[”~ Comparable carbonyl metal compounds are not yet known.

Table 1. Metal isocyanide complexes of 3d transition metals

Like CO, isocyanides (CNR) are good .n acceptors; not only have various metal(o) complexes of the type [M(CNR),] ( n = 4 , M = C r , Mo, W; n = 5 , M = F e , Ru, 0 s ; n = 4 , M = Ni) been synthesized,[711 but also very recently the first anionic complex, [Co(CNR),I0 (R = 2,6-Me,C,H,)[721 (i.e., an analogue of [Co(C0),le). Binuclear complexes such as [Co,(CNR),] and [Fe,(CNR),], which resemble the corre-

1082 Angeew. Chem. lnr. Ed. Engl. 29 (1990) f077- 1089

sponding metal carbonyls not only in molecular formula but also structurally, are also known.[71i As a historical reminis- cence, we may mention that the most direct relatives in this series to the parent Ni(CO),), the compounds [Ni(CNR),] (R = C,H,, p-EtOC,H,), were obtained exactly 40 years ago in the same city (Munich) by two groups working fully independently of one another, in both cases via ligand substitution reactions starting from [Ni(CO),] . [731

4 CNR [Ni(CO),] -- [Ni(CNR),] - 4 c o

In the case of L= CNR, however, this synthetic method (formation of [ML,] from [M(CO)J and L) is extremely unusual and can only be used in the absence of a catalyst when a labile starting material is available. Under purely thermal conditions, the reaction of metal hexacarbonyls, [M(CO),J (M = Cr, Mo, W), and of iron pentacarbonyl, [Fe(CO),], with isocyanides often leads only to monosubstitution even at higher temperatures and with long reaction times.[71e1 The use of phase-transfer catalysts can improve the situation.[74] However, it is even more favorable to carry out the reaction of [M(CO),J or [Fe(CO),J with CNR in the presence of a heterogeneous catalyst such as CoCI,, activated carbon, or metallic platinum on an oxide support. This method, devel- oped by Coville and co-w~rkers,[’~] has made it possible to replace all the CO ligands in the otherwise inert Fe(CO), by CNR and also to substitute more than half of the carbonyl groups in metal clusters such as [Ir4(CO)12]. Apart from metal carbonyls, arene and cyclodi- and -triene complexes have been used as starting materials for reactions with isocyanides; the synthesis of [Cr(CNR),] from bis(naph- tha1ene)chromium and CNR (R = nBu, C,H 1) is a good

The coordination properties of CO and isocyanides are similar. The most generally encountered bonding situations are compared in Figure 11, top; it should be mentioned that,

termlnal p2 bridge p3 bridge

Fig. 11. Coordination possibilities for CNR.

in some cases, terminally bonded isocyanide ligands can be bent at the N

The ligand CNR can also bridge two or three metal atoms, in the manner shown in Figure 1 1, bottom. A good example of the p2-C,N form is the nickel cluster [Ni,(CNtBu),] described by Muetterties et A comparable interaction of CO with two or three metallic centers has been discussed for the adsorption of carbon monoxide at surfaces ;[78i the possible threefold bridging seems particularly likely when M is a transition metal and M’ an electropositive metal such as K, Mg, or Al.

CO and CNR are also closely analogous in their reactions. Both isocyanide and carbonyl ligands are able to undergo insertion into M-C (3 bonds; this process can take place either spontaneously or in the presence of a Lewis base L (see Fig. 12). Whether the acyl or imidoyl residue formed is bonded in a mono- or bidentate manner depends ultimately on the electronic configuration and the required coordination geometry of the metal.

Fig. 12. Insertion of CO and CNR into M-C 0 bonds.

There is one interesting reactivity difference between structurally analogous acyl and imidoylcobalt complexes of the half-sandwich type formed by CO and CNR insertion, respectively (see Fig. 13). While the compound [CpCoMe- (CO)(PMe,)]I, obtained from [CpCo(CO)(PMe,)] and methyl iodide in toluene, undergoes only isomerization to [Cp- Co{ C(O)Me)(PMe,)I] when acetone is added,[791 a cycloaddition reaction takes place when the corresponding methyl isocyanide complex is dissolved in this solvent. A five-membered metallaheterocycle is formed, probably via an intermediate with the partial structure Co-C(Me) = NCH,; the final product has been characterized by an X-ray structural analysis.[801 Furthermore, [3 + 21 cycloaddition reactions occur with acetaldehyde and benzaldehyde as well as with a number of nitriles R C N (R’ = Me, Ph, cyclo-C,H,, CH = CH,, CMe=CH,, CH=CHMe, NH,, NMe,, SMe); the five- membered ring of the type [Col-C(CH,) = NR-CR = N shown in Figure 13 is thermodynamically unstable and iso- merizes to [CoJ-C( = CH,)-NR = CR’-NH, independent of the nature of the group R.[811 With CS, it is even possible to realize a double cycloaddition, leading to the dimetallaspiro- heterocycle depicted in Figure 13; this compound has been structurally characterized.[*’] The difference in behavior between coordinated CO and CNR documented by this and other resultsr831 is probably due to the basicity of the CNR nitrogen atom being higher than that of the oxygen in CO; moreover, the building block [MI-C(Me) = NR formed by a methyl shift is a better 1,3 dipole than the corresponding unit [MI-C(Me) 0.

r

Angew Chem. Int . Ed. Engl. 29 (1990) 1077-1089 1083

'/.

c o = [ C O ] M e p '

O=CMez / Me

[co 1 ~ c' \\

Fig. 13. Reactivity of [CpCo(CO)(PMe,)] and the CNR analogue

/

Me lcol-c' I \\

N \ ,o"- ' 7 R '

The greater polarity of the C-NR bond compared with the CEO bond causes metal isocyanide compounds to be suitable starting materials for the formation of aminocar- bene complexes. The pioneering work in this area was done by Chaff and ~o-workersI~~1, who showed that platinum(n) isocyanide compounds of the type cis-[PtX,(L)(CNR)] (X = CI, Br, I; R = Me, Et, Ph, p-Tol; L = PMe,Ph, PEt,, PEt,Ph) react with primary alcohols and amines even under very mild conditions to give cis-[PtX,(L)(C(NHR)(OR'))] and cis-[PtX,(L)(C(NHR)(NHR')}], respectively. Some- what later, this synthetic principle was extended to isocyanide complexes of iron,@'] rhodium,[881 ruthenium,[891 osmium,[901 and gold;[911 a large number of carbene complexes of these metals could thus be

The observations (confirmed by kinetic studies) that both primary and secondary amines react faster than alcohols and that electron-withdrawing groups R at the isocyanide nitrogen atom accelerate the reaction indicate that the formation of the carbene ligands C(NHR)(NHR) and C(NHR)(OR') occurs via an initial attack of the amine or alcohol at the carbon atom of the isocyanide.

/L /OR' R O H L N H R' ,L ,NHR'

< NHR X-Pt=C, t- X-P<=C=NR 1 X-Pt=C,

/ L' NHR L

A similar conversion of a coordinated carbonyl group to an amino(hydroxy)carbene ligand C(OH)(NHR) or C(OH)(NR,) is not known; the nucleophilicity of a primary or secondary amine is apparently not sufficient to make this possible. However, Fischer and Koiimeier showed almost simultaneously with the work carried out by Chatt and co- workers (see Fig. 14) that lithium dialkylamides do undergo

N H R 2 NHR' M-CZNR' - M=C:

N R 2

LiNR2 ,@Lie R E X ,OR' M-CSO - M=C M =C

'NR, 'NR,

Fig. 14. Nucleophilic addition to CNR and CO metal complexes.

nucleophilic attack at a CO group bound to a metal (e.g., in [Cr(CO),]); subsequent alkylation gives an alkoxy(amin0)- carbene ligand.[931 This variation of the classical Fischer method for the formation of metal carbene complexes has found further application[92] and has proved useful in or- ganic synthesis.[94]

5. The Completion of the Series CO, CNH, CCH,: Vinylidenes as Building Blocks for Metal Complexes

Like CNH, vinylidene (CCH,) is in a strict sense isoelectronic with CO; in addition, it is similarly labile in the free state, in analogy to its nitrogen relative. The energy barrier for the conversion of C=CH, to its isomer acetylene HC = CH has been calculated by Schaeffer et al. to be only ca. 4 kcal mol-' and could in fact be even lower because of zero point energy effects.[951 The lifetime of C=CH, can thus be estimated to be lo-" to 10- l2s; this is in good agreement with the results of trapping experiments.[961

In contrast to isocyanides (CNR), vinylidene derivatives of the general type C = CHR and C = CR, are also unstable and thus not available as starting materials for synthesis of complexes. It is therefore absolutely necessary to generate the ligand C = CH,, C = CHR, or C = CR, at the metal; as will be shown below there are various methods for doing so.

The first results in this direction were obtained only two years after the discovery of the carbene complexes; again, as so often, luck played an important role. Mills and Red- house[97] had attempted to prepare an iron carbene complex by irradiating a benzene solution of [Fe(CO),] and diphenyl- ketene; however, they obtained a product with the molecular formula [Fe,(CO),(C,Ph,)J, the X-ray structural analysis of which showed it to be a bridged vinylidene complex. The first mononuclear compound with a vinylidene Iigand was de-

1084 Angew. Chrm. In [ . Ed. Engl. 29 (1990) 1077-1089

scribed in 1972 by King and Saran, who isolated the compounds [CpM{C=C(CN),)L,Cl] (M = Mo, W; L = PPh,, AsPh,. SbPh, , etc.) in the course of studies on the reactivity of 1 -chloro-2,2-dicyanovinyl complexes of molybdenum and tungsten.[981 The extremely high thermodynamic stability of the vinylidene-metal bond observed by them has also been confirmed for many other examples; it is thus not surprising that today vinylidene complexes of almost all the transition metals are known.[991

Very exact information on the structures and bonding modes of these complexes is available. As can be seen from the comparison in Figure 15, not only vinylidene and car- bony1 complexes but also vinylidene and isocyanide com-

Fig. I S . Coordination possibilities for C =CH,

plexes show common structural features. The vinylidenes as terminally bonded ligands have excellent r-acceptor properties, which according to a comparative spectroscopic investi- gation are better than those of CO and only a little worse than those of CS and SO,.['oo1 It is remarkable that the rotational barrier for the M = C bond is very small for both isocyanide and vinylidene so that optical isomers have not yet been detected for allene analogues of the type [LL'M = C = CRR'] .I i

A large number of compounds with doubly bridging vinylidene ligands are known; these can be obtained both directly (see the iron complex [Fe,(CO),(p-C = CPh,)] already mentioned)[971 and stepwise via addition of a coordinatively unsaturated metal compound to an M = C = C R R unit."91 The latter method is important for the formation of binuclear complexes with two different metal atoms. The p,-C,C bridges (Fig. 15) are also by no means unusual and are found in particular in ruthenium and osmium trinuclear clusters.[' 031

The aforementioned extraordinary lability of the compounds C = CRR' leads naturally to the question how complexes containing the structural unit M = C = CRR' can be prepared. Two methods have so far proved generally useful; these start either from alkynylmetai compounds or from coordinatively unsaturated or substitution-labile metal complexes and I-alkynes. Other methods, which employ vinylidene sources such as ketene~,~'~' 1 ,I -dichloroolefin~,['~~] diazoolefins,[' 06] and a-(chlorovinyl)silanes,[' 07] can be used in special cases but have not attained general importance.[99]

The road to vinylidene complexes starting from alkynylmetal compounds was opened in particular by Davison and co-workers['081 and Bruce and c o - w o r k e r ~ ~ ~ ~ ~ ~ (see Fig. 16). Cyclopentadienyl iron and ruthenium compounds of the general composition [CpML,(C= CR)) can undergo not

only protonation but also alkylation [using MeOS0,F or [ORJBF, (R = Me, Et)]; the yields are practically quantitative, indicating that the 0-C atom of the alkynyl ligand and not the metal is the preferred site for attack by the elec- trophile (even for L= PMe,). The observation that anionic alkynyl compounds can undergo electrophilic addition reactions even more readily than neutral compounds corresponds to expectations; it has been used by Berke and co- workers, in particular, for the synthesis of the vinylidene manganese complexes [CpMn(CO),(C = CRR')] .['

M = F e , R u

M = Mn, R e

Fig. 16. Synthesis of vinylidenemetal complexes from alkynylmetal compounds and 1-alkynes.

The pioneering work on the conversion of the thermodynamically much more stable I-alkynes (HC=CR) into the isomeric vinylidenes (C = CHR) in the coordination sphere of a metal was done by Antonova, Kolobova, and their co- workers.['00* ' ' ' I They found that the photochemical reaction of [CpM(CO),] (M = Mn, Re) with phenylacetylene does not give the expected alkyne metal compounds [CpM- (C0),(q2-PhC = CH)] but instead the isomeric vinylidene complexes [CpM(CO),(C = CHPh)]. The IR spectra record- ed during the reaction indicated that the alkyne is first coordinated, but attempts to isolate the intermediate [CpM- (CO),(q'-PhC-CH)] in an analytically pure form were

" 'I. Similar results were obtained later in the reactions of the systems [CpML,]@ (M=Ru, Os), [Cr(CO),], and ~(CO),(R,PC,H,PR,)] (all of which are related to [CpM(CO),] (M = Mn, Re)) with 1 -alkynes reported by Bruce,['ogb- 21 Berke," ' 31 and Templeton[' 14]

and their co-workers. In each case vinylidene rather than I-alkyne complexes were formed.

The question thus raised regarding the mechanism of the conversion of 1 -alkynes to metal-bound vinylidenes could at first be answered only in a speculative manner. The sugges- tion made by various authors, in particular by Antonova and co-workers[' ' ' I (see Fig. 17), was that subsequent to the coordination of the alkyne an intramolecular oxidative addition takes place, the alkynyl(hydrido) intermediate then un-

I 1

Fig. 17. Proposed mechanisms for vinylidene formation from I-alkynes.

Angeu Chem In1. Ed. Engl. 29 (1990) 1077-1089 1085

dergoing a 1,3-H shift to give the vinylidene complex. Al- though this appeared plausible, there was at first no experimental evidence for it. In addition, a very detailed study by Silvestre and Hofmann[1151 showed that the isomerization of [L,MH(C = CR)] to [L,M(C = CHR)] would have a very high activation energy, so that the stepwise conversion of the alkyne to the vinylidene complex shown in the upper part of Figure 17 would be improbable. They suggested that a slip- page of the I-alkyne into a position in which only one carbon atom of the triple bond interacts with the metal should be more favorable; this is reminiscent of the bonding situation in a vinylcarbenium ion (see Fig. 17).

Evidence that a stepwise interconversion is possible after all and that, with the correct choice of ligands, the three possible isomers can be isolated was produced in the course of our own work on the reactivity of coordinatively unsaturated triisopropylphosphane rhodium and iridium complexes. In the initial stages we were able (see Scheme 5) to trans-

The final reaction products shown in Schemes 5 and 6 behave in part very similarly to their CO relatives. For example, reaction of both the compounds trans-[IrCl(C = CHR)- (PiPr3)2] and the Vaska complex tran~-[IrCl(CO)(PPh,)~] with HBF, results in protonation at the metal,['201 whereas the half-sandwich compounds [CpRh(C = CHR)(PiPr,)] (R = H, Me, Ph) and [CpRh(CO)(PMe,)] react with benzoyl azide to give the same type of five-membered metallahetero- cycIes.['211

The parallels in reactivity are even closer for isocyanide and vinylidene complexes, however. The method discovered by Chatt and co-workers for the preparation of metal alkoxy(amino)- and diaminocarbene compounds (see Sec- tion 4), which is based on the nucleophilic attack of a Lewis base such as ROH or RNH, on the carbon atom of the i so~yanide , [~~I can readily be extended to vinylidene complexes, in particular when these are cationic. The cl-carbon atom of the vinylidene ligand is then even more electrophilic than in comparable neutral compounds, so that, for example, starting from [CpFe(C = CH2)(CO)(PPh3)J@ and alcohols, amines, mercaptans or hydrogen halides, it is possible to obtain various types of iron carbene complexes. Figure 18 contains a summary of the results described by Hughes et

Scheme 5. L = PiPr,.

form the square-planar alkyne compounds trans-[RhCl- (RC= CH)(PiPr,),] into the octahedral alkynyl(hydrid0) complexes [RhHCl(C E CR)(PiPr,),(py)] by adding pyridine and to treat these with CpNa to give the half-sandwich compounds [CpRh(C = CHR)(PiPr,)];" a little later, using iridium as the central atom, we were able to synthesize the complete series of isomers shown in Scheme 6." ''I Accord- ing to more recent studies, there is no doubt that an analogous stepwise conversion of HC = CR to C = CHR also occurs for rhodium.['02, ''I

Scheme 6. L = PiPr,. Middle: 6(IrH) % -43.

These results seem at first to be in complete disagreement with the MO calculations of Silvestre and H o f f m ~ n n . ~ ~ ~ ~ ] However, these authors started from complex building blocks [ML,] such as [CpMn(CO),] (or more simply [MnHJ4@) in which the metal has a 16-electron configuration; thus, addition of the alkyne gives a filled shell with 18 electrons. This requirement is not met, however, by the fragment [MCl(PiPr,),] (M = Rh, Ir): here, the metal has only a 14-electron configuration, so that a free coordination site is still present after addition of the alkyne. For this reason the hydride transfer from the metal to the 8-C atom of the alkynyl ligand in the intermediate [MHCI(C = CR)(PiPr,),] could occur intermolecularly ; this is in agreement with pre- liminary kinetic data (for the example M = Ir).[1191

m ,CI [ Fe] =C

'CH3

m ,NHR m SR [ Fe]=C [Fe]=C:

'CH3 '"3

[Fe] = C p ( C O ) ( P P h 3 t F e

Fig. 18. Reaction of [CpFe(C =CH,)(CO)(PPh,)lm with nucleophiles

Another feature common to the parent compound CO and its near relatives CS and C = CH, should be mentioned. Since the beginning of this century it has been known that metal carbonyls react with thermally or photochemically generated 16-electron complex fragments to give binuclear compounds in which the CO acts as a bridge; the prototype of this reaction is the formation of [Fe2(C0),] from [Fe- (CO),] and [Fe(CO),] . [ lZ3] Using the isostructural series [CpRh(CX)(PR,)] with X = 0, S and CH, as an example, we were able to show that these three complexes react in exactly the same way with the 16-electron species [CpMn- (CO),] generated by irradiation of [CpMn(CO),] and stabilized by THF (Fig. 19).L65* 1241 It is remarkable that not only

X = 0, S , CH2

Fig. 19. Reaction of the isostructural compounds [CpRh(CX)(PR,)] with KpMn(CO),I

1086 Angew. Chem. Ini. Ed. Engl. 29 (1990) 1077-1089

the CO but also the CS and C=CH, bridges occupy an unsymmetrical position between the two metal atoms; this is presumably due to the dissimilar electron configurations at rhodium and manganese.

6. On to the Second Century: The Search for the Missing Relatives

What does the future look like? Will the organometallic family which is now celebrating its birthday increase in size, and in which direction could this perhaps take place? Which will be the next metal-bonded relative of CO?

One very promising candidate is certainly SiO. On the basis of earlier work by Schmid et al.,[1251 Zybill et al. have very recently shown that dichlorosilylene, SiCl,, can be generated at a 16-electron complex fragment just as can a carbene and that the M-SiC1, bond can be stabilized by means of adduct formation with suitable Lewis bases such as hexa- methylphosphoric triamide (HMPT)." The X-ray structural analyses of [(CO),CrSiCl,(hmpt)] and [(CO),FeSiCl,- (hmpt)] show that the M-Si distance is relatively short and that the HMPT molecule is only weakly If it were possible to displace the HMPT molecule and replace it by H,O and thus to eliminate two molecules of HCI, one could generate the CO analogue SiO. A very similar conversion of CC1, coordinated at osmium to CO has been described by Roper and co-worker~ . [~~] In order to fix the silicon monoxide ligand it may be necessary to use an electron- richer complex building block such as [C,Me,Re(PR,),], [Os(PMe,),], or [C,Me,Ir(PMe,)] rather than [Cr(CO),] or [Fe(CO),]; such building blocks could stabilize the M-SiO bond by means of stronger 71 back-bonding.

D

If it were possible to fix SiO, then why not also BO@? In Section 2, we referred to the existence of the extremely short- lived species CH,BO, which is isoelectronic with BH,CO; the former can be thought to be built up from CHF and BOO. According to H~fSmann,[ '~ '~ CHF is isolobal with [Mn(CO),]@ and [Co(CO),]@, so that compounds such as [Mn(CO),(BO)] and [Co(CO),(BO)] (i.e., analogues of [Cr- (CO),] and [Fe(CO),]) would be plausible synthetic goals. Attempts to synthesize these complexes could build on the work of Noth and Schmid, who reported the preparation of the readily decomposing compound [(CO),CoBCl,] as early as 1966." Addition of a Lewis base such as HMPT would give an intermediate similar to that formulated above for Six,. Careful protolysis could then convert an ElX, ligand into an El0 ligand (El = B). Boron compounds with a coordination number of two are no longer curiosities today, as has been shown clearly in the last decade, in particular by Noth, Parry, Paetzold, and Berndt." "I

What other possibilities are there apart from SiO and BOO? What is the situation regarding CFB, which we have already mentioned in Scheme 2, or BF, which is also isoelectronic with CO? Is it realistic to think of such ligands becom- ing available? If intuition is not deceiving, a compound of the type L,M(CF) will soon be described in the literature. The question will then be only whether this molecule is better described as a carbyne complex [L,M = C - F] than as a car- bony1 analogue [L,M = C = F]; this question can only be answered by an X-ray structural analysis, together with MO calculations. The signposts on the way to [L,M(CF)] have probably already been set up by Roper, who, during the course of extremely interesting studies, has been able to synthesize several compounds containing the fragments M(CC1,) and M(CF,).[' 301 Although compounds such as [RuCl,(CF,)(CO)(PPh,)] and [Os(CF,)(CO),(PPh,),] have not yet led to products containing an OsCF the compound [OsCl(CPh)(CO)(PPh,),] obtained by the same group['321 shows that the goal of synthesizing a neutral complex [OsCl(CF)(CO)(PPh,),] or a cation [Os(CF)(CO),- (PPh,),]@ is quite realistic. The existence of trinuclear clusters with p,-CF bridges such as [((CO),CO),(~-CF)][ '~~~ and [((CO),Fe),(p,-CF),]['341 is also encouraging in this respect. These complexes resemble the corresponding carbonyl compounds with triply bridging CO ligands in both their structure and their bonding features.

A complex such as [Fe(CO),(BF)] or [(C,R,)Os(L)(BF)] also seems attainable. Schmid, Petz, and Noth [13'] described compounds of the formula [Fe(CO),(BNR,)] (R = Me, Et) as early as 1970; while these are extremely thermally labile, there is no doubt that they do exist. Although the road from [Fe(CO),(BNR,)] to [Fe(CO),(BF)] will not be an easy one, it does appear passable. Reactions at osmium also appear feasible; the building blocks [C,H,Os(PR,)] ,I1 361 [C,H,OS(CO)],~'~'~ and [(C6H3Me3)O~(CO)],[1381 used by us, could well prove suitable for the attachment of a BF ligand.

One hundred years have passed and a new era lies before us. One does not really need to be a prophet to forecast that the second century will also be an exciting one, that further surprises await us, and that the present survey will certainly not be the last occasion on which the serendipitous discovery made by Ludwig Mond will be remembered with reverence.

I thank Heinrich Vahrenkamp for suggesting this subject and Ernst Otto Fischer for valuable information regarding the material to be consulted. Oliver Niirnberg helped in the preparation of the reaction schemes, figures, etc., and I thank him heartily for this help.

Received: February 14, 1990 [A 781 IE] German version. Angew. Chem. 102 (1990) 1109

Translated by Prof. 7: N . Mitchell. Dortmund

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