The closed metal carbonyl clusters

31 T H E CLOSED METAL CARBONYL CLUSTERS

P. Chinl lstituto di Chimica Generale ed lnorganica dell'Universit~ Milan, Italy

CONTENTS I. II. III.

Introduction T a b u l a r Survey of CMCC . . . . . . . . . . . . . . . . . . . . Syntheses of C M C C . . . . . . . . . . . . . . . . . . . . . . A. C o n d e n s a t i o n of Coordinatively Unsaturated Species . . . . . . . . . I. Pyrolysis of Metal Carbonyl C o m p o u n d s . . . . . . . . . . . . 2. Photolysis . . . . . . . . . . . . . . . . . . . . . . . 3. Halogen Abstraction from Halo-carbonyl C o m p o u n d s . . . . . . . . 4. Oxidation of Carbonylmetallates . . . . . . . . . . . . . . . . 5. Electrons Redistribution in a Carbonylmetallate Containing a Transition Metal Cation . . . . . . . . . . . . . . . . . . . . . 6. Ligand Abstraction from a Substituted Metal Carbonyl M(CO),Ly . . . . B. C o n d e n s a t i o n b e t w e e n a Carbonylmetallate and a Metal Carbonyl . . . . . C. Double Exchange Reaction b e t w e e n a Carbonylmetallate and a Halomctalcarbonyl . . . . . . . . . . . . . . . . . . . . . IV. Structural Description of CMCC A. CMCC with Thre¢ Transition g i e i a l ' A t o m s i i i i i i i i : i i i 1 1. CMCC 6 / 6 / 6 without H e t e r o a t o m s . . . . . . . . . . . . . . . 2. CMCC 6 / 6 / 6 w i t h H e t e r o a t o m s . . . . . . . . . . . . . . . . 3. CMCC 6/7/7 . . . . . . . . . . . . . . . . . . . . 4. CMCC 7/7/7 . . . . . . . . . . . . . . . . . . . . B. CMCC with Four Transition Metal Atoms . . . . . . . . . . . . . I. CMCC 6 / 6 / 6 / 6 . . . . . . . . . . . . . . . . . . . . 2. CMCC 6/6/7/7 . . . . . . . . . . . . . . . . . . . . 3. CMCC 6/7/7/7 . . . . . . . . . . . . . . . . . . . . 4. CMCC 7 / 7 / 7 / 7 . . . . . . . . . . . . . . . . . . . . 5. CMCC 6 / 8 / 8 / 8 . . . . . . . . . . . . . . . . . . . . 6. CMCC 9 / 9 / 9 / 9 . . . . . . . . . . . . . . . . . . . . C. CMCC with Five Transition Metal Atoms . . . . . . . . . . . . . . D. CMCC with Six Transition Metal Atoms . . . . . . . . . . . . . . E. CMCC w i t h more than Six Transition Metal Atoms . . . . . . . . . . V. Bonding in CMCC • • • . . . . . . . . . . . . . . . . . . A. CMCC and the Noble Gas Formalism . . . . . . . . . . . . . . . B. 4
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Reviews 1968

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31 32 34 34 34 35 35 35 35 35 36 36 36 36 36 36 37 37 38 38 38 38 38 39 39 39 39 40 40 40 41 43 43 45 46 46 47 47 48 48 49 49

INTRODUCTION

According to a definition given by Cotton ~ a cluster compound is ,~ a finite group of metal atoms which are held together entirely, mainly, or at least to a significant extent, by bonds directly between the metal atoms even though some non-metal atoms may be associated intimately with the cluster >>. On the basis of this definition the metal carbonyl derivatives can be divided in cluster and non-cluster compounds, depending on the presence of significant metal-metal bonds. These two categories can be further subdivided into two classes, as it is shown in the following scheme:

.

I

J

nor~ ~uslcr compounds (no sig'nif~canl metal-metal boadl}

I

A) monoe.mrk: compounds Cr(COk; Ni(CO)~ PR))

B) poiymerk compounds

[Ru(COhCh]=; [M~(CO)~R].

metal carbonyl ~rivlliw's

I_

I

g'3 ~ ¢luslers Mn~CO).; Co, COb

I

D) ¢1o*¢d¢luslCrl Fe,(CO).: RMCO).

cluster comp(~m~ll ($11~i1~¢1n| I11~[I[-ITJGII| bOn~)

32

P. Cntm

This classification is not only a formal one; the compounds of classes A, B and C, despite a few exceptions in class C such as FeACO)9 and Os3(COh0X2 ( X = C I , Br, I) '~°, represent the derivatives in which the ratio between the number of monodentate ligands (or the number of simple bonds in the case of polydentate ligands) and the number of transition metal atoms is the highest, and whose chemistry is fairly well known. On the contrary the compounds of class D do not present the highest values of such ratio and only in the last years important advances have been made in syntheses, structures, bonding and reactivity of these compounds. They form the subject of the present review. Some compounds having a bent open shape are on the borderline between classes C and D. This is the case o f Fe~(CO)gE2 ( E = S , Se, Te) ~ shown in Figure 1, of Fe3(CO)9(Ph2CN2)2, H of Fe~(CO)s(C2PhD (black form) 77 shown in Figure 31, of Re3(CO),4H ~ , and of MnRe2(CO)I4H ~39 shown in Figure 2. In all these cases one of the metal-metal distances is very long and nonbonding.

o

O

0

c

c

0~ C~ F*"-~. S~

"¢~'C~0

o~C ~ ~'~ c~ o

\

o

Figure I. Molecular structure of Fe3(CO)~S2.~

selective to what was deemed relevant in the opinion of the reviewer. Metal carbonyl compounds have been recently reviewed by Calderazzo, Ercoli and Natta ~, and halogen cluster compounds by Kepert and Vrieze m. Throughout the text and the tables the following abbreviations have been used: Bu Bz Cp CDas Diars Diphos DMF En Et M Me Ph Pr Py R p-Tol THF

n - Butyl Benzyl Cyclopentadicnyl 1.2-Bis(dimcthylarsine)tetrafluorocyclobutcne o-Phcnylcnbis(dimethylarsine) 12-Bis(diphenylphosphine)ethane Dimethylformamide Ethylendiamine Ethyl Generical metal Methyl Phenyl n-Propyl Pyridine Generical organic radical p-Tolyl Tetrahydrofuran

II. TABULARSURVEYOF CMCC Closed metal carbonyl clusters, or CMCC, are usually found in the transition metals of group VIII, but less common examples can be found with metals of group VI and VII. In table 1 we have collected the compounds known or reasonably believed to be CMCC, together with the principal references to their synthesis, infrared spectra and structure determination; additional information on the mass spectra is found in ref. (42) and on raman spectra in ref. (191, 223). It has been necessary to divide the CMCC containing transition and non-transition metal atoms in two sub-classes: sub-class E includes the CMCC in which it is possible to individuate, a closed cluster formed only by the transition metal atoms, sub-class F indudes the CMCC in which one or more non-transition metal atoms are necessary in order to obtain the closed geometrical shape. Examples of sub-classes E and R

I

(ce)~c(@o~~co (cob (C0)3 Figure 2. Molecular structure of MnRe2(CO),H. (H. D. Kaesz, R. Bau and M. R. Churchill, I. Am. Chem. Soc., 89, 2775 (1967). Reproduced by permission

Figure 3. Example of sub-class E: Co)(CObSnR.'~

(C0)4I ~ e ~ Cluster compounds are one of the more rapidly growing fields of inorganic chemistry at the present moment, so that more than half of the literature quoted in this review appeared in the last four years. An effort was made in order to cover the literature up to August 1968. The coverage was meant to be

Figure 4.

~

Fe(CO)(

Example of sub
33

The Closed Metal Carbonyl Clusters Table I. Compounds known or believed to be CMCC Transition metal

Cr Mo

w

Mn Tc Re Fe

Ru

Os

Co

CMCC

Preparation

[Cr)(CO),,] 2[Mo,(CO),,]'-

18 19

Struc. data

126 15

[w,(co).]'-

]9

[W)(CO)XH)(OH)(OMe)(MeOH)] 3[W)(CO))(H)(OHh(H:O)] ~- (a) [W((CO),~(H20),]'- (a) Mn)(CO)uH) Mna(CO)9(PPh2h Tc)(CO).H~ Rea(CO)uH) [ae,(CO),,]'[Re(CO),]. Fe)(CO)~ [Fe,(CO).] 2- (a,b) Fe3(CO).(NMeDH Fej(CO).(PPI~) Fe)(CO)11[ P(OMe),] Fe)(COho(CDAs) Fe)(CO)to(SCH2S) Fe,(CO)to[P(OMe)02 FeSCO),lP(OMehJ3 Fe3(CO)9(SBzhS Fe~(CO)t(C2Phz) Fe~(COh(CzPhD violet Fe,(CO)t)] 2- (a,l:) e4(CO)~S2 Fe,(CO),(lrCp)( F~(CO).C Ru)(COh2 [Ru)(CO)~Hj]- (a) Ru)(CO),o(SR)H R = E t , Bu Ru)(COh*(C4Hn) Ru)(CO)Io(AsPh~h Ru)(CO)dPRt)3 R=Bu, Ph, OPh Ru~(CO),(GRD2 R = P h , pCIC,H, Ru)(CO),(GH,)2 RudCO)nH2 Rus(CO)nH4 (g and 13 forms) Ru~(CO)ITC Ru~(CO)~,(arene)C [Ru(COhNO], Ost(CO). Os~(CO)t,(OH)H Os)(CO)~ OMe)H Oss(COhdOMe)z Os~(CO)**H, Os)(CO),(PPh)), Os3(CO),(Diarsh OsdCOh,H2 OsdCO).H, Os((CO)~O, Co,(CO),CR (d)

104 104 104 86 3 138 126 15 183 I00 100,110,114 192 9,74

104 104 131 3 138 126,196a 15 183 76,182 8,81,I I0 192 9

189a

189a

67 112 189a 189a 79 27 122 97,105,108,110,111,118 79 150 38 33,41,45 189 130 46 40 46,129,187 49,195 65 131,216 128,131,135.216 133 ,217 133 ,217 46 35,117,134,215 134,215 i 34 ,215 134,215 134,215 35 35 134,215 134,215 132 31,44

67

Co)(CO),]2C2

o,(COb(SiCH = CH:) Co)(CO)9(SnBu) Cos(CO)9(SR) R = Ph, pTol Co~(CO).S Co](CO))~ Co~(CO),(CSD Co~(CO),(PMe2h [Co~(CObS]2S2 Co~(CO)dSR)S R=Et, Bz Co,(COb(SEt),. Co2(CO),(SEt) Co)(CO)s(SEt),. Co3(CO)6S Co,(CO),(SR), R = E t , Ph Co)(COh(SPh)s. Co)(Co)~ Coj(COh(C,F(SD3 Co3(CO))('Ir,-Cp)2 [CoKCOh(areneh] + Co,(CO)12 Co,(CO).(EPh,) E = P,As,Sb CO((CO)mS2 Co,(CO),dCSD CodCO)~(C~l~) (c) Co~(CO),,

(b) ~Co,(CO),,]'Co.(CO),,]'-

Reviews 1968

Infrared

205 26 74,221

18% 18% 27 81,110 150 38 16,39,45 189

27 77 78 181 38 60,169

46 46,129,187 195 65 131,216 128,131.216 133,217 217 134 95,215 95,215 95,215 95,215 35 35 134,215 134,215

21,220

170 61 135 135

31,44

199

6,31

31

6

145 127 154 167

145 127 154 167

156 96 167 155 209 155 154-155 154 148,152 150 54,87 48 167 156,168 158

156 96 167 155 209 155 155 1 148 150 54 30 48 167 156,168 158

5!

51

5

50,57 51

50,57 5!

5,218 4,219

83,179

206 71 197 209 210 207

205 199 72

34 Treble I.

P. Cram (continued). Rh

Rh,(COh(~-Cp)3(isomer 1) Rlh(CO)d~-Cph(isomer 2) Rlh(CO)a2 RhdCO)~6

Ir

[Rh.(CO)~] ~-

5~

In(CO),2

47 163 163 163 163 103 88 10J 22 15i 103 28 28 28 28 8,192a 85 214 214 214

[lr.(CO),,H]-

Ni

Pt

Mn-Fe Re-Fe Fe-Ru Fe-Co Ru-Os Ru-Co Os-C.o Co-Rh Co-Ni

185 177 28a,56,109 28a,47,56,109

lr4(CO)~0(PPboh lr.(CO),(PPh~)~ [lrdCO)j,]'[Ni.(CO)] a- (a) Nil(COh(x-Cph [Ni,(CO),]'- (b) Ni4(CO),FP(C2I-LCNh], Nh(COh[C2(CFj)Q2 [Ni,(CO).] 2Pt,(CO),(PPh~Bz), Ph(CO)~(PR~), PR3= PPh3,PPh2Me Ph(CO)s(PR3)4 PI~= PPh3.PPhMe2 fPt(COh], [MnFe~(CO), 0[ ReFe2(CO)u]FeR~(CO)n FeaRu(CO)a FeRudCO)~3H2 FeCo2(CO)~e FeCo2(CO)gS [FeCo,(CO)a]- (a) Ru:Os(CO),2 RuOs2(CO)n RuCodCO)t2H OsCodCO)t2H Co~R}h(CO)n Co3Rl~(COh~ CoNi~(COh(~-Cp)~ CoNh(CO)(S)(~-Cph [Co,Ni~(CO),,]~-

185 185 16 51 53 47 163 163 163 i 63 103 88

185 177 208 62 4,53 208 5-a 5-a 71,t 20

103

22 151 28 28 28 28 85 85 214 214 214

146 52 130 130 171 172 55 198

146 52

22

68 68

71 206

171 55

55

55

55 7I 71

* The corresponding neutral hydrido compound has also been reported, b Derivatives of the corresponding hydrido monoanion have been reported, c Derivatives with Call2, C2Et2, C2PIh, Ca(Ph)(COOMe) and C,(H)(SiMe3) have been reported, a Derivatives in which R is H, F, CI, Br, I, Me, Et, Bu, Bz, CHzCH2Ph, CF3, COOH, COOMe, COOEt, CH(OAch, CH=CHOOH, CH~CHr COOH have been reported.

F are reported in Figure 3 and 4. Compounds of subclass E are also reported in Table I. Compounds of subclass F are beyond the aim of the present review.

IlL

SYNTHESES OF CMCC

Many CMCC contain non-metallic heteroatoms such as sulphur, phosphorous, etc. These compounds are generally obtained by the action of particular reactants able to bridge the cluster by formation of more than one bond. Such reactions apparently cannot be classified and we shall limit ourselves to recalling a few of the most important examples. Some of these reactants are: sulphur and organic thiocompounds 2"1~'167, diphosphines ~, chloroform and other halocarbons ~1 and some acetylenic derivativesu'm. For the synthesis of CMCC in which such heretoatoms are not present, there are three general methods (A, B and C). Low to moderate temperatures should be employed, since CMCC are usually thermally sensitive compounds. Whether the kinetics of formation will be favourable at a chosen temperature cannot be easily foreseen. The striking importance of kinetic factors is examplified by the direct formation of Rl~(COh2 in the

reduction of Rh~(COhCI2 both with carbon monoxide-KOH and with copper powder. The carbonyl Rl'h(CO)t6 is formed only by further transformation of R1~(CO)12 ss. This fact probably implies that the entropy factor of the activation energy can control the mechanism of reaction, the probability of putting together four reduced Rh(CO)x groups being much greater than the probability of putting together six similar groups. A.

CONDENSATIONOF COORDINATIVELY UNSATURATED SPECIES

One method consists in the condensation of low valence, coordinatively unsaturated species, which can be either monomeric or polymeric. These species will condense with themselves or with other coordinatively saturated species. Since the coordinatively unsaturated species can be produced in many different ways, we may distinguish the following procedures.

1. Pyrolysis o/simple metal carbonyl compounds. This reaction was employed in 1910 in order to obtain Co4(CO),,: ~ 2 Co2(CO), 600)Co,(CO)u+4 CO

Inorganica Chimica Acta

TheClosed Metal Carbonyl Clusters The same reaction was used for obtaining Rub(CO)t,, t~ Ru6(COhTC,m Os~(CO)u, tit Fe4CO)~0t-Cp)(, ~° Cot (CO),(~-Cph ~s° and Ni~(CO)2(x-Cp)~.~ More recently Ru2Os(CO)t2 and RuOs2(CO)t~ have been obtained by heating Ru3(COh2 and Os~(CO)u in xylene under CO pressure at 175".m

2. Photolysis. The synthesis of Fez(COb by irradiation of Fe(CO)s was reported in 1905; 7s more recent application of the method are the syntheses of Fe2Ru(COh2 and FeRu2(COh2a~ and of the anions [Fe~(CO),]~- m and [ReFe2(CO)u]- as and the reaction: 3 Rh(COh(n-Cp) h~)Rh~(CO)~(~t-Cph+3 CO This method which has very broad possibilities will probably be increasingly used.

3. Halogen abstraction from halo-carbonyl compounds. In the presence of carbon monoxide the halogen abstraction is a general method for the synthesis of metal carbonyl compounds. It gives CMCC only when these are more stable than the corresponding simpler carbonyls. Recently this method has been applied to the synthesis of Ru3(CO)tzf Rh((CO)t2,~'~'s6 Rlh(COh6, na'~'ss Ir4(CO),~ 47'I~ and [Pt(CO)2],. zs A related synthesis of Os3(CO)tz starting from OsO) has been also reported?s'm. Generally the reaction is carried out starting from the metal halides, but the formation of intermediate halo-carbonyl compounds is probable. It is possible to use several different halogen acceptors. In heterogeneous conditions powdered metals (such as Cu, Zn and Ag) have been frequently used; in homogeneous conditions carbon monoxide itself can be used, especially in the presence of water and alkali. A typical example of heterogeneous reaction is the synthesis of Rh((COhv This compound can be obtained in good yield from anhydrous rhodium trichloride and zinc under carbon monoxide pressure at 25*-60*,~'~°9 or by reduction of Rh2(CO)(CI~ both with zincs~ or with copper-bronze. ~ 2R1~(CO),C12 + 2Zn + 4CO

25* ) Rh4(CO)u+2ZnC12 100 atm

At temperature higher than 80* Rib(CO)t6 is formed.~.t°9 Similarly the low pressure synthesis of Ru~(CO),2 from ruthenium trichloride, carbon monoxide and zinc consists of a halogen abstraction from the intermediate [Ru(COhClz],fl A good example of homogenous reduction in very mild conditions is the synthesis of the anion [Rhtz(CO)~] ~- in the presence of alkali (for example sodium acetate) and carbon monoxide: ~ 6 Rhz(COhCIz+14CH3COONa+ 13CO+7H20

water-alcohol 25°, 1 atm

35

example of a related reaction is: m 2Rh2(CO),Cl:+Co2(CO),

n-hexane 25" ) Co2Rl~(CO),,+ ?

4. Oxidation o! simple carbonylmetallates. The method was first employed in 1924 for the synthesis of Fe3(COhz by means of copper(II) salts9t and later improved by the introduction of an heterogeneous oxidant like MnOz: too 3[Fe(CO),] ~_+2Mn,. water 25*) [Fe~(CO).]2_ + 2 M # ÷+ C O ?

A similar method has been employed for the synthesis of compounds of the type Fe~(CObEz, where E = S, Se, TeJ to

5. Electrons redistribution in a carbonylmetallate containing a transition metal cation. This method has been recently applied to the synthesis of the [Co~(CO)ts] 2- anion: so 2[CoB,][Co(CO)412 60* Co,(CO),,+12B Co,(CO),,+2[CO(CO),]-

B

B=ethanol

) [ Co,(CO),,],- + Co, CO),+ c o

The tetracarbonylcobaltate reacts as it was a Co~(COh species. A related synthesis of Co((CO),~ was previously described: u Co(C,H,sCOO)2+ 2HCO(CO),

) ICon(COb]+ 2CTHtsCOOH

21Co)(CO),~+ Co~(CO), ~

2Co.(CO)n

This method is also suitable for the preparation of ~
)

) [FeCo)(COhz]-+ ~Coz(COh+CO Similar reactions are probably responsible for the RuCo3(CO)tzH 171 and OsCo3(CO)nH syntheses,m

6. Abstraction o / a ligand L from a metal carbonyl derivative o] the type M(CO)xLy. This method appears to be of a great interest, but it was so far applied only to the synthesis of platinumphosphinecarbonyl compounds, ~ where the reaction is especially easy because of the low stability of the starting material and occurs through redistribution of ligands: 15Pt(COh(PPhDz

78* ,

3Ph(COh(PPhD,+ 2Pt,(CO)(PPh0,+ 19120

) Naz[Rh,(CO)~]+ 12NaCl+ 14CH~COOH+7CO2 In the absence of carbon monoxide this last type of synthesis has been considered only recently,s~ An Reviews 1968

The synthesis of the nickel cluster compound Ni,(CO)6[P(CzH(CNh]( u may involve a similar reaction.

36

P. CHINI

B. CONDENSATIONBETWEEN A CARBONYL METALLATE AND A METAL CARBONYL COMPOUND

According to the present state of knowledge, it can be assumed that the mechanism of formation of some anionic CMCC, when the reaction is performed in alcoholic alkali or in suitable Lewis bases such as pyridine, is based on a condensation between a carbonylmetallate and a metal carbonyl compound. The condensation may take place through a nucleophilie attack of the type .7'~ [Mn(CO)s]- + Cr(CO),

160%170*

)

[MnCr(CO)t.]- + CO

and: m [ Fea(CO),,]~- + Fe(CO)s

) [Fe~(CO).]~- + 3CO

Such a primary condensation may be followed by disproportionation (116). This behaviour is examplified by: 2[Fe~(CO).]a- H~O ) [Fe(CO).H]- + [Fe~(CO).] ~-+ CO+ OHAlso simultaneous condensation of compounds formed by hydrolysis is possible: "~ 3[Fe(CO)~H]-

H~O) [Fea(CO).]'-+2I'h+CO+OH-

Condensations of the latter type are perhaps very important when the reaction is carded out in tetrahydrofuran in the presence of sodium tetrahydridoborate ~s'"'m or when the CMCC is obtained as a hydride derivative upon final acidification, m Reactions where a condensation due to nueleophilic attack by a carbonyl-metallate on a metal carbonyl compound is probable, but where disproportionation and condensation of less stable hydridie species cannot be ruled out, were applied for the preparation of CMCC of iron, m'l~'t°B'm'm'm'"s such as [Fe3(CO).] ~- and [ F ~ ( C O ) . ] ' - , nickel, m such as [Ni3(COh] 2-, [Ni4(CO)9] ~- and [Nis(COb] 2-, chromium ~s and molybdenum) 9 such as [Cr3(CO)t(] 2- and [Mo~(COh(] 2-, ruthenium, m such as Ru4(CO)uH2 and Ru4(CO)nH4, osmium, m such as Os3(COh0Hz and Os3(COh0(OR)H, manganese," such as Mn3(CO)uH~, rhenium, ~s'm such as Re~(CO)uI-Ia and [Re~(COh6] ~-, iron-manganese? such as [MnFe-ACO)u]- and iron-rhenium, ~ such as [ReFe~(CO)l~]-. Although the scope of the method seems broad, the outcome of these preparative reactions is never foreseeable a priori. The influence of apparently minor changes in experimental conditions may be very great. C. DOUBLE EXCHANGEREACTIONS BETWEEN A CARBONYLMETALLATEAND A HAL,OMETALCARBONYL Double exchange reactions between a carbonylmetallate and a halometalcarbonyl having together the amount of bonded carbon monoxide required for the formation of CMCC have been reported only recently; Co2RI~CO)n has been obtained with the re-

action: 5s toluene

Zn[Co(CO),]2+ Rh2(CO).Clz

IV.

)' CoaRh2(CO)u + ZnCl2

25*

STRUCTURAL DESCRIPTION OF CMCC

The CMCC are characterized by structures of high symmetry. Generally the transition metal atoms describe a regular geometrical structure such as a triangle (three metal atoms), a tetrahedron (four metal atoms), a square pyramid (five metal atoms) and an octahedron (six metal atoms). The CMCC can therefore be classified according to the number of transition metal atoms. It is useful to do a further classification according to the /ormal coordination number, or bonding number, shown by the transition metal atoms. This number corresponds to the sum o/ the number o/ simple

bonds /ormed with the ligands and o/ the possible simple bonds with other metal atoms. Such a classification is based on the knowledge of the structure of the concerned compound. The structure is known with certainty only in a few istances, while in many cases it can be tentatively assigned on the basis of infrared spectra and by analogy with compounds of known structure. A symbolism such as 6 / 6 / 6 indicates a CMCC with tl~ree transition metal atoms, each metal atom having a formal coordination number of six.

A.

CMCC WITH THREE TRANSITION METAL ATOMS

1. CMCC 6 / 6 / 6 (without heteroatoms). The structure of Os3(COh2, 61 of Ru3(CO)12Is and of R u t (CO)4(CsHs)z 2~ common to this type of cluster is shown in Figure 5. This class probably includes the following compounds: Ru3(CO~PR3h, Os3(CO~(PR3h, Osr (CO)dDiars), FeRu2(CO)u, Ru2Os(CO)n and RuOs:(COb2. 0

I o

"c~!/c

I

o

o

o

I o

Figure 5. Molecular structure of Os,(CO)u?'

2. CMCC 6 / 6 / 6 (with heteroatoms). The structure of Co3(CO)9CCH3 I~ is shown in Figure 6, and it is typical of this type of cluster. Compounds included in this class are: (CO)9Co3C-CCo3(CO)9, ~ Co3(CO~SiR, Co3(CO)gSnR, Co3(CO)gSR, FeCo2(CO)~S, ~ FeCor (CO)gSea and the paramagnetic Co3(CObS ~ and Cot (CObSe/I The compounds Co3(CO~S(SR) and [ C o r lnorganica Chimica Acta

37

The Closed Metal Carbonyl Clusters

(CO)TS]2Sz~97 also have a structure related to this type of cluster.

H~!/I4

/i\

(OC}3Co~Co

One of the two isomeric forms of Rh3(CO)3('~-Cph belongs also to this class ~s5 (Figure 9), as well as probably C03(CO)3(~-Cp)3. The structure of the violet form of Fe3(CO)8(C2Ph2)2~7 and of Fe3(CO)9(C2Ph2) z7 are shown in Figure 31.

ICOI 3

\X/

1C0)3

Figure 6. Molecular structure of Co)(CO),CCH3.t"

3. CMCC 6 / 7 / 7 . The structure reported in Figure 7 belongs to Fe3(CO)t2. ~ The same type of structure has been found in Fe~(CO),(PPh3) 74 and it is probably present in the compounds Fe3(CO)~2.~[P(OMeh]~ ( x = l , 2, 3), RuFe.,(CO)~2, and in the anions [MnFez(COh2]- and [ReFe2(COh2]-.

0 C 0

Figure 9. Molecular structure of Rh3(COh(~-Cp), first isomer. (E. F. Paulus, E. O. Fischer, H. P. Fritz and H. Schuster-Woldan, 1. Organometallic Chem., 10, P 3 (1967). Reproduced by permission.

C°

4. CMCC 7 / 7 / 7 . Figure 10 shows the structure of the other isomeric form of Rh3(COh(~-Cph. In A structure related to that of Figure 10, buth with hydrogen bridges between the metal atoms is probably present in Mn3(CO)I2H3 and in Re3(CO)t2H3.

c

c°

o °c /

0

%

Figure 7. Molecular structure of Fe3(CO)n, (L. F. Dl~.l and I.. F. Blount, Inorg. Chem., 4, 1373 (1966). Reproduced by

C-

__ Rh (Tr-Cp)

C -

-- Rh ('IT- Cp )

permission.

Figure 8 shows the structure of the anion [Fe3( C O ) , H ] -26 in which an hydrogen atom substitutes a brdiging carbon monoxide group of Fe3(COh.,. In the compounds Os3(COho(OR), and Os3(CO)zoH2 both bridging carbon monoxide groups are substituted by two OR or two H bridges. ~3s 0 C

o

0C

I o

°c

Figurc 8. Molecular structure of the anion [Fej(CO).H](L. F. Dahl and L. F. BIount, lnorg. Chem., 4, 1373 (1966). Reproduced by l~ermission. Reviews 1968

// 0

Figure 10. Molecular structure of Rhj(COh(~-Cp),, second isomer.'"

The compound C03(COh(SEt)s, ~ in which five bridging SEt groups and a bridging CO group are present, belongs also to this class; its structure is shown in Figure 11. A similar type of cluster has been proposed for Co~(COh(C4F4S~)3. The <, clusters Cos(CO):o(SEt)s and C06(CO),(SEt)4S have strictly related structures, based on a C03(CO)s(SEth molecule analogous to C03(CO)4(SEt)5. The sulphur atoms of the cluster C03(CO)s(SEth are used for donation to a Co~(CO)s(SEt) and to a C03(CO)6S fragment. ~,210 Between the two parts there are no metal-metal bonds.

38

P. Cram

ocRS

/$R~_ ,,~SRcO

caNI

o\

I

caH~/

±c.

c

,o

-c.. °

oiC"

0

0 Figure 1I. Molecular structure of Co~(CO),(SEt)s.~' The compounds Ni3(COh(~-Cph, t2° Ni3(CO)S(~-Cph 7~ and Ni2Co(COh(~-Cph 71 have a structure with ~face~ bridging carbon monoxide groups; this structure is shown in Figure 12. In the second compound a carbon monoxide group is substituted by a sulphur atom. The ion [Co3(COh(areneh] + also probably has the same structure.

Figure 14. Molecular structure of Co,(CO),,(C2Eh).n

O

H

(w-Cp) N

~N~

i('ff-i(1T-Cp) cp)

fl 0 Figure 12. Molecular structure of Ni3(COh(~-Cp),.m

B.

Figure 15. Molecular structure of the anion [Re,(CO),~] 2 (R. Bau, B. Fontal, H. D. Kaesz and M. R. Churchill, ]. Am. Chem. Soc., 89, 6375 (1967). Reproduced by permission.

3. CMCC 6 / 7 / 7 / 7 . This type is the most common and it is exemplified by the regular tetrahedron of Co4(COh2 ~ in Figure 16. Other compounds with the same structure are Rh4(CO)m ~ Co2Rh2(COh2, ss Ir4(CO)t0(PPh3)2, s" Ir4(CO)9(PPh3)35" and probably the anion [FeC03(CO)~]-.

CMCC WITH FOUR TRANSITION METAL ATOMS

A tetrahedron is generally found in CMCC with four metal atoms, though it is highly distorted in a few

cases.

OC~.~0 CO

cO ~cd/

oo.

\

oc/i

co\

1. CMCC 6 / 6 / 6 / 6 . This type of structure was reported for Ir4(C0)12 ~ and it is schematically shown in Figure 13.

(CO)3 Ir

Figure 16. Molecular structure of Co~(CO)~2.~

4. CMCC 7 / 7 / 7 / 7 . This type of structure has been found in Ni4(CO)6[P(CzH4CNh]~ zz and is reported in Figure 17.

(COl3

Ni

Figure 13. Molecular structure of In(CO),2.~

2. CMCC 6 / 6 / 7 / 7 . There is a strong distortion of the tetrahedron formed by the cobalts atoms in the structure of Co4(COh0(C2Et2) 7z shown in Figure 14. A similar structure may be present in Co4(CO),oS2Y9 The limiting distortion to a perfect planar arrangement of four metal atoms is reached in the anion [Re4(CO),~]2 ,~5 whose structure is shown in Figure 15.

R3P--Ni~ N i ) o

-PR3 ~3

Figure 17. Molecular structure of Nh(CO),[P(C2H,CNh],. n

lnorganica Chimica Acta

The Closed Metal Carbonyl Clusters

5. CMCC 6/8/8/8. This type is shown by the structure of the anion [Fe~(COh~] ~-7~ in which there are three <,edge>} bridging and one ~face}} bridging carbon monoxide groups (Figure 18). The < bridging groups form asymmetric bridges, the two C-Fe distances being 1.82 and 2.27 ,~.

C.

39

CMCC WITH FIVE TRANSITION METAL ATOMS

CMCC with five metal atoms are uncommon Fer (COhsCs was shown to have square pyramidal structure, as it is shown in Figure 20. Less well defined are Fes(COh0Bh, ~°6 Fes(COh0Sn~~ and the derivatives of the anion [Nis(CON] ~-. 0 c..O

C-o o Figure 20. Molecular structure of Fes(CO),C.a

Figure 18. Molecular structure of the anion [Fe4(CO),,]'-. ~s

There is no agreement on the location of the hydrogen atom in the compounds FeCo~(COh~H, RuCor (COh~H and OsCo~(COh~H at the center of the tetrahedron of transition metal atoms, m For FeCo~(COpH the acidity of the hydrogen atom,u the formation of a covalent mercury derivative,s~ the mass spectrum m and the ka/kv isotopic effecttz~ can be explained with a structure formally derived from that of [Fe4COh~] ~by substitution of the ~face}} bridging carbon monoxide by an hydrogen atom.

6. CMCC 9/9/9/9. This type of structure has been found only for Fe4(CO)4(~t-Cph)SL The suggested structure is schematically shown in Figure 19. Probably Ru4(CO)tzH4 (one of the isomeric forms) and Os(CO)t~H4 belong to the same class, the ~face}} bridging carbon monoxide groups being substituted with hydrogen atoms.

°.

'It C~Fe ~

D.

C M C C WITH SIX TRANSITION METAL ATOMS

CMCC with six metalatoms have octahedral structures. The 7 / 7 / 7 / 7 / 7 / 7 class has been found in Rth(COh4(arene)C '7° (not taking in account the central carbon atom) and it is shown in Figure 21. Only one bridging carbonyl group is present; RudCO)17C is believed to have the same structure. (C0)2

\gy,.,.°., \g,/

(co)~ Figure 21. Molecular structure of Ru~(COh,(eurcne)C.'~° The 8 / 8 / 8 / 8 / 8 / 8 class has been found in the isomorphous carbonyls Co6(CO)l~ and RI~(CO)I~ whose structure is shown in Figure 22. Four ~face}} bridging carbonyl groups are present. The anion [Co~(CO)ts] 2- belongs to the same class and its structure is shown in Figure 23; in this anion there are three <
q

C

q'c Figure 19. Molecular structure of Fe,(COh(~-Cp)4."' (,) Note added in prOO/. Another example of 919/9/9 cluster ts the [Re,(CO)uJ4,]2- anion, where a regular tetrahedron of Re(CO) 3 groups is auoclat~d with 6 hydrogen atoms reigned to the 6 edge IH-.~. Ix~itlons. TI~ R e - Re distance is ~.160(7) A, a value Intermediate between R e - - R e (2.987 in [R¢4(CO)~]2-, Figure 14 and ref. 15, and 3.02 )i in Re2(CO)~0, ref. 69) and Re-H-Re (3.39 A in MnRe2(CO),4H, Figure 2 and ref. 1~9). Similarly In the triangular 8/7/7 [Re](CO),2H2]anion the rhenium atoms are connected by o n e R e - Re bond (;.055(7) A) and two Re"H~Re bonds ¢:1.177 ~) ¢M. R. Churchill, P. H. Bird, H. D. Kaesz, R. Bau, B. Fontal, S. W. Kirtley, I. Am. Chem. Sot.. in press). Clearly the hydrogen atom Is able to bddu¢ two metal atoms by both complete and partial insertion in the metal-metal bond.

Reviews 1968

O\

C_

\

~ h-~..c'O

0 \" "C~'

ki2"-..,

o

I h"...... "(:"o

Figure 22. Molecular structure of Rh,(CO),, (E. R. Corey, L. F. Dahl and W. Beck, ]. Am. Chem. Soc., 85, 1202 (1963). Reproduced by permission.

40

P. Cnm~

9 o

_old"

Figure 25. Molecular structure of the anion [Rhu(CO),0]z-.' Figure 23. Molecular structure of the anion [Co,(CO),s]2-? '' V.

The 9 / 9 / 9 / 9 / 9 / 9 class has been found in the anion [Co6(COh4] 4- 4 whose structure is shown in Figure 24; eight ~face>~ bridging carbonyl groups arc present. It is worth mentioning that the [Co6(COh4] 4anion exists in more intcrconvcrtible forms 5s and it is believed that the ~> bridging carbonyls groups can easily rearrange to ~edge~> bridging groups. This also seems to be the case of the isoelectronic [Ni2Co4(COb.] 2- anion, s5

Figure 24. Molecular structur¢ of the anion [Co,CO,]'-.'

E.

CMCC WITH MORE THAN SIX METAL ATOMS

The only structurally characterized compound of this type is the salt [NMe4]2[RhI:,(CO)30], 4 whose structure is shown in Figure 25. Two Rh6 octahedrons, having the basic structure of Rh6(COh6, are joined at an apex. A similar type of structure, based on two tetraheddcal units, is believed to be present in [Its(COb0]'-.

A.

BONDING IN CMCC

CMCC AND THE NOBLE GAS FORMALISM

The number of electrons on each transition metal atom can be calculated by dividing the total number of electrons brought by the ligands (5 for ~-Cp and 6 for arene) by the number of metal atoms and adding the electrons present in the valence shell of the metal, without considering eventual metal-metal bonds. This gives the results examplifie,d in table 2. It is apparent that with 17 electrons a dimer is formed, with 16 a trimer' and with 15 a tetramer. Since in these cases it is possible to envisage respectively one, two and three metal-metal bonds, the noble gas configuration is obtained in each case and it is believed that such a tendency to reach a noble gas configuration can be explained by the necessity to back-donate electronic charge to the antibonding orbitals of carbon monoxide. This back-bonding is believed to be greatest whC"if all the low energy orbitals of the metal are formally ~illed. The only examples o f paramagnetic CMCC are C03(CO)9S, j~7 C03(CO)gSe, 71 Ni3(CO)2(~-Cph 162 and Ni3(CO)(S)(~-Cph; a they have one electron in excess over the noble gas formalism and their stability can be ascribed to c0nformational factors. The diamagnetic compounds C03(COh(SEt)s, Cos(CO)t0(SEt)s and C06(CO),(SEt)4S also deviate from the noble gas rule; in these cases on the C03(COh(SEt)s or Co~(CO).4SEth part of the cluster there is an excess of two electrons. ~'~'2~° Other exceptions are Os3(COh0H2 (two defect electrons) Osa(COh0(OR)2 (two excess electrons) m and Pt4PPhMe2h(CO)s (four defect electrons). ~ The hexameric CMCC, moreover, show systematic deviation from the noble gas rule. In every case these clusters have two excess electrons. Despite this the 86 electronic configuration is evidently a stable one; oxidation and reduction .reactions performed with the anion [C06(COh5] 2- 5 showed that, in contrast with previous theoretical expectations, ~''" such reactions eccurred without loss or gain of electrons giving the isoeleetronic species C06(CO)16 and [CodCO),4] 4-. An analogous exception to the noble gas rule is found in [ Rhl_,(COho ] 2- . It is therefore clear that the noble gas rule is a formal and limited empirical rule, and that its use in predicting formulas of CMCC should be only accepted with reserve. Inorganica Chimica Acta

41

The Closed Metal Carbonyl Clusters Table II.

The noble gas formalism

Number of transition metal atoms in the cluster

2 Fe2(CO)9

Number of electrons for each transition metal atom. without considering the metal-metal bonds

Possible metalmetal bonds

17 ~

Fe~(CO), v

,

Co,(CO),,

--

If one assumes proportionality between force constants and bond strenghts, the existence of a range of absorptions for each type of carbon monoxide groups is expected also to correspond to a continuous change in the carbon-oxygen distances. It is possible to compare stretching frequencies and bond distances, but this comparison involves considerable errors due both to the unsatisfactory accuracy in measuring distances between light atoms in the presence of heavy ones, and to the different interactions of the normal modes in each particular symmetry. Nevertheless some examples, believed to be typical, are reported in Table Ilia. In these cases an higher electronic density on the metal atom is associated with a longer carbon-oxygen distance, a lower stretching frequency and a shorter metal-carbon distance. In pictorial terms this corresponds to a higher contribution of the resonance form B: M--C--~--O

15

~ M=C=O

A

B

Comparison between bond distances and stretching frequencies of terminal carbon monoxide groups.

Table lll.a

5 Fes(COhC

6

RhdCO),,

B.

Compound

4x15 + lx14

14,33

0

+2

<
In the structures of CMCC we have found three different types of carbonyl groups; terminal carbon monoxide, carbon monoxide bridging two metal atoms (edge bridging) and bridging three metal atoms (face bridging). The carbon-oxigen distance increases in this order and agrees with the observed stretching frequencies. Table 1II shows the range of the C-O distances and stretching frequencies. Although in the presence of electron donor ligands or of negative charges a considerable lowering of the frequencies is possible (values in brackets) the stretching frequency range of the different types of carbonyl groups is fairlv characteristic and this property can be exploited to obtain limited structural information.

TaMe IlL Range of C-O distances and stretching frequencies

C-O group

Distance A

Frequence cm-'

terminal *edge~ bridging *face~ bridging

1.12-1.19 1.165-1.20 1.19-1.22

2150-1950(1850) 1900-1750(1650) 1800-1700(1600)

Reviews 1968

d C = O A vC=O(a)

dM=CA.

Ref.

Fe(CO)s [ Fe,(CO),~]~-

1.12(2) 1.18(4)

2034,2013 1.79(2) 78a, 39 1967 1.72(4) 78, 81

Ruj(CO)~2 Ru,(C,H02(CO),

1.14(2) 1.19(3)

2059,2029 1.91(2) 1996,1920 1.81(3)

[Co,(CO)~sl"Co,(CO),,]'-

1.15(3) 1.17(1)

1982 1910

39, 169 65, 21

1.74(4) 50, 5 1.70(1) 51, 4

a Strongest bands.

The number of bands observed in the stretching carbonyl region of CMCC is generally smaller than the number of possible bands calculated on the basis of symmetry considerations. Generally the infrared spectrum is mainly a very useful finger print for identification. The bridge formed by carbon monovide can be asymmetric, as for example in [Fe4(COh3]2-, 7s (Co,(CO)~o(EhC2), rz and Fe3(CO)n(PPh3), TM so that all intermediate cases between that of a perfectly symmetric bridge and that of a terminal carbon monoxide are possible. This asymmetry can be easily interpreted in terms of the valence bond formalism, by taking into account the following mesomeric structures: o

M/

M (a)

0

"M (b)

M (c)

If structures (b) and (c) do not contribute to the same extent the bridge will be asymmetric. In these two structures the electron pair of a metal-carbon bond has been depicted as a metal atom lone pair. This indicates a possible mechanism of ~,internal nucleophilic attack,, for transfer of terminal or bridging carbon monoxide groups from one site to another.

42

P. C H I N !

An equivalent approach by three center bonds considers two metal equivalent orbitals in a C2~ symmetry:

O2( r r ) C2v metal orbitals

/

"rF Q

orbitals at 120" (and 109"), the angles between the interacting orbitals can be corrected, and the geometry of coordination of the metal atom can be approximated to a better extent.

CO 0

~

\

II

ff co

A1 (0")

T w o of the four necessary electrons are taken from

the carbon monoxide, and two from the two metal atoms .36 For the carbon monoxide bridging to three metal atoms a similar system of mesomeric forms may be envisaged: o

/c

o Ii

I

o II

/c~

c~

bl

..M

~

= Id

0 U (.}./C

0 I CN~.)

M~'-~;-M ~----~ M.

0 C I

M ~

M-~--e~-IOl

Inspection of the latter three resonance structures gives an immediate idea of how carbon monoxide contributes two electrons to every three metal atoms and the three metal atoms themselves supply the bonding with four electrons. An equivalent molecular orbital approach uses one orbital on each metal atom. In the C3~ symmetry of the M3 triangle, the three metal orbitals pointing to the carbon atom give rise to combinations A ~ + E . The A~ combination is filled by the ligand lone pair, while the E doubly degenerate combination is filled by four metal electrons, which back bond into the vacant CO(~*) orbitals. ~'m

E

'rT

*

'

-

.

.

.

- - - , .

,,

,'

"

C=O

An important consequence of considering <, bridging carbonyl groups as three-center bonded and <
u

M

Figure 26. Angles between atoms and orbitals in polycentric bonds: a=measured angle, b=assumed angle between orbitals.

Another consequence is that, owing to the overlap of orbitals of different metal atoms, there is a slight character of metal-metal bond obtained through the polycentric bond with the ligand. An experimental evidence of this effect can be found in the shortening of the metal-metal bond distance related to the presence of multiple bridging CO groups. This shortening generally amounts to only 0.06-0.1 A, as it is shown in the data in Table V. No shortening has been observed in the case of simple bridging groups, as in C04(CO)12 and in Rh4(COh2. ~-a~

Table IV. Some values of the MCM angles in metal carbonyl

derivatives

~edge~ MCM angle

CMCC CoACOh Fe2(CO),(~--Cph Ruz(CO),Ot--Cp)z Fe~(CO)) Rhz(CO),('rt--Cp)2 Rh3(COh(~--Cp)3 Fe~(COh(C2Ph2h Ni.(CO).[ P(CH2CH2CN),], Co2(CO)7(CH,O2) [Co.(CO).] 2[Rhu(CO)~] 2Ni3(CO)2(~--Cp)3 [Fe,(CO)-] ~Rh,(COh6 Fe,(CO),(~--Cp), [Co,(COt,,]'-

83* 85* 87* 87* 84" 82* 79* 83" 79* 81" 80*

~face~ MCM angle

Ref.

78° 78" 77* 79* 79*.5' 80* 77*

92 173 176 190 178 177 77 22 175 218 4 120 78 62 78 4

Comparison of simple M-M distances with diassociated with multiple bridges

Table V. stances

Compound Rh,(CO)3(~--Cph Feb(CO)t2 [Fe,(CO),,] ~[Co,(CO),] 2-

M-M associated with multiple M-M without bridges (A) bridging groups A 2.62 2.577(3) 2.500(6) 2.46 (1)

2.66-2.71 2.68500) 2.580(5) 2.52 (I) a

Ref. 185 205 78 218

a single ,faceJ bridging group lnorganica Chimica Acta

The Closed Metal Carbonyl Clusters

43

Table Vl. Ionization potentials and stretching frequencies in simple metal carbonyls Ionization Pot. (ev) Mass Spectrum Photoionization

Configuration

Compound

vC = O(cm "-1)

vM = C(cm -I)

W(CO), Fe(CO)3

Octahedron Trig.Bipyr.

8.562" 8.53:"

8.18 TM 7.95 ~

1997.5 ~ 2013.5

374 u' 474

2034.4 ~

431 n

Ni(CO),

Tetrahedron

8.64 m

8.28 z°~

2057.6 '9

422 TM

C.

BACKBONDING

AND

COORDINATION

The donation of an electron pair from carbon monoxide to the metal (with formation of acr bond) and the back donation of metal d electrons into the vacant CO(re*) (with formation of ~ bond) produces multiple metal-carbon bonding. The availability of the full d orbitals along the bond concerned depends on energetic and geometric factors. Although a discussion of these two factors is possible only in a rough, qualitative fashion, nevertheless one can consider the limiting case for which the electronic energy of the metal is relatively low as for Fe(CO)s, or relatively high, as for [Fe(COh] 2-. The value of the ionization potential for the process:

The levelling effect due to the negative charge is also shown in Figure 27. In this figure the strongest infrared absorption of the terminal carbonyl groups of several anionic CMCC is plotted against the ratio between the number of negative charges and the number ot transition metal atoms (for example this ratio is 1/3 for [Co6(COhs]2-). The levelling effect is shown by the rough independence on the particular transition metal considered and the geometry of the cluster. The diagram can also be used tentatively to obtain stoichiometric information from the infrared spectra of unknown CMCC anions. 16 t7

18

0.9

M(CO), ---)M ( C O ) , + + e

is assumed as an index of electronic energy. This value may be regarded almost constant in the series W(CO)6, Fe(CO)s and Ni(CO)4 and is reported in Table VI. If the electronic energy is constant, the CO stretching frequencies may be thought to reflect the influence of cOOrdination numbers (geometric factors) on the extent of backbonding. The decrease of the stretching frequency in the sequence tetrahedron, trigonal bipyramid, octahedron, shown in Table VI, indicates that the orbital overlap making up the ~x component of the multiple bond is the greatest in an octahedral configuration in which only three orbitals (d,y, d,~ and dye) are available for the six ligands. In the trigonal bipyramidal Fe(CO)s four d orbitals are shared by five ligands, while in Ni(COh five orbitals are shared by four ligands. It follows that the geometry of overlap is even more important than the number of suitably filled d orbitals. A similar effect is to be expected ~a ]ortiori,~ for a smaller number of d orbitals (2 or 1) interacting with a larger number of ligands in less favourable geometrical conditions (hepta- and octacoordination). If the d electron energy is sufficiently high, as it may be assumed for doubly negatively charged carbonyl metallates, backdonation will be pronounced even from less sterically suitable d orbitals. In such a case the extent of back bonding will be roughly independent of the coordination number. A levelling effect of this type can already be found in the stretching frequencies of the carbonyl group in the series of the mononegatively charged carbonyl metallates: Carbonyl

Metallate

I

V(CO),]-

Mn(CO),]Co(CO),]-

Reviews 1968

~C = O(cm -I) 1859 1895, 1863 1886

Ref. 17 80 80

1o 11 12 12

11 lJ

o,3 o.2 o,I

210(X)

2O5O

I 1950

I 1900

Y" t cm-'m|

Figure 27. CMCC.

Strongest stretching frequency in some anionic

n" of charles n" of metal atoms

0.25 0.25

1 Rha(CO).]' [ FeCo~CO)., ] [IrdCO)~ I '

0.3] 0.55 0.35 0.5 0.5

[ NbCo,( Cq ))..]~ [ MnFedCI ) ) . ] [ReFc~CC ) . ] [ Fe,(CO),, I ' [ R e , CO). r -

0.5

[MnW(CC ) . ]

0.66

[Fe,(CO)., I'

1.0

[W~(CO). ~

1.0

[ FeKCO),

0.166

o.3s

o.s o5

0.5 0.66 I.O

D.

Anion

[Co,(CO),, ]'

[Rec~co ~][FeCo(CO ~]

[CrCo(CO~]

[Co(CO), ]'- (=) rcr~co~, '-

v c= 0 2040 2020 2020 1°~2 1977 1990 1991 1967 1969 1961 1953 1948 1936 1941 1925 18°J0 1685 1866

Solvent THF acetone THF THF THF THF THF DMF acetone THF CH~CI~ THF CH~ij DMF THF THF THF DMF

Rcf. 53 52 163 ~0 55 85 85 81 15 7 192-1 7 1924 81 51 96a g6-a 81

Point i1" I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18

THE METAL-METAL BOND, FORMAL COORDINATION NUMBER AND NUMBER OF LOCALIZED BONDS

The CMCC can be considered as finite parts of closed packed metallic structures. Our understanding of the different combinations of metal orbitals involved in the closed packed structures of metals is unsatisfactory and ambigous 2a~ and it is not surprising that the same difficulty is found in the metal orbitals requirements of the several geometrical types of CMCC. The main difficulty arises from multiple metal-metal bond formation. The largest metal-metal bond distances have been found in some dimeric carbonyls,

P. C m N t

44

such as Mm(CO),o, 7° Mn2(CO),(PEt3h,"" Re2(CO),o,s To(CO),0," [Fez(CO),]2-, TM Moz(CO)6(~-Cph TM and Co2(CO)dPBuD2, 'n" in which there are no bridging groups. It seems reasonable to assume these distances as the closest to the single metal-metal bond. m. In Table VII we have compared some of these distances with the corresponding distances in several CMCC and metals; the significant shortening of the metal-metal distance in metals and in many CMCC is assumed to be an index of the multiple bond character.

T~l~e¥1l. Evidence

metal-metal distances Compound Rea(eOho R e metal

of multiple metal-metal bonds through M-M, distances A

Reference

2.24 b

69 201 15 64 64

[Fe,(COh]'Fe metal Fe,(CO), Fel(CO)n [ Fe4(COh3]'-

2.88 2.482 2.46 2.55-2.685 2.50-2.58

174 201 190 205 78

Co2(CO),(PBu,h C o metal Co2(CO)3 Co((COh, Co,(CO),s]'-

2.66 2.506 2.52 2.49

127 a 201 92 205

2.46-2.52 2.47-2.53

218 4

[

R~(CO),.]'Re,CI,2] ~-

Re2Ch]2-

Co,(CO),]'-

3.02 2.75 2.98 2.47 a

a Estimated bond order of 2." b Estimated bond order of 4?'

Although it is possible to account qualitatively for such multiple bonding in CMCC, we prefer, in first approximation, to neglect the multiple character of the metal-metal bond. If we consider now the need of justify the stereochemistry at the metal atoms it seems that two different pictures of CMCC are particularly useful. In the first one we fix the stereochemitry choosing suitable hybrid orbitals and we assume that both metalligand and metal-metal bonds are conveniently represented as essentially localized bonds. In this case we expect metal-ligand and metal-metal bond energies of not very different value. In the second one only the ligands stereochemistry is explained starting from V.B. hybrid orbitals, the metal-metal bonds are assumed to be more conveniently represented in M.O. terms starting from non hybridized orbitals, A situation which generally implies a gap of energy between the two types of bonds, and reminds the usual picture of a and ~ bonds in aromatic compounds. The bond between two transition metal atoms can be therefore conveniently represented using outer hybrid or inner d orbitals. When the two metal atoms are supposed to share outer hybrid orbitals the bond can be represented as an electron density mainly concentrated betwen the two partners; this outer localization give rise to a stereochemically active bond. When the two metal atoms are supposed to share inner d orbitals there is much less localization owing to the particular shape of these orbitals and this bond,

which has nodal planes along the bond axes, can be considered stereochemically inactive. The two description are equivalent, inner metal-metal bonds being useful in explaining both unusual stereochemical situations and delocalization over the whole system of metal-metal bonds. Generally it is a matter of convenience to choose a particular type of description. For example the Co3(CO)gCR compounds (average OCCoCO angle of 100.5 °) can be described assigning to the cobalt atoms both octahedral and tetrahedral hybridization. In the first case there are outer d%p3 metal-metal bonds, while in the second there are inner d metal-metal bonds. 37 This diffrentiation between outer metal-metal bonds and inner metal-metal bonds implies the possibility of using different values for the bonding number (or formal coordination number) and for the number of equivalent metal orbitals (hybridization number) used in the outer sphere. The metal-metal bond can be represented using outer and inner components which can generally point not only along the straight line connecting the two metal atoms, but also along other directions connecting the atoms in a bent way. This equivocal situation is well illustrated by the case of Co2(COh, whose structure is shown in Figure 28.

O°C C:O O " c ~

C=0 C'O

O:-C

C~.O

Figure 28. Molecular structure of Co(CO)s ~

In such a case there are two different bonding schemes in agreement with the octahedrical hibridization on each cobalt atom. A bent outer metal-metal bond is obtained assuming the electronic configuration (d3)6(d2sp3)3, an inner straight metal-metal bond is obtained from the electronic configuration (d3)S(dZsp3)4.n There are some semiempiric postulates useful in establishing the more convenient way of representing metal-metal bond in CMCC. The hybridization numbers 4, 5, 6, and 7 would be the more favourable for backbonding, higher hybridization numbers such as 8 and 9 seem out of question. This means that bonding numbers such as 8 and 9 can be reasonably reduced to lower and more acceptable hybridization numbers. Moreover it has been assumed 36 that generally the metal-metal bond is an inner bond sterically inactive when bridging carbonyl groups are present. This is also reasonable if one assumes that in CMCC the bonding angles between hybridized orbitals (suitable for allowing backdonation) cannot be less than about 900.60*. According to these postulates in Co4(CO),, the apical cobalt atom is octahedrically coordinated and presents stereochemically active metal-metal bonds, but the bonds between the three basal octahedrically lnorganica Chimica Acta

The Closed Metal Carbonyl Clusters

coordinated coba.h atoms are inner and sterochemically inactive. Six electrons are assigned to one At bonding and two E degenerate antibonding combinations of octahedral hybrid orbitals pointing below the basal triangle, and energies are taken in such a way that no residual metal-metal bond interaction is left on these bent combinations. The bonding scheme of the three basal cobalt atoms is the following: electronic configuration

(d')'(d~sp~)s (d~)' ~

metal-metal bonds

(d')'

~(d') '~

From this point of view the bonding situation of Co,(CO)12 is very similar to that of [Fe4(COh3] 2-, since in both cases the basal atoms are octahedrically coordinated and linked by inner bonds. This fact points out how formal are the bonding numbers of 6 / 7 / 7 / 7 and 6/8/8/8. We also reasonably assume that in Rla~(CO)to and in Co~(CO)~ the metal has tetrahedral coordination to the carbon monoxide groups and that these clusters can be explained on the base of an hybridization number of four.57 The tetrahedrons are oriented in such a way that the dx, and d~, atomc orbitals of each metal atom may be chosen to point along the edges of the octahedron and their combination in the Oh symmetry gives four triply degenerated molecular orbitals (Tts, T2~, Tt,, T2u). Moreover the d~ atomic orbitals point on the center of the octahedron and their combination gives three molecular orbitals (A~g, E, and T~,). The required number of 86 electrons in the valence shell is obtained as follows: 48 electrons are accomodated using the six tetrahedral sets of sp~ hybrids, 24 are located on the twelve essentially non bonding d~_y~ and d.,y metal orbitals (these orbitals are also used in backdonation), 12 electrons on the triply degenerate Tt, and Tz~ molecular orbitals and 2 on the At~ molecular orbital, s7 Similarly the At~ and Ttu molecular orbitals are responsible for the bonding of the carbon atom at the center of the oetahedron in RudCOh~(arene)C. t~° A bonding scheme in terms of molecular orbitals has been also recently proposed for Co~(CO)4(SEt)s.~ It is of high interest that structure determinations of some homologous diamagnetic and paramagnetic clusters have shown a significant increase of the M-M distances associated with the presence of one excess electron: 7t diamagnetic compounds

paramagnetic compounds

M-M )[

A A

Co3(CO)~S

2.554(7) 2.637(7)

0.08

Co2Fe(CO),S

Co2Fe(CO)~Se

2.59 Co)(CO)~.Se 2.61 Ni)Co(COh(~--Cp)j 2.358(4) Nis(COh(~--Cp)~ 2.389(2)

0.02 0.02

This shows that the assumptions on the delocalizability of the metal-metal bonds are substantially correct and that contrarily to previous theoretical consiReviews 1968

45

derations m the excess electron lies in a molecular antibonding orbital principally associated with the metal atoms. The equivalent orbitals treatment of Kettlere't" is based on several different assumptions such as the fixed presence of metal-metal bonds in edge and face position. For the less symmetrical CMCC, such as Fe3(COh2 and Co,(CO)t2, this description seems subject to objection, and for RhdCO)t6 it is presently inadequate. Our experimental knowledge of the metal-metal energies in metal carbonyl derivatives is extremely poor; bond energies (in kcal/mole) of 18.2-22.1 have been proposed for Mn2(CO)t0,z*,~ of 51 for Re2(CO)t0,2~ of 61.5 for MnRe(CO)10 ~° and of 11.5 for Co2(CO)~,25 from mass spectra data. However such data have been questioned owing to the possible formation of radicals in excited state, u'93 The metal-metal bond overlap increases in going down a subgroup and therefore the formation of increasingly stable stereochemically active bonds is expected to increase in the same order, ts4 For example Ru(CO)s and Os(CO)5 are not stable and easily turn to the CMCC Ru3(CO)t2 and Os3(CO)n. A similar trend is observed in the Cobalt subgroup where Co2(CO)8 is stable, but the Rhodium and Iridium comparably stable compounds are RhdCOh6 and Ir4(COh2. The mass spectra of polynuclear metal carbonyls confirm this view: the ions containing two metal atoms represent 60% of the ions in the case of Mn2(CO)to in comparison with 96% in the case of Re2(COh0, and the ions contaifaing three metal atoms represent 35% in the case of Fe3(COh2 in comparison with 92% in the case of Ru3(COh2J2 It is noteworthy to point out that such spectra show a strong tendency to maintain the metallic cluster. The extreme case is shown by the apparence of the simple cluster metal ion as C04+ from C04(CO),,, to Ru3+ from Ru3(CO)m t~ Os3+ from Os3(CO)m TM Os4+ from Os4(CO)~204,TM FeCo3+ from FeCo3(COh2Ht7~ and RuCo3+ from RuCo3(COh2H.m

E.

CONFORMATIONAL EFFECTS IN CMCC

Usually the linear dimeric metal carbonyls present a staggered configuration of the radial carbonyl groups, in order to minimize steric interaction. A staggered configuration has been found in Mnz(CO)10,7° Mn2(CO)r (PEt3)2,2u Tc2(CO)10,12 Co2(CO)dPR3hy"'1~ Hg[Co(COh]2 j~ and Zn[Co(CO)4]2J ~° The eclipsed configuration has been found only in the presence of a linear M-H-M bond, in [Cr2(COh0H] - ~ and in RezMn(COh4H, t39 and it is possibly related to some particular properties of this bond. The structure of RezMn(COh4H is shown in Figure 2 and illustrates both the staggered and the eclipsed case. The shorter Re-Mn bond (2.960 A) corresponds to the staggered situation, and the longer Re-H-Re bond (3.39 ,~,) to the eclipsed one.

We shall now consider the presence of similar conformational forces in CMCC. The triangular clusters Fe3(COh2, Fe2Ru(COhz, FeRu2(CO)~. Ru3(CO)t2 and Os3(COh2 show different

46

P. CmNI

structures: in FeRu:,(CO)m Ru3(CO)~, and Os3(CO),~ there are no bridging groups and the metal atoms are reasonably octabedral with hybridization number six, but in Fe2Ru(COhz and in Fes(COh2 the presence of bridging groups can be associated with a sterically inert metal-metal bond again ascribing an hybridization number of six to the iron atoms. The tetrabedral dusters Co4(CO)m Rh((COh2 and Ir((CO)tz present an analogous situation; here Ir4(CO)u has no bridging groups and octabedral iridium atoms; Rh((CO)s2 and Co((CO)~2 present the same hybridization together with sterically inert metal-metal bonds due to the presence of bridging groups. In Fe3(CO)m Fe2Ru(CO)12 Co4(CO)u and Rlu(COh2 the oxygen atoms describe an icosahedron; in the other cases they describe the more stericaUy crowded shapes of a truncated trigonal bipyramid (Oss(CO)st and Rus(CO),~) and of a truncated tetrahedron (Ir4(CO)~). From a simple point of view of the sterical crowding the icosabedral structure appears to be better, but, owing to the fall of backdonation to the carbonyl groups in the series face >) bridging > ~ edge )~ bridging > terminal, such structures with bridging groups have higher backdonation requirements than the structures with only terminal groups. Therefore it seems probable that only the metal atoms with lower ionization potentials (Fe, Co) are able to satisfy the conditions of low sterical crowding, while, for the metals with higher ionization potentials (Os, Ir) the backdonation is the driving force in determining the structure. This interpretation agrees with the presence of bridging groups in Ir4(CO)9(PPh0s and Ir4(CO)~0(PPh3h,~-~ where, owing to donation from the phosphine ligands, stronger backdonation to the carbonyl groups is possible. The importance of the effective electronic density on the metal atoms is well stressed by the changes in structure due to the substitution of the carbon monoxide with electrons donating ligands or with negative charges. Some typical examples in which there is a loss of the higher backdonation requirements are:

Ir~(CO)~2

PPh3 ~ Ir.(CO),(PPh,), -CO

+

6/6/6/6 m

6/7/7/7" Co2(CO), +PPh,> Co2(CO),(PPh))2 -CO

6/6 n

5/5 ,m.t~ Fez(CO),

7/7 ~

•-Jf-e

-CO

~ [ Fea(COh]z5/5 m

The introduction of negative charges or electron donating ligands therefore has a significant effect on the metallic cluster compound and often large structural rearrangements can be observed. Another example of such rearrangement can be found in Co6(CO)16 and [Co6(CO)ts]z-? In this case the substitution of a face ~ carbon monoxide with an electron pair, would formally lead to the formation of a free electron pair on each of the three metal atoms previously bonded

by the ~ face >, carbon monoxide, two electrons being acquired and four being released from the polycentric bond. The three electron pairs would then attract three terminal carbon monoxide groups, thus' forming three new ~ edge ~> bridges between the metal atoms of the original triangle.

Vl.

REACTIVITY OF CMCC

The reactivity of CMCC is a difficult subject, because in many cases too little has been reported in the literature, the main interest having been devoted to the preparation and not to the reactivity of these compounds. We have divided the subject according to six main types of reactions: oxidation, reduction, electrophilic, nucleophilic Chard), nucleophilic (soft) and catalytic. In the uncharged CMCC we assume the presence of a residual negative charge --~ on the transition metal atoms and of a residual positive charge +~ on the carbon atoms of the carbonyl groups; these polarities originate from differences in donation and back donation and are schematically shown in Figure 29.

tf

c ~ c ÷g/y

c/-

c_jc / Figure 29. Polarities in a CMCC M,(CO), A.

OXIDATION

REACTIONS

Many oxidizing agents have a destructive effect on CMCC. This result is related to: a) the low valence state of the metals, b) the relatively low energy of the metal-metal and metal-carbon bonds, c) the relatively high energy of the metal-oxygen and metal-halogen bonds. Also important is the fact that oxidizers, picking up electrons from the metallic cluster, are able to diminish the metal-carbon strength by lowering the backdonation. Generally uncharged CMCC show an increasing stability to air oxidation as the Pauling electronegativity of the metal diminishes; negatively charged CMCC are not stable in the air. For example iron, cobalt and nickel zerovalent compounds are easily oxidised, but the ruthenium, osmium, rhodium and iridum compounds are generally stable. There are some noticeable exceptions such as the air stability of the Cot (CO)oCR compounds. 167 The first stage of the reaction with halogens seems to be the breaking of metal-metal bonds, and intermediate carbonyl-halides have been reported. For example: m.t~0 Ru,(CO),2+3X2 --* 3 cis Ru(CO)oX2 --* [Ru(CO),X,], Inorganica Chimica Acta

47

The Closed Metal Carbonyl Clusters

More recently the linear compounds X-Os(CO)c--Os(CO)4-(CO)4Os-X have been obtained by the breaking of only one metal-metal bond in Os3(COh2:3° The reaction with iodine can be used for the determination of the carbon monoxide content only when the carbonyl halides are not stable. This is the case of iron, cobalt and nickel in the presence of pyridine, but in the case of rhodium it is necessary to add an excess of 1,2-bis (diphenylphosphine)ethane after the iodine.~ The reaction with weak oxidizers such as FeCI~ and HgCI~ is strongly dependent on the reaction medium. In organic solvents such as acetone and methanol a complete oxidation of iron, cobalt and nickel carbonyls has been reported: ~ acetone

[ HNR)] [ HFe,(CO),,] + 8FeCI, ) 1IFeClz+ I ICO+ HCI+ [HNR,]Ci In water the same reaction has been used to prepare intermediate insoluble zerovalent compounds: s~ water

[Co6(CO),s]2- + CO + 2FeCl) Co~(CO),)+ 2Feel2 + 2C1-

)

A formal oxidation occurs in several reactions o f Ru3(CO)12 with halocarbons: ~'ag~ CHCh or CCh 9O"

• [Ru(CO))CI2])

the reduction of the [Co6(CO)~s] z- anion to the [Co6(COh~] (- anion) ~ The second type of result is very common, for instance as with Fe3(CO)t2,1s3 Ru3(CO)m ~ Rh4(CO)lz ~ and Ir4(COh2:l~ THF Fe,(CO)u+6Na ~ 3Na2[Fe(CO),] Sometimes the reaction is more complicated: s~ 9/4Co4(CO),+ 5Na ~ Na2[Co6(CO),,] +3Na[Co(CO),] It is known that some organometallic zerovalent compounds, such as dibenzenechromium and cobaltocene, can be compared to alkali metals, in respect to their high tendency to form the corresponding monovalent cations. There is an immediate reaction between cobaltocene and a toluene suspension of Co~(CO)m the primary reaction is probably the following: ~ 2(Tr---Cp)2Co+ 2Co,(CO)tz ---. --) [(~--Cp)~Co].[Co,(CO)~] + 2 c o Sodium tetrahydridoborate, NaBl-h, has been used to reduce Os3(COh2 131 and Fe3(CO)m43 in this last case the anion [ F e 3 ( C O ) . H ] - is formed. The compound Mn3(COh0(BH3hH, whose structure is shown in Figure 30, has been obtained as a by-product in the reduction of Mn2(COh0 with NaBI-I4fl°

60% 0

\c

CHBr)

Ru)(CO).

) [Ru(COhBr2])

9'

%$I

o

ID

0

o

70".80•

CH2= CH--CH~X 80"-90"

/

c

(n-AII)Ru(COhX 60-90% X = CI, Br, I

/

HI I ~

~A

A similar type of reaction is the related insertion of R3SnH and RaSiH compounds in metal-metal bonds) 9 as for example:

o

/ ~c~,~o '%o /c 0

Ru3(CO),~ Me)Sill [Me)Si--Ru(CO),]2

B.

REDUCTION REACTIONS

Many CMCC can be compared to aromatic organic compounds, both having filled and unfilled delocalized molecular orbitals. Therefore we expect the unfilled molecular orbitals to accept easily electrons from alkali metals, and the thermodynamic balance of the process to be strictly dependent on the reaction medium, that is on the alkali solvation. The reaction of the neutral CMCC with alkali metals in solvents such as diethylether, tetrahydrofuran and diethylenglicoldimethylether is a very easy one and no dependence on the reaction medium has been reported yet. This reaction can follow two different patterns depending on the breaking of metal-carbon bonds or metal-metal bonds. The first type of reaction is rare, one example being Reviews 1968

Figure 30. Molecular structure of Mn,(CO),o(BH0~H. (H. D. Kaesz, W. Felmann, G. R. Wilkes, and L. F. Dahi, 1. Am. Chem. Soc., 87, 2753 (1965). Reproduced by permission.

C.

REACTIONS WITH ELECTROPHILIC REAGENTS

Very little is known on this type of reactions. In the old literature many zerovalent CMCC are reported to be unreactive with non-oxidizing concetrated acids, but these observations are of limited value owing to the insolubility of the compounds in the polar medium. The study of the protonation reactions of CMCC is an open field. There are many reports on the reaction between cluster carbonylmetallates and acids, but this obvious reaction is not of interest in the present context. It is worth mentioning the formation of an addition compound between Co((COhz and AIBr3.u

48

P. CltlNI D.

REACTIONS WITH
The strongest ~ hard ~ nucleophilic reagents, such as O H - , O R - , H - and R-, react at the positive charged carbon atom of a carbonyl group. This type of mechanism has been recently proposed by Kruck m and agrees with the isolation of carbene derivatives: ~ M.(CO),+ OH-

M,(CO),-,C\o

-

+

40~

[Fe(en),][Fe,(CO),,] en

9O*

) [Fe(en),][Fe,(CO),] ~

[Fe(en),][Fe(CO),] 145" en = etylendiamine "~ 3[FePy,][Fe,(CO),,] +21CO '"

H20

This type of mechanism also agrees well with the high reactivity of very few soluble compounds such as Ir4(CO)t2 ~ and Rh~(CO)t6; ~ both react easily at room temperature with an alcoholic suspension of sodium carbonate. The reaction is seldom simple and often condensation or fragmentation processes are superimposed; examples are: Na2COr-EtOH lr,(CO)~, ) [lr((CO).H]- m CO Fe)(CO),2+4KOH - ) Ka[Fes(CO)u]+K~COs+2HIO 1® KOH

NHs ), [Fe(NH~),] [Fe,(CO),,] ''° --33*

5Fe,(CO),z+ laPy - ~

[M,(CO),_,]'- +CO,

Ru,(CO),,

Fe,(CO). ~

/ O H OH~ ) M.(CO),_,C N o /O-

)

Fe3(CO),2

The reaction with pyridine oxide and dimethylsulfoxide are similar to the reaction with pyridine, m Examples of reactions of the cobalt compounds are: 3CO,(CO),, +24B 25*), 4[COB,] [Co(CO),], + CO

B = Methanol, ethanol, s7 pyridine m and ammonia? ° Co,(CO),,+ 12B a=~ 2[CoB,][Co(CO),], B = Methanol, acetone, pyridine, s~ 4[Co~(arene),(CO),]X + 6Py [CoPy,] [Co(CO),]2+ 7Co +2CoXz+ 12arene ~

E.

REACTIONS WITH t~ SOFT ~> NUCLEOPHILIC REAGENTS

) [Ru((CO),,]'- + [Ru((CO),,]'- 'st

Co,(CO)t~ KOH) [Co(CO),]- u With RI'u(CO)n the reaction is particularly sensitive to the reaction medium and to the nucleophilic agent, different results being obtained in alcohol, alcohol-water and water. 5s In alcohol-water with sodium or potassium acetate the anion [Rh12(CO)~] 2is formed. Weaker ~ hard >> nucleophilic agents, such as alcohol, ethers and amines, seem to be able to react only with iron and cobalt CMCC, leading to a change in the oxidation number of the transition metal atoms and to the breaking of some metal-metal bonds. The high reactivity of the derivatives of these particular metals can be due to the low ionization potentials of these metals and to the associated easier polarizability of the negative fractional charge present on the metal skeleton of the cluster. Alternatively the same reason can be related to a high mobility of the carbon monoxide groups; the time of half exchange with uCO is 300 hrs for Fe~(CO)n and 80 hrs for Co((CO)m ~4~m but unfortunately no similar data have been reported for CMCC of the noble transition metals. Generally metal-metal bond breaking is much easier for the lighter transition metal CMCC than for the heavier ones. Examples of reactions of the iron compounds are the following: 6Fes(CO),, ROH) [Fe(ROH),][Fes(CO).H]2 + 25* + 10Fe(CO)s+ Fe(OR)z R = Me, Et ~°~

The most important < nucleophilic agent is surely carbon monoxide itself and it is astonishing to realize how limited is the information reported on the reaction between CMCC and carbon monoxide. The entalpy of the process: Co,(CO)tz+4CO a:~ 2Co~(CO), has been obtained ( - 3 3 kcal/mole) by measuring the equilibrium pressure at different temperatures, a This reaction has been recently observed at room temperature and atmospheric pressure, using isopropanol as reaction medium, u Recently evidence has been obtained by I.R. spectroscopy for the equilibrium: Ru)(CO)~2+3CO l ~ C

3Ru(CO)s

It has also be shown that, at 150" and 150 atm carbon monoxide partial pressure, Ru(CO)((PPh3) is quantitatively obtained from Ru~(CO)9(PPh3h; lu conversely, the homologous Os3(CO)9(PPh3h gives free triphenylphosphine and Os((CO)tz at 180 ° and 150 atm carbon monoxidefla All the octahedral clusters of cobalt react easily with carbon monoxide: solutions of Co6(COh6, [Co#CObs] z-, [C06(COh(] (- and [Ni,Co((COh4] ~react rapidly at room temperature and atmospheric pressure. 5°'sl'ss In the case of [C06(COhs] 2- in tetrahydrofuran the I.R. spectrum shows the following primary process: 57 [Co,(CO),~]'- + 9 c 0 --, 2[Co(CO),]- +2Co2(C0h lnorganica Chimica Acta

The Closed Metal C~bonyl Clusters

On the contrary the derivatives of the anion [Fej(CO).] ~- and [Fe4(CO).] ~- are reported to react with carbon monoxide only under pressure at elevated temperatures (I00"-200"C) giving derivatives of the anion [Fe(COh]~-. ~ The chromium anion [Cry(CO).]'- reacts at 150"C under pressure according to the equation: ~s [Cr,(CO)u+3CO

~

[Cr(CO),]'-+2Cr(CO),

The most extensively investigated reaction with <
o:

i

o

\

o

o

+%~c~ OC"--'F°~~' :~ co i ~oc/ :.~o '' -3c° '.__¢...

~

0

(,) F~ (co),2

(b)

o

.,re~

,

%~

_jl a, ~'|

,,, ISOHERIZATION

/~

The reactions between unsaturated hydrocarbons and metal carbonyl compounds have been also recently reviewed, n,m and we will recall only the extraordinary reactivity of Ru~(COhTC with several aromatic compounds, m as well as the formatioq of a ~ chain by reaction of Fe3(CO)9(CzPhz) with an excess of diphenylacetylenefl The last reaction is shown in Figure 31. Nitric oxide is reported to react with C04(COhz at a temperature at which decomposition of Co(COhNO occurs? s° and with Rh6(CO)t6 there is a similar high temperature reaction with formation of rhodium metal. u By the reaction of RudCO)n with NO a product formulated as [Ru(COhNO]. has been obtained; 4~ Fe3(COh2 gives Fe(CO)z(NOh. ~

F.

CATALYTIC

APPLICATIONS

Generally CMCC are stable only in limited conditions of temperature and carbon monoxide pressure and this behaviour has drastically limited their application to catalytic reactions in which carbon monoxide is involved. The only well-known case of this type of catalysis is the synthesis of alcohols starting from a mixture of olefin, carbon monoxide and water and using derivatives of the [Fe3(CO)HH]- anion as catalyst. This reaction has been thoroughly studied in the case of propilene, with the following results: m

/

CH,CH,CHjCH~OH

(85%)

CHr-CH = CH=+ ]CO + 2H.XZ)

+ 2CO2

"~(CH=).~HCH,OH

(I 5%)

(~'mlper~mre = 9&-IIO'C; CO partial p~uure = IO-15 atm; ytld : 90%).

Organic radicals are formed by reaction of Co~(CO)tz with CCh or CBr4 and they can be used to start the polymerization of vinyl monomers such as methylmethacrylate. On the contrary Co2(COh behaves as a polymerization inhibitor, possibly it picks up the radicals very rapidly, ts Polycentric ligand-metal bonds and deloealized metal-metal bonds are probably common features of CMCC and of molecules chemisorbed on metal surfaces; the importance of CMCC as models for catalytic reactions on metal surfaces is emerging at present.

Ce,HsCa~Hi, Fe:j(CO)9

+ % ~ c ~ ~ - co i

o

49

o¢

g

oo

--tr-w"-

t

o

Acknowledgments. The author is particularly grateful to Dr. P.S. Braterman, Prof. F. Calderazzo, and Dr. D. Giusto, for helpful criticisms and for several important suggestions. He also wishes to tank Prof. L. Malatesta for his encouragement to undertake this work, Prof. V. Scatturin and P. Corradini for many helpful discussions and the Italian Research Council (C.N.R.) for financial assistance.

Co

has O

(¢1) (C.QI~CaC.4H~Fe=(CO), (bi*¢k)

REFERENCES (C) (C~H~CaC.~H~)zFe3 (CO)s (vide)

Figure 31. Formation of a C, chain from diphenylacetylene (L. F. Blount, L. F. Dahl. C. Hoogzand and W. Hubel, ]. Am. Chem. $oc., 88, 292 (1966). Reproduced by permission. Reviews 1968

(I) E. W. Abel., B. C. Crosse, a n d O. V. Hutson, 1. Chem. Sot., (A), 2012 (1967). (2) E. W. Abel and B. C. Crosse, Organon,wtalll¢ Chem. Rev., 2,

495 (1%7).

(5) E. W. Abel, and I. H. Sabherwal, I. Or~anometalUc Chem., I0,

491 (1967).

50

P. CHINI

(4) V. Aibano, P. L. BeJlon, P. Chinl, and V. Seatturtn, ,~Proceedings Symposium on Metal Carbonyls, lnorg. Cldm. Acta, Ed. Venice 2-4 Sept., 1968, B-4. (3) V. Albano, P. Chini and V. Scatturin, Chem. Comm., 163 (1968). (Sa) V. Albano, P. L. Bellon and V. Seatturtn, Chem Comm., 730 (1967). (6) G. Allegra, B. Mostardini-Peronaci, and R. Ercoli, Chem. Comm., 549

(19f~).

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Reviews

1968

51

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