Carbonyl clusters fragmentation

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

Small Co metal clusters Cora (ra = 2-8) react with CO, with sequential addition leading to the saturated Co species [Co2(CO)7], [Co3(CO)i0], [Co4(CO)i2], [Co5(CO)13] and [Co6(CO)i5]-.71 This points towards one of the features of low-valent Co carbonyls a tendency to form stable clusters. Reactivity of Co with 02 is higher but leads to cluster fragmentation, whereas N2 is essentially unreactive. Entry into carbonyl chemistry of low-valent Co is frequently via the well-known dimer Co2(CO)8. A range of reactions commencing with this compound has been developed, as follows. 

Heteronuclear compounds containing gold(I) and other metal atoms which present Au -M interactions are well represented in the area of metal carbonyl clusters. The addition of a AuPR3+ or Au2(/u-P-P)2+ fragment to a metal cluster results in the formation of Au—M bonds often with retention of the cluster framework. Several reviews have been reported recently,3153-3155 and so it will not be treated here. Some representative examples are found in Figure 26. 

Cluster fragments, e.g., M(CO)3, can display a variety of bonding capabilities even within the same molecule. This is the case in the structures of the binary carbonyls of osmium. The simplest cluster, illustrated in (a), is Os3(CO)12. This consists of an equilateral triangle... 

These ideas are not restricted simply to binary carbonyls and may apply equally to all conventional cluster fragments. On hydrogenation, Os3(CO)12 reacts first to produce H2Osa(CO),0 and then (at slightly higher temperatures) t Os COl. In this case, the polymerization step H2Os3(CO)10 — H4Os4(CO)i2 may be viewed as... 

Carbonyl dissociation will diminish in the presence of external CO. Carbon monoxide will also tend to trap out the unsaturated low-cluster fragments and therefore prevent cluster formation. The formation of Os5, Os8, Os7, and Os8 clusters from Os3(CO)12 is [CO]-dependent, the relative yields of the lower or higher clusters depending on the CO pressure. 

As already shown in Eqs. (3), (4) and (5), fragmentation of carbonyl cluster anions, on reaction with carbon monoxide becomes easier with increasing negative charge. It is therefore not surprising that reduction of tetranuclear clusters with alkali metals in excess, gives rise to simple mononuclear derivatives [M(CO)4]-, as found for Co4(CO)12 in liquid NH3 13, e Rh4(CO)12 in THF under C056) and Ir4(CO)12 in THF under CO 17S ... 

Stereochemistry of Monohydrido- and Dihydrido-Dodecanickel Carbonyl Clusters Containing a Hexagonal Close Packed Nickel Fragment... 

BROACH ET al. Carbonyl Clusters Containing a hep Nickel Fragment 97... 

Tetranuclear clusters Pt4(CO)5L4 and Pt4(CO)5L3 have been prepared from the reaction of CO on the trinuclear species.336,347 For tetrametallic compounds examples are found with Pt2Mo2, Pt2Co2 and Pt2M2 (M = Cr, Mo, w).348,349 These latter complexes show an irreversible two-electron reduction leading to the rupture of the metallic core into identified fragments. These complexes are of the type [PtM(jU3-CO)(ji2-CO)2Cp(PPh3)]2, and the first two oxidation waves indicate one-electron processes.349 A tetrametallic carbonyl cluster PtOs3(fi-H)2(CO)io(PR3) has also been characterized.350... 

The vacuum pyrolysis of [Ru3(CO)12] with PhSeSePh at 185°C affords the tetraruthenium cluster [Ru4(CO)n(/i,4-Se)2] (86) (Fig. 28) as the only isolable product.75 By contrast, the reaction of [Os3(CO)10(NCMe)2] with PhSeSePh under mild conditions produces [Os3(SePh)2(CO),0] (87). Apparently, only the Se-Se bond of PhSeSePh is cleaved in this process.7677 In the reaction with the kinetically more labile [Ru3(CO),2] (compared with [Os3(CO)i2]) and with the use of more-forcing pyrolysis conditions, the Se-Se and Se-C bonds rupture and fragmentation of the carbonyl cluster takes place. Isolation of 86 as the major component suggests that this may... 

The potential participation of HRh(C0)4 In the reactions of high nuclearlty rhodium carbonyl clusters could be analogous to the formation of CNi2 (COgH]- upon fragmentation of CN112(C0)21H2 2— by carbon monoxide (equation 7)(12/... 

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