Clusters metal carbonyl

Whereas a large number of metal-sulfur clusters are present in nature, carbonyl clusters are exclusively products of chemical synthesis. They have been widely used in industrial catalytic processes17 and some of these processes are triggered by the redox aptitude of these species.lc g As for the metal-sulfur clusters, we will briefly discuss their structures and their propensity to donate/accept electrons in order of increasing nuclearity. We will consider only homonuclear and homoleptic metal-carbonyl derivatives. However, it is noted that heteronuclear derivatives are gaining considerable interest due to the synergistic effect of metal-metal bonds possessing a polar character.lc,ld 

In CH2CI2 solution this compound displays two successive one-electron reductions, only the first of which is chemically reversible (Eq/- = -0.35 V Ep-12- = -0.85 V (vs. Ag/AgCl)). On the basis of the electrochemical quasireversibility of the [Fe3(CO)i2]/[Fe3(CO)12] process (AEP = 110 mV, at 0.2Vs 1), it seems reasonable to assume that 

Although these two derivatives are isoelectronic with [Fe3(CO)i2], both exhibit irreversible reduction processes. This observation is in agreement with the theoretical prediction that the LUMO of [M3(CO)i2] compounds, which are electronically saturated (48 valence electrons), is metal-metal antibonding, and the antibonding character follows the order Os Ru Fe. 

In the case of [Fe3(CO)n]2-, a triply bridging carbonyl group is asymmetrically positioned (as deduced by the bond distances) above the Fe3 triangle, which is not found for [Ru3(CO)i i]2-,18 and [Os3(CO)i i]2-.15 In all the three compounds there is a carbonyl group bridging two metal atoms. 

Comparison with the corresponding dianion reveals important structural differences. In fact, the triply bridging apical carbonyl is no longer present and one of the bonds of the bridging carbonyl (namely, Fe3-Cl) weakens (becoming half-bonding). This asymmetric disposition of the ligand leads to an inequivalence of all the three Fe-Fe distances. 

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Fig. 13. The stmctures of closo metallaboranes where O represents BH , CH (a) [< /(9j 0-( q -C H )Ni(B22H22)] (b) closo-l]l-[v[-Q ]) -l]l-53i] pri Closo metallaboranes can also be formed by the direct interaction of polyborane and metal carbonyl clusters. For example.

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

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. 

M ,(CO) Metal carbonyl clusters with bridging CO groups M-C(0) M... 

Table 25.9 Some metal carbonyl clusters with interstitial atoms...

The relationship between metal carbonyl clusters and supported metal catalysts. J. Evans, Chem. Soc. Rev., 1981,10,159-180 (94). 

The closed metal carbonyl clusters. P. Chini, Inorg. Chim. Acta, Rev., 1968, 2,31-51 (223). 

Novel reactions of metal carbonyl cluster compounds, R. D. Adams and 1. T. Horvarth, Prog. Inorg. Chem., 1985, 33,127 (200). 

The limited magnetic measurements of very mixed -metal clusters are summarized in Table XIII. The magnetic behavior of some anti-ferromagnetic very mixed -metal carbonyl clusters (Fig. 82) has been studied by Pasynskii and eo-workers. Temperature dependences of the magnetic susceptibilities of Cr2Co(/t3-S)3(/i-SBu )(CO)2() -C3H4R)2l (R = H. Me) have been determined us-... 

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