Some Metal-Carbonyl Clusters

That transition metal-carbonyl clusters, which contain an apparent abundance of electrons, might have much in common with boranes and carboranes, notorious for their deficiency of electrons, appears at first sight improbable. However, the structural and bonding relationship between them becomes apparent if one considers certain metal-carbonyl clusters for which localized bond treatments are unsatisfactory. 

All these molecules contain 86 electrons associated with the valence shells of the 6 metal atoms (the core carbon atoms of the carbides are considered to contribute all their valence shell electrons to these clusters). In the hydride H2Rufl(CO)i8, for example, the metal atoms supply 48 electrons, the carbonyl ligands 36, and the hydrogen atoms 2. Formally, this hydride maj be regarded as derived from the anion [Ru(CO)3]g , which, in turn, can be shown to be formally related to the c(oso-borane anion RgHe as follows (199, 200). 

If 18 electron pairs are assigned to the eighteen metal-carbon bonds (3 on each metal), there remain six AO s on each metal atom, and 25 electron pairs, for use in metal-metal bonding. Since well over half the 

For simplicity, one can allocate 3 electron pairs to nonbonding orbitals on each metal atom, which leaves three AO s on each metal atom for cluster skeletal bonding, and 7 electron pairs to be accommodated in the bonding MO s these AO s generate. This description is reminiscent of that outlined above (Fig. 7) for EgHa . Each Ru(CO)j unit of H2Rue(CO)i8, like each BH unit of can contribute three 

Similar bonding descriptions are possible for the other octahedral metal carbonyl clusters. In Rha(CO)i6, for example, 4 electron pairs can be allocated to the four metal-carbon bonds each metal atom is involved in (each metal has two terminal and two triply bridging 

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

Reactivities and Reaction Mechanisms of Some Metal-Carbonyl Clusters... 

Some metal carbonyl clusters have vertices of post-transition elements such as phosphorus, sulfur, and their heavier congeners. Let us consider an alkyl-phosphinidene vertex R-P. The phosphorus atom of a normal alkylphosphinidene vertex uses only its 5" and p orbitals for chemical bonding leading to four tetrahedrally hybridized orbitals, one of which is an external orbital for bonding to the alkyl group and the remaining three of which are internal orbitals for the cluster skeletal bonding. However, in addition, hypervalent alkylphosphinidene vertices are... 

Some metal carbonyl clusters such as HFe3(CO)n, Ru3(CO)i2 and Ir4(CO)i2 have been studied in an alkaline solution as possible catalysts for the water gas shift reaction. A plausible mechanism has been proposed that involves nuleophilic attack by H2O or OH on an electrophilic metal center of the cluster to form an unstable carbohydroxy metal complex, which is then decarboxylated to give a metal hydride from which H2 is eliminated. 

Some metal carbonyl clusters in zeolites have been reported as active phases for the water gas shift reaction under mild reaction conditions. Iwamoto et al. sug-gested that [HFe3(CO)n] in NaY yielded a catalytic activity for the reaction, at 337-453 K and 1 atm, which was comparable to that in solution at 453 K and 40 atm. Kinetic and spectroscopic results indicated that the reaction between [HFe3(CO)ii] and H2O is rate determining, as it is in solution. 

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-... 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

These types of clusters represent some of the more modest sizes and geometries detected in homo- and hetero-metal carbonyl clusters. From dimetallic up to pentadecametallic clusters have been defined by crystal structures, and assembly of the metal centers in these clusters adopt a number of well-defined arrangements.83 Redox activity in these polymetallic clusters is anticipated and has been observed. Routes to large carbonyl polymetal clusters have been reviewed 83,84... 

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. 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

Another class of molecules that will be discussed contains cluster compounds such as the polyhedral borane anions, B H -", and some metal containing species such as the metal carbonyl clusters. 

Fig. 2. Schematic molecular structures of some high nuclearity metal carbonyl clusters based on octahedra of metal atoms (section a).

There is much interest in transition-metal carbonyl clusters containing interstitial (or semi-interstitial) atoms in view of the fact that insertion of the encapsulated atom inside the metallic cage increases the number of valence electrons but leaves the molecular geometry essentially unperturbed. The clusters are generally anionic, and the most common interstitial heteroatoms are carbon, nitrogen, and phosphorus. Some representative examples are displayed in Fig. 19.4.3. 

A limitation of supported metal nanoclusters prepared from molecular metal carbonyl clusters is that, so far, clusters of only several metals (Ru, Rh, Ir, and Os) have been made in high yields (80 to 90%, with the likely impurity species being mononuclear metal complexes). However, this disadvantage is offset by the advantage of the characterizations, which show that some clusters are stable even during catalysis, at least under mild conditions. 

Kinetics of Reactions of Some Tetrahedral Metal Carbonyl Clusters. Iri,(CO) j2 has been shown to undergo CO substitution with PPI13 by a predominantly bimolecular reaction (62) and this is... 

Studies of thermal reactions of other tetranuclear metal carbonyl clusters seem to be rather rare. Reports on some interesting reactions of complexes such as H Riii, (C0)i2 (68, 69) have recently appeared as have descriptions of fragmentation kinetics of complexes such as H2RU4(CO)13 (70). 

Many phosphane-substituted transition-metal clusters have been synthesized from late transition-metal carbonyl clusters and the appropriate phosphane using reductive ETC catalysis with reductive initiation [318-333]. Indeed such an initiation provides an exergonic cross electron-transfer propagation step. Most syntheses were carried out using a cathodic initiation or sodium benzophenone radical anion. The method was successful because it turned out that the first substitution of a carbonyl by a phosphane proceeds with high yield and coulombic efficiency in homoleptic metal carbonyl clusters and some others. 

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