Carbonyls clusters

Many carbonyl clusters have strnctnres similar to boranes. To what extent is the approach nsed to describe bonding in boranes applicable to bonding in carbonyl clusters and other clnsters  

According to Wade, the valence electrons in a cluster can be assigned to framework and metal-ligand bonding.  

Total number of number of electrons number of electrons 

As we have seen, the number of electrons involved in framework bonding in boranes is related to the classification of the structure as closo, nido, arachno, hypho, or klado. Rearranging this equation gives 

Number of electrons total number of number of electrons involved in framework = valence electrons - involved in metalbonding in cluster ligand bonding 

When transition metal carbonyl cluster compounds are photolyzed, fragmentation of the cluster is frequently observed. Examples of such reactions 

These reactions leading to lowo nuclearity metal clusters occur via sequential photocleavage of metal-metal bonds. The degree of aggregation of the products can in some cases be controlled by changing the reaction conditions. An example is the photochemical reaction of Ru3(CO)i2 with either l,2-bis(dimethyl-phosphino)methane (dmpm) or l,2-bis(diphenylphosphino)methane (dppm). Irradiation of a cyclohexane solution of Ru3(CO)i2 with excess dmpm gives the trinuclear complex Ru3(CO)io(/t-dmpm)  

For the less basic phosphine dppm, photolysis of equimolar amounts of Ru3(CO)i2 and dppm gives the analogous cluster compound Ru3(CO)io(//-dppm). When, however, 2 or 3 equivalents of dppe are used, cluster fragmentation occurs and the product is the dimer Ru2(/t-CO)(CO)4(A -dppe)2.  

A unified model has been presented for rationalizing the photoreactions of the unsubstituted clusters M3(CO)i2 (M = Ru, Os), and it may be feasible to adapt such a model to explain and predict the photoreactions of other unsubstituted cluster carbonyls. Excitation of M3(CO)i2 at shorter wavelength leads to CO dissocia- 

Photoreactions can also be used to both synthesize and fragment mixed-metal oligomers and clusters. An example of such a transformation is the photochemical extrusion of mercury from the heterotrimetallic cobalt-mercury cluster complex HgCo2(CO)8. Photolysis into the MMCT transition of HgCo2(CO)g results in the formation of cobalt carbonyl, Co2(CO)g, and elemental mercury  

Metal Carbonyl Vertices and Electron Counting in Metal Carbonyl Clusters 

A single external orbital bonding to a carbonyl group, an isocyanide ligand (R-N =C ), or a trivalent phosphorus ligand [RyP, (R2N)3P, (RO)jP, etc.] 

Three external orbitals forming a bond to all of the carbon atoms in a planar pentagonal (cyclopentadienyl), hexagonal (benzene), or heptagonal (tropylium or cycloheptatrienyl) ring 

A single external orbital containing a lone electron pair without bonding to an external group such an external orbital is conveniently called a nonbonding external orbital 

In all known cases at least some of the external orbitals of heavy vertex atoms are nonhonding most frequently the number of such nonhonding external orbitals is three. The common types of vertex groups involving transition metals (M) as vertex atoms are metal tricarhonyl vertices M(CO)3 and cyclopentadienylmetal vertices C5H5M both of these types of vertex groups have three nonbonding external orbitals. 

The valence electron counts corresponding to the various structural classifications 

---

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. 

Heteronuclear Carbonyls and High Nuclearity Carbonyl Clusters... 

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 many related compounds, including rhodium carbonyl cluster anions, which are present in the solutions cataly2ing ethylene glycol formation and which may be the catalyticaHy active species or in equiUbrium with them (38). 

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... 

Figure 25.11 Metal frameworks of some high-nuclearity binary carbonyl and carbonylate clusters of osmium (a) Os5(CO)i6 (trigonal bipyramid) (b) Os6(CO)ig (bicapped tetrahedron, or capped trigonal bipyramid) (c) [Os6(CO)ig] (octahedron) (d) Os7(CO)2i (capped octahedron) (e) [Osg(CO)22] (bicapped octahedron) (f) [Osi7(CO)36] (3 shaded atoms cap an Osu trigonal bipyramid).

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

A general property of these carbonyl clusters is their tendency to behave as electron sinks , and their redox chemistry is extensive. [OsioC(CO)24]" has been characterized in no less than five oxidation states (n = 0-4) though admittedly this is exceptional. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

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). 

Phosphine palladium and phosphine platinum carbonyl cluster compounds. N. K. Eremenko, E. G. Mednikov and S. S. Kurasov, Russ. Chem. Rev. (Engl Transl.), 1985, 54, 394 (69). 

The carbonyl clusters provide an interesting set of models for the chemisorption of CO on nickel. It is very interesting that, for the Ni (CO) assignments, a plot of Pco versus 1/n for the three-center-bonded CO moieties extrapolates to 1950 cm for = oo (the "chemi-... 

This reaction is diffusion controlled in solid CO. The binuclear carbonyl is unstable and even at these low T decomposes to Ag2 or higher clusters. Similarly, Au forms Au(CO)[ or 2, which does not dimerize. However, Cu cocondensed with CO and Ar forms compounds Cu (CO) (n = 1-4), which after warming to 35 K decompose to larger Cu carbonyl clusters of indeterminate composition, the IR spectra of which resemble CO chemisorbed onto bulk Cu. 

---