Polynuclear complexes counting

Metal-Metal Bonding and Electron Counting in Polynuclear Complexes... 

Metal-metal bonds in polynuclear complexes containing more than four metals are not appropriately described as localized bonds. Wade and Mingos have developed a scheme (supported by theory " ) for predicting polyhedral shapes by counting skeletal electron pairs and Teo has described a topological electron-coimting scheme. - These schemes are beyond the scope of this book. 

All the standard texts listed in Section A.3 of the Appendix have lengthy sections on coordination and organometallic compounds of the transition elements. See also the books listed in Sections A.10 and A.ll. Bell (1977) gives the fullest account of the chelate effect. Cotton and Wilkinson (1988) (Section A.3) is best for the catalytic applications of complexes. Jolly (1984) (Section A.3) discusses electron-counting in polynuclear carbonyls in some depth. 

An example of the application of this procedure to a complex chromatogram is shown in Figure 6.13 [34]. The sample is a mixture of polynuclear aromatic hydrocarbons from river sediments fractionated by capillary gas chromatography in the laboratory of Dr. M. L. Lee [40]. A count of peak maxima yields p = 145. Component numbers estimated from the slope and intercept give m = 234 and 267, respectively, yielding an average estimate of m = 250 for the number of components, which exceeds the number of peaks... 

These and related methods of structure prediction depend upon the allocation of specific electron counts to different framework geometries and these applications have been very successful in assigning structures to cluster carbonyls and their derivatives. However, for the higher polynuclear carbonyls as the molecularity of the compounds increases the predictive power of the theories becomes less decisive in differentiating between alternative structures. In essence the Wade-Mingos approach assumes that the frontier orbitals of the complex primarily involve metal orbitals so that any variation in the electron occupation will be reffected in a structural change in the metal framework. The Wade theory also requires that the structure of the complexes are based on triangulated polyhedra as found for the boron hydrides. 

Extrusion of carbon monoxide, as shown in reaction 9, has been seen from a few diphosphine bridged metal complexes. This sort of reaction is also found when simple metal carbonyls condense to form polynuclear clusters (e.g., the conversion of Fe(CO)s to Fe2(CO)9). In electron-counting terms, all of the examples available involve loss of carbon monoxide from two 18-electron centers. If a metal-metal bond is accepted as part of the... 

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