Metal Bonding and Electron Counting in Polynuclear Complexes

Metal-Metal Bonding and Electron Counting in Polynuclear Complexes 

Tlie maximum number of formal metal-metal bonds in pol)muclear complexes has often been determined from the total number of valence electrons. This coimting scheme assumes that each metal will adopt at maximum an 18-electron configuration and that a metal-metal bond is a shared pair that contributes to the 18-electron configuration of both metals. As noted below, these restrictions do not apply to metal pol3muclear complexes. However, when these restrictions do apply, the number of metal-metal bonds is 18 x (the number of metals) - (the total number of actual valence electrons), and the number of formal metal-metal bonds (shared pairs) is given by Equation 1.1. 

M = number of metals N = total number of valence electrons 

This equation does not predict the appropriate number of metal-metal bonds when the compound contains two-electron three-oenfer bonds. Many complexes containing more than one metal contain bridging hydride or alkyl ligands that participate in this type of bond. In these cases, the simple formula in Equation 1.1 typically overestimates the M-M bond orders. 

M M = m m Half-arrow description for a bridging hydride ligand. 

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