Ligand stereochemistry of metal carbonyl clusters: Steric vs electronic effects

The homogeneous spreading of valence electrons and free charges in metal carbonyl clusters is not ensured by the best steric disposition of the ligancls; this is the major pitfall of all the theories of ligand stereochemistry based on purely steric arguments. In order to compute local formal charges, which are essentially a mathematical device for taking into account the distribution of local valence electrons and free charges, the electrons donated by a (η1-C) carbonyl ligand have been partitioned between adjacent metal atoms, according to a "bond valence" approach. Cotton's charge equalization principle has been formulated quantitatively and used in molecular mechanics computations of metal carbonyl clusters which have been modeled as if they were mainly under the influence of (i) valence forces, constraining the CO to float on the equipotential surface maintaining the CO vector approximately perpendicular to the surface, (ii) van der Waals interactions, conveying both the (few) strong repulsions responsible for nonpenetrability of bodies and the (many) weakly attractive interactions, and (iii) local charge interactions, addressing the fulfilments of the local electron bookkeeping but also favoring the conformations associated with the better spread of the total charge on the cluster. The performance of the method has been discussed by comparing the experimental and computed stereochemistries of a series of metal carbonyl species ([FeCo(CO)8]-, Fe2(CO)9, Fe2(CO)7(bipy), CrOs(CO)10, Fe2(CO)4Cp2, MnRh(CO)4Cp2, CrNi(CO)4Cp2, V2(CO)5(Cp)2, [Fe3(CO)11]2-) deliberately chosen to assess local charge effects, which are relevant (and visible) whenever the best steric conformation of carbonyl ligands does not correspond to a reasonable distribution of local valence electrons and/or charges. © 1995 American Chemical Society.