Chemical bonding in transition metal carbonyl clusters: complementary analysis of theoretical and experimental electron densities.

In the last few years. the quantum theory of atoms in molecules has become the paradigm for interpreting theoretical and experimental electron density distributions. Within this framework, the link between bonding modes and topological properties has been fully achieved for 'light atom' molecules. However, the derived correspondence rules cannot be extended straightforwardly to organometallic compounds since bonds to a transition metal display a different and much narrower spectrum of topological indexes. The complementary usage of theoretical computations on a set of prototype transition metal molecules and experimental determinations of the electron density in transition metal carbonyl clusters are discussed. Since these compounds are characterised by weakly bound metal cages and fluxional carbonyl ligands, the focus is on the nature of metal-metal and metal-carbonyl interactions as well as on the evolution of three-centre-four-electron M(mu-CO)M bonds along the (CO)M-M <----> M(mu-CO)M <----> M-M(CO) conversion path. The interpretation of the electron density distribution here proposed could be extended reasonably to a wider class of organometallic compounds. (C) 2002 Elsevier Science B.V. All rights reserved.