What can tell topological approaches on the bonding in transition metal compounds

摘要

The bonding in molecules containing transition metal atoms is generally described and explained by model based on orbitals. Alternatively, topological approaches such as the Atoms in Molecules (AIM) theory or the Electron Localization function (ELF) analysis offer an orbital independent framework; enabling the partition of the molecular position space into basins of attractors bearing a chemical signification. It is then possible to recover familiar chemical objects such as atoms, bonds and lone pairs. We present an overview of the possibility offered by the ELF analysis to investigate the bonding in the transition metal compounds ranging from isolated atoms to solids. The M-shell population of the ground state atoms is always less than the expectation (Z-12 or Z-11) whereas that of the TV-shell is greater than 2 or 1 in the case of Cr and Cu. The bonding in the TM di and tri-halides has been investigated and it shown that the populations in the MF_3 series (M = Sc,..., Zn) can be rationalized by invoking the contribution of resonance structures in which the fluorine atom forms a dative bond with the metal and therefore does not fulfil the octet rule. The bonding in carbonyl complexes is mostly characterized by the population and the spin density population of the V(C, M) basin which accepts most of the net density transfers from the metal whereas the sum of the populations of the remaining carbonyl moiety remains almost equal to that calculated in free carbon monoxide. In bimetallic complexes, such as M_2(HNCHNH)_4 (M = Nb,..., Pd) our analysis shows that there is a huge delocalization between the two metallic subunit which is revealed by the value of the variance of the corresponding populations. This delocalization appears to be a direct consequence of the symmetry and of diamagnetism of these molecules. In all these examples, the localization of the spin density is a clue to interpret the bonding. Examples of multicenter bonds favoured by the size of the transition metallic cores are presented which shed light onto the nature of agostic hydrogen interaction as well as on planar tetracoordinated carbons and of bulk metals. Results concerning chemisorption on a catalyst surface and the bonding in bulk metal oxides are discussed.