Metal-ligand bonding in metallocenes: Differentiation between spin state, electrostatic and covalent bonding

We have analyzed metal-ligand bonding in metallocenes using density functional theory (DFT) at the OPBE/TZP level. This level of theory was recently shown to be the only DFT method able to correctly predict the spin ground state of iron complexes, and similar accuracy for spin ground states is found here. We considered metallocenes along the first-row transition metals (Sc-Zn) extended with alkaline-earth metals (Mg, Ca) and several second-row transition metals (Ru, Pd, Ag, Cd). Using an energy decomposition analysis, we have studied trends in metal-ligand bonding in these complexes. The OPBE/TZP enthalpy of heterolytic association for ferrocene (-658 kcal/mol) as obtained from the decomposition analysis is in excellent agreement with benchmark CCSD(T) and CASPT2 results. Covalent bonding is shown to vary largely for the different metallocenes and is found in the range from -155 to -635 kcal/mol. Much smaller variation is observed for Pauli repulsion (55-345 kcal/mol) or electrostatic interactions, which are however strong (-480 to -620 kcal/mol). The covalent bonding, and thus the metal-ligand bonding, is larger for low spin states than for higher spin states, due to better suitability of acceptor d-orbitals of the metal in the low spin state. Therefore, spin ground states of transition metal complexes can be seen as the result of a delicate interplay between metal-ligand bonding and Hund's rule of maximum multiplicity. (c) 2006 Elsevier B.V. All rights reserved.