Preference for sulfoxide S- or O-bonding to 3d transition metals - DFT insights

Abstract Complexes of 3d transition metals with sulfoxide ligands bound by sulfur were searched on the Cambridge Structural Database. DFT calculations reproduced the experimental geometries and the lowest energy isomer (S or O bound) matched the experimentally observed one. Distances, Wiberg indices and charges (Mulliken, Atomic Polar Tensor and Natural Population Analysis) were compared, showing an elongation and weakening of the S-O bond with O-coordination. Metal charges became more positive with O-coordination, accompanied by a more negative sulfur charge. Examples of O-bound sulfoxides were studied for comparison. An Energy Decomposition Analysis was performed in a series of more relevant complexes, in order to compare the two isomers and identify the reason for the structural preference. In complexes with one S-bound sulfoxide, the attractive interaction terms (electrostatic and orbital) compensate the large Pauli repulsion by a larger amount in the S- than in the O-bound isomer. In the complex with O-bound sulfoxide this tendency was reversed by a very large reorganization energy (to bring fragments from their preferred geometry to the geometry they have in the complex). In the complexes with two ligands, S,S or O,O binding was preferred, and in one example they coexist. The mixed S,O coordination type led to a large orbital stabilization, but required large reorganization energies.