Are Octahedral Ruthenium(II/III) and Osmium(II/III) Complexes Always Low-Spin?

Abstract

The influence of geometrical changes on the spin multiplicity of the ground states of the octahedral ruthenium(II/III) and osmium(II/III) complexes is investigated using the TD-DFT and MRCI methods. On the example of the [RuCl6]4- complex, we show that only after the optimisation of the molecular geometry in a solvent (using a polarised continuum model), which shortens the M-L bond lengths by ~0.2 Å, is the correct order of spin states obtained (i.e. a singlet is correctly predicted to be the ground state). On the contrary, in terms of the in vacuo optimised geometries of this negatively charged species, both the DFT and MRCI calculations predict a quintet ground state. This finding is further analysed by calculating the low- and high-spin potential energy curves corresponding to an elongation of the M-L distance, which makes it possible to predict the critical point at which the crossing of the two spin states occurs. Finally, it is complemented by the TD-DFT calculations of the lowest excited states in each spin multiplicity for a series of prototypical ligands. It is demonstrated that the calculated results correlate well with the known strengths of the ligand field. The two findings presented in this work are a small contribution to our understanding of the electronic structure and properties of the octahedral ruthenium(II/III) and osmium(II/III) complexes, which are relevant both in biomolecular and material sciences.