Stability rules for d5/d6 mixed-valent dimers. Effects from the donor/acceptor capability of the metal (ruthenium vs osmium) and from the occupancy of the mediating ligand orbital (LUMO vs HOMO)

Electrochemical stability constants were determined for the mixed-valent intermediates in the redox series [(bpy) 2 M(bptz)M-(bpy) 2 ] 4+/5+/6+ and [(bpy) 2 M(adc-Me 2− )M(bpy) 2 ] 2+/3+/4+ ; M=Ru, Os; bptz=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine, adc-Me 2− =1,2-diacetylhydrazido (2−). The trends observed allow us to rationalize in a consistent fashion the stability of the mixed-valence forms toward disproportionation: The equilibrium constant K c depends on the π-donor/π-acceptor charater of the metals and on the occupancy and electron population at the coordinating centers of the interacting ligand π orbital