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). Inorganic Chemistry, (29)23:4696--4699, 1990. [PUMA: HOMO acceptor d5 d6 dimer donor ligand orbital;LUMO osmium rule ruthenium stability;stability]