Spectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes: intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres

A series of binuclear ruthenium and osmium complexes [(bipy)(2)Ru(qpy)Ru(bipy)(2)](4+) (1), [(bipy)(2)-Os(qpy)Os(bipy)(2)](4+) (2), [(bipy)(2)Ru(pytr-bipy)Ru(bipy)(2)](3+) (3), [(bipy)(2)Ru(pytr-bipy)Os(bipy)(2)](3+) (4), [(bipy)(2)Os(pytr-bipy)Ru(bipy)(2)](3+) (5) and [(bipy)(2)Os(bpbt)Os(bipy)(2)](2+) (6) {bipy = 2,2'-bipyridyl; qpy = 2,2': 5', 5 '':2 '', 2'''-quaterpyridyl; pytr-bipy = 3-(2,2'-bipyrid-6-yl)-5-(pyrid-2-yl)-1,2,4-triazolato, and bpbt = 5,5'-bis-(pyrid-2 ''- yl)-3,3'-bis-1,2,4-triazolato} are reported. Analysis of the electrochemical data focuses on structural factors and on determining the extent of electronic communication between the metal centres in the mixed valence oxidation state. Intervalence charge transfer (IT) bands could be identified in the spectra of the complexes 4 and 6 only. Analysis of their spectroelectrochemical data leads to the conclusion that the IT is superexchange mediated through the HOMO of the bridging ligand.