Redox characteristics and anion association behaviour of stereoisomeric forms of mono- and oligonuclear metal complexes using high pressure electrochemistry

The effect of the presence of the anions PF6-, BF4-, ClO4-, and tosylate(-) on the potentials of the Ru-III/II redox processes in a series of mono-, d i-, and trinuclear complexes involving polypyridyl ligands are reported. Th e anions gives rise to a cathodic shift in the respective redox potentials in the sequence PF6- < BF4- approximate to ClO4- < tos(-), with the magnitu de of the shifts being largest for the tri-nuclear and smallest for the mon onuclear species. In one dinuclear complex, a variation of anion was also o bserved to affect the difference in potentials between the [6+/5+] and [5+/ 4+] couples. These effects are consistent with known specific association o f the anions with metal complexes of this type. In the presence of PF, high -pressure electrochemical studies revealed the molar volume changes for suc cessive redox couples in di- and trinuclear complexes were almost invariant (<Delta>V-complex = 24 +/- 2 cm(3) mol(-1)) and consistent with the value for mononuclear species. This constancy is interpreted in terms of isolated Ru-III/II processes in all complexes and implies that the electrostrictive effects on solvent are limited to a very short range.