The salts [NEt4][Ru(CN)(CO)2L(o-O2C6Cl4)] {L = PPh3 or P(OPh)3}, which undergo one-electron oxidation at the catecholate ligand to give neutral semiquinone complexes [Ru(CN)(CO)2L(o-O2C6Cl4)], react with the dimers [{Ru(CO)2L(µ-o-O2C6Cl4)}2] {L = PPh3 or P(OPh)3} to give [NEt4][(o-O2C6Cl4)L(OC)2Ru(µ-CN)Ru(CO)2L′(o-O2C6Cl4)] {L or L′ = PPh3 or P(OPh)3}. The cyanide-bridged binuclear anions are, in turn, reversibly oxidised to isolable neutral and cationic complexes [(o-O2C6Cl4)L(OC)2Ru(µ-CN)Ru(CO)2L′(o-O2C6Cl4)] and [(o-O2C6Cl4)L(OC)2Ru(µ-CN)Ru(CO)2L′(o-O2C6Cl4)]+ which contain one and two semiquinone ligands respectively. Structural studies on the redox pair [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)]− and [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)] confirm that the C-bound Ru(CO)2(o-O2C6Cl4) fragment is oxidised first. Uniquely, [(o-O2C6Cl4){(PhO)3P}(OC)2Ru(µ-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)]− is oxidised first at the N-bound fragment, indicating that it is possible to control the site of electron transfer by tuning the co-ligands. Crystallisation of [(o-O2C6Cl4)(Ph3P)(OC)2Ru(µ-CN)Ru(CO)2{P(OPh)3}(o-O2C6Cl4)] resulted in the formation of an isomer in which the P(OPh)3 ligand is cis to the cyanide bridge, contrasting with the trans arrangement of the X–Ru–L fragment in all other complexes of the type RuX(CO)2L(o-O2C6Cl4).
![Graphical abstract: Homobinuclear cyanide-bridged linkage isomers containing the redox-active unit [(µ-XY)Ru(CO)2L(o-O2C6Cl4)] (XY = CN or NC)](http://hg.y866.cn/compound/lib/scimg/usr/1/B700648A.jpg)
