A new photochemical synthetic route to species of the type [Ru(terpy)(pp)(CO)]²⁺ has been elaborated [terpy = 2,2′:6′,2″-tetrapyridine; pp = a bidentate α,α′-diimine ligand such as 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 4,4′-dimethyl-2,2′-bipyridine (dmbpy) or 4-(2-methylpropyl)-2,2′-bipyridine (mpbpy)]. For the species [Ru(terpy)(mpbpy)(CO)]²⁺ containing the new unsymmetrically substituted ligand (mpbpy) the two possible geometric isomers were separated by cation-exchange chromatography. The carbonyl groups can readily be removed and the vacant site substituted with pyridine, triphenylphosphine and chloride, with retention of the stereochemistry at the metal centre. By the correct choice of monodentate ligand at the sixth co-ordination site, the MLCT absorption maximum in the electronic spectra can be shifted by up to 150 nm, and the Ru^III-Ru^II redox couple by over 1 V.