Photoinduced ligand dissociation of pyridine occurs much more readily in [Ru(tpy)(Me(2)bpy)(py)]2 than in [Ru(tpy)(bpy)(py)](2+) (tpy = 2,2':6',2 ''-terpyridine; bpy = 2,2'-bipyridine, Me(2)bpy = 6,6'-dimethyl-2,2'-bipyridine; py = pyridine). The S-0 ground state and the (MLCT)-M-3 and (MC)-M-3 excited states of these complexes have been studied using BP86 density functional theory with the SDD basis set and effective core potential on Ru and the 6-31G(d) basis set for the rest of the atoms. In both complexes, excitation by visible light and intersystem crossing leads to a (MLCT)-M-3 state in which an electron from a Ru d orbital has been promoted to a pi* orbital of terpyridine, followed by pyridine release after internal conversion to a dissociative (MC)-M-3 state. Interaction between the methyl groups and the other ligands causes significantly more strain in [Ru(tpy)(Me(2)bpy)(py)](2+) than in [Ru(tpy)(bpy)(py)](2+), in both the S-0 and (MLCT)-M-3 states. Transition to the dissociative (MC)-M-3 states releases this strain, resulting in lower barriers for ligand dissociation from [Ru(tpy)(Me(2)bpy)(py)](2+) than from [Ru(tpy)(bpy)(py)](2+). Analysis of the molecular orbitals along relaxed scans for stretching the Ru-N bonds reveals that ligand photodissociation is promoted by orbital mixing between the ligand pi* orbital of tpy in the (MLCT)-M-3 state and the d sigma* orbitals that characterize the dissociative (MC)-M-3 states. Good overlap and strong mixing occur when the Ru-N bond of the leaving ligand is perpendicular to the pi* orbital of terpyridine, favoring the release of pyridine positioned in a cis fashion to the terpyridine ligand.