Widely differing photochemical behavior in related octahedral {Ru-NO}6 compounds: intramolecular redox isomerism of the excited state controlling the photodelivery of NO.

trans-[(NC)Ru(py)(4)(mu-CN)Ru(py)(4)(NO)](3+) (py = pyridine) is a stable species in aqueous solution. It displays an intense absorption in the visible region of the spectrum (lambda(max) = 518 nm; epsilon(max) = 6100 M(-1) cm(-1)), which turns this compound into a promising agent for the photodelivery of NO. The quantum yield for the photodelivery process resulting from irradiation with 455 nm visible light was found experimentally to be (0.06 +/- 0.01) x 10(-3) mol einstein(-1), almost 3 orders of magnitude smaller than that in the closely related cis-[RuL(NH(3))(4)(mu-pz)Ru(bpy)(2)(NO)](5+) species (L = NH(3) or pyridine, pz = pyrazine, bpy = 2,2'-bipyridine; phi(NO) = 0.02-0.04 mol einstein(-1) depending on L) and also much smaller than the one in the mononuclear compound trans-[ClRu(py)(4)(NO)](2+) (phi(NO) = (1.63 +/- 0.04) x 10(-3) mol einstein(-1)). DFT computations provide an electronic structure picture of the photoactive excited states that helps to understand this apparently abnormal behavior.