The influences of R, the alpha -diimine, and the transition metal M on the excited-state properties of the complexes [M(SnR3)(2)(CO)(2)(alpha -diimine)] (M = Ru, Os; R = Ph, Me) have been investigated. Various synthetic routes were used to prepare the complexes, which all possess an intense sigma-bond-to-ligand charge-transfer transition in the visible region between a sigma (Sn-M-Sn) and a pi*(a-diimine) orbital. The resonance Raman spectra show that many bonds are only weakly affected by this transition. The room-temperature time-resolved absorption spectra of [M(SnR3)(2)(CO)(2)(dmb)] (M = Ru, Os: R = Me, Ph: dmb = 4,4'-dimethyl-2,2'-bipyridine) show the absorptions ? of the radical anion of dmb, in line with the SBLCT character of the lowest excited state. The excited-state lifetimes at room temperature vary between 0.5 and 3.6 mus and are mainly determined by the photolability of the complexes. All complexes are photostable in a glass at 80 K, under which conditions they emit with very longs lifetimes. The extremely long emission lifetimes (e.g., tau = 1.1 ms for [Ru(SnPh3)(2)(CO)(2)(dmb)]) are about a thousand times longer than those of the (MLCT)-M-3 states of the [Ru(Cl)(Me)(CO)(2)(alpha -diimine)] complexes; This is due to the weak distortion of the former complexes in their (SBLCT)-S-3 states as seen from the very small Stokes shifts. Remarkably, replacement of Ru by Os hardly influences the absorption and emission energies of these complexes; yet the emission lifetime is shortened because of an increase of spin-orbit coupling. The quantum yield of emission at 80 K is 1-5% for these complexes, which is lower than might be expected on the basis of their slow nonradiative decay.