The irradiation of ruthenium-sulfur dioxide complexes of general formula trans-[Ru-II(NH3)(4)(SO2)X]Y with laser light at low temperature results in linkage isomerization Of SO2, starting with eta(1)-planar S-bound to eta(2)-Side S,O-bound SO2. The solid-state photoreaction proceeds with retention of sample crystallinity. Following work on trans-[Ru-(NH3)(4)Cl(eta(1)-SO2)]Cl and trans-[Ru(NH3)(4)(H2O)(eta(1)-SO2)](C6H5SO3)(2) (Kovalevsky, A. Y.; Bagley, K. A.; Coppens, P. J. Am. Chem. Soc. 2002, 124, 9241-9248), we describe photocrystallographic, IR, DSC, and theoretical studies of trans-[Ru-II(NH3)(4)(SO2)X]Y complexes with (X = Cl-, H2O, or CF3COO- (TFA(-))) and a number of different counterions (Y = Cl-, C6H5SO3-, Tos(-), or TFA(-)). Low temperature IR experiments indicate the frequency of the asymmetric and symmetric stretching vibrations of the Ru-coordinated SO2 to be downshifted by about 100 and 165 cm(-1), respectively. Variation of the trans-to-SO2 ligand and the counterion increases the MS2 decay temperature from 230 K (trans-[Ru-II(NH3)(4)(SO2)Cl]Cl) to 276 K (trans-[Ru-II(NH3)(4)(SO2)(H2O)](Tos)(2)). The stability of the MS2 state correlates with increasing sigma-donating ability of the trans ligand and the size of the counterion. Quantum chemical DFT calculations indicate the existence of a third eta(1)-O-bound (01) isomer, the two metastable states being 0.1-0.6 eV above the energy of the ground-state complex.