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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 η¹-planar S-bound to η²-side S,O-bound SO₂. The solid-state photoreaction proceeds with retention of sample crystallinity. Following work on trans-[Ru(NH₃)₄Cl(η¹-SO₂)]Cl and trans-[Ru(NH₃)₄(H₂O)(η¹-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 …