Binuclear Cyclopentadienylmetal Methylene Sulfur Dioxide Complexes of Rhodium and Iridium Related to a Photochromic Metal Dithionite Complex

The photochromic dithionite complex Cp*2Rh2(μ-CH2)2(μ-O2SSO2) (Cp* = η5-Me5C5) is of interest because it undergoes an unusual fully reversible unimolecular photochemical rearrangement to the isodithionite complex Cp*2Rh2(μ-CH2)2(μ-O2SOSO). In order to obtain more insight into these systems, a comprehensive density functional theory study has been carried out on isomeric Cp2M2(CH2)2(SO2)2 (M = Rh, Ir) derivatives. The experimentally observed rhodium complexes with coupled sulfur dioxide (SO2) units to give dithionite or isodithionite ligands are surprisingly high-energy kinetic isomers in our analysis, reflecting the need for dithionite rather than SO2 for their synthesis. Many isomeric structures containing two separate SO2 ligands are found to lie at lower energies than these dithionite and isodithionite complexes. In the lowest-energy Cp2M2(CH2)2(SO2)2 isomers, the two methylene groups couple to form an ethylene ligand that can be either terminal or bridging. In slightly higher energy structures, a form...