Disproportionation of sulfur dioxide to octasulfur and sulfur trioxide on Cp*2Cr2S5. Crystallographic and sulfur-34-labeling evidence for an oxygen-transfer mechanism and observation of facile exchange of Cr2S5 core sulfurs with 34S. Structures of Cp*2Cr2(.mu.-S)(.mu.-.eta.2-S2)(.mu.-.eta.1-SS.cntdot.SO2).cntdot.SO2 and Cp*2Cr2(.mu.-S)(.mu.-.eta.2-S2)(.mu.-SSO3)