Comparison of the optimized D-2h structures for the free S-4, S-4(-) and S-4(2-) With the experimentally observed structure id the [{Rh-2(eta(5)-C(5)Me(5))(2)(mu-CH2)(2)}(2)(mu-S-4)](2+) complex shows that the S-4(-) unit in the complex is close to the free S-4(-) anion-radical. Calculations for [{Rh-2(eta(5)-C5H5)(2)(mu-CH2)(2)}(2)(mu-S-4)](2+) followed by population analysis give the total charge on the S-4 unit as about -0.7 e. The bonding character of S-4 With metal atoms in the complex has been described as sigma-back-donation from Rh atoms to the S-4 unit accompanied by pi-donation onto Rh atoms. For the free S-4(-) anion-radical the trans C-2h isomer is the most favorable structure, and the rectangular D-2h isomer is only 1:3 kcal/mol higher. For the free S-4(2-) dianion, the nonplanar gauche C-2 structure is calculated to be the most stable form while the D-2h structure lies 16.3 kcal/mol higher.