"Half-bonds" in an unusual coordinated S(4) (2-) rectangle.

Don't be square! A rare S(4) (2-) rectangle bridging two M(2)Cp(2)(mu(2)-CH(2))(2) (M=Rh, Ir) fragments is found to contain two "half-bonds" with S-S distances of 2.70 or 2.90 A. Computational studies explore the connection between these "half-bonds" and a Jahn-Teller distortion, as well as possible intermediates that form M(4)S(4) (2+) clusters having the S(4) (2-) rectangle rotated by 90 degrees. The bonding of a rare S(4) (2-) rectangle coordinated to four transition metals (synthesized by Isobe, Nishioka, and co-workers), [{M(2)(eta(5)-C(5)Me(5))(2)(mu-CH(2))(2)}(2)(mu-S(4))](2+) (M=Rh, Ir) is analyzed. DFT calculations indicate that, while experiment gives the rectangle coordinated with its long edge parallel to Rh-Rh bonds and perpendicular to the Ir-Ir bonds, either orientation is feasible for both metals. Although rotation of the S(4) rectangle is likely a multi-step process, a calculated barrier of 46 kcal mol(-1) for a simple interconversion pathway goes through a trapezoidal, not a square, transition state. An argument is presented, based on molecular orbital (MO) calculations, that the long S-S contacts (2.70 and 2.90 A) in the rectangle are in fact two-center, three-electron bonds (or "half-bonds"). Moreover, the 2- charge on the S(4) rectangle is related to a Jahn-Teller distortion from a square to a rectangle. Finally, DFT is used to explore possible stable intermediates in the oxidative process giving these M(4)S(4) (2+) compounds: for Ir, the coupling of two Ir(2)S(2) (+) molecules appears feasible, as opposed to a possible two-electron oxidation of a neutral Rh(4)S(4) molecule.