`Half-Bonds' in an Unusual Coordinated S-4(2-) Rectangle

The bonding of a rare S42 rectangle coordinated to four transition metals (synthesized by Isobe, Nishioka, and co-workers), [{M2(h5-C5Me5)2(m-CH2)2}2(m-S4)]2+ (M=Rh, Ir) is analyzed. DFT calculations indicate that, while experiment gives the rectangle coordinated with its long edge parallel to RhRh bonds and perpendicular to the IrIr bonds, either orientation is feasible for both metals. Although rotation of the S4 rectangle is likely a multi-step process, a calculated barrier of 46 kcalmol1 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 SS contacts (2.70 and 2.90 ) in the rectangle are in fact two center, three-electron bonds (or "halfbonds"). Moreover, the 2 charge on the S4 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 M4S4 2+ compounds: for Ir, the coupling of two Ir2S2 + molecules appears feasible, as opposed to a possible two-electron oxidation of a neutral Rh4S4 molecule.