“Half‐Bonds” in an Unusual Coordinated S 4 2− Rectangle

Abstract Don′t be square! A rare S 4 2− rectangle bridging two M 2 Cp 2 (μ 2 ‐CH 2 ) 2 (M=Rh, Ir) fragments is found to contain two “half‐bonds” with SS distances of 2.70 or 2.90 Å. 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°.The bonding of a rare S 4 2− rectangle coordinated to four transition metals (synthesized by Isobe, Nishioka, and co‐workers), [{M 2 (η 5 ‐C 5 Me 5 ) 2 (μ‐CH 2 ) 2 } 2 (μ‐S 4 )] 2+ (MRh, 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 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 SS contacts (2.70 and 2.90 Å) 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.