“Half‐Bonds” in an Unusual Coordinated S₄²⁻ Rectangle

Abstract

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.

S42− Rings, Disulfides, and Sulfides in Transition‐Metal Complexes: The Subtle Interplay of Oxidation and Structure

“…The local geometry of the binuclear metal components is virtually unchanged in 1. Our theoretical analysis [5] shows that the most dramatic effect of coupling (if there were no oxidation) is the destabilization of the s* combinations formed, in the S 4 plane, by the p k * and p k S 2 populated levels (s 2 * and s 1 * in Figure 2 a, b). These are already in antibonding relationships (therefore destabilized) with the populated d*-d* and d-d metal combinations: s 2…”

“…[6] The alternative arrangements of the S 4 2À unit in 1 and 2 ( Figure 1 a) have an intuitive justification. [5] In an isolated S 4 2À square, the two lowest s* levels are degenerate, that is, the p k -p k and p ? -p ?…”

Chemistry of Tetrasulfur Including Mysterious Rectangular S4 Metal Complexes

“…8 Their DFT calculation suggests that, while experiments give the rectangle coordinated with its long edge parallel to RhRh bonds and perpendicular to the IrIr bonds, either orientation is feasible for both metals. The most stable structure for both Rh and Ir is that with the long SS bond parallel to the MM axis; however, the differences are less than 4 kcal mol ¹1 in each case.…”

“…Here, it is worth quoting the suggestion made by Mealli, Hoffmann, et al: "we also begin to see how the nature of the main-group element and of the metal influence 2 S 2¹ /S 2 2¹ and 2 S 2 2¹ /S 4 2¹ systems which are probably relevant and regulate important biological processes based on the inner electron-transfer process." 8,9 Chemistry of both double-charged S 4 species, S 4 2+ and S 4 2¹ , has made steady progress based on experiments and theoretical calculations. As concerns the former species, the new concept of "lone pair aromaticity" arising from its D 4h square geometry deserves special mention.…”

“…6,7 Stimulated by our findings the Mealli and Hoffmann group has done theoretical studies of the two complexes and clarified that both S 4 units are stable forms and two disulfide units form an unusual S 4 2¹ rectangle through a two-electron oxidation of two disulfide S 2 2¹ . 8,9 Oxidation of elemental sulfur by certain powerful oxidants in solvents of low basicity produces the square-planar colorless cation S 4 2+ which is isolated in crystalline salts with various anions and characterized, inter alia, by X-ray crystallography and theoretical calculation. 10 On the other hand, reduction of elemental sulfur by alkali metals gives the pale yellow tetrasulfide anion S 4 2¹ which is also isolated in various compounds of main group and transition elements and structurally characterized by X-ray diffraction on single crystals.…”