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

Abstract

A sulfur atom, with its six valence electrons, may reach an octet by association into S n rings. Two-electron reduction of the rings usually leads to polysulfide chains S n 2À . Small, neutral S n rings are rare; witness S 4 , for which there is scant evidence [1] and continuing theoretical uncertainty about its structure.[2] As is the case for S 3 , the S 4 rings instability is presumably due to lone-pair repulsion. On the other hand, coordinated S 4 2À most certainly exists, and as we will see, not only as a simple chain (of which Na 2 S 4 is an example [3] ). This paper moves toward an understanding of the variety of complexed S 4 2À structural types and in the process shows that some compounds which hitherto have been considered as disulfide complexes may be profitably seen as containing D 2h -distorted S 4 2À rings. Their formation is attributable to a coupling redox process induced by either an external oxidant or inner oxidation of coordinated metal centers. The analysis leads us to the problem of tuning the S 2 2À /2 S 2À interconversion as well. The electronic structure of some compounds in the literature is reinterpreted, and, we believe, there emerges the outline of a consistent way of thinking about redox coupling of sulfides and disulfides. . [4] In these chair-like complexes, the S 4 rectangle has rather different S···S sides (e.g., 2.049(3) vs. 2.894(3) in 1), the shorter ones being either along (in 1) or across (in 2) the MÀM bonds. Compound 1 is obtained upon 2-e À oxidation of the binuclear precursor [Cp*Ir(m-CH 2 ) 2 (m-S 2 )IrCp*] (3), [4] in which the d 7 metal centers reach saturation with a formal Ir À Ir bond (whose actual length is 2.642(1) ). 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

A Bonding Quandary—or—A Demonstration of the Fact That Scientists Are Not Born With Logic

“…Then, the related part of the manuscript produced by Roald, Carlo, and co-workers is faithfully reported as it eventually appeared in Angewandte Chemie. [5] Santiago: The molecular ion A has interesting similarities with the Cu z catalytic site of nitrous oxide reductase, a tetracopper cluster with a tetrahedral m 4 -sulfido bridge. [6] The Cu 3 S portion of Cu z resembles half of A, with almost identical Cu À S and Cu À N bond lengths.…”

“…Further support for S 2 coupling comes from the computational result that for simpler ligands on each Cu center (two ammonia molecules), the S À S distance falls to 2.2 . [5] Terms for Making It a Fair Fight Roald, Santiago, Carlo: Dozens of e-mails wended their way between Firenze, Barcelona, Ithaca, and the many places the protagonists found themselves traveling to, both before and after the paper was published. We do not give the actual historical account of the debate, which was fraught, as any human action is, with many digressions.…”

“…That avoided crossing is also an indication that a multiconfigurational computation (which we have not yet attempted) might yield a description that melds both metal configurational assignments. [5] [2] The sulfur-sulfur distance was to me too long to be considered a bonding distance. I went to the Cambridge Structural Database (CSD) [9] for an overview and found that the distance between two sulfur atoms supported by two or three transition metals presented the bimodal distribution shown in Figure 5.…”

A Definitive Answer to a Bonding Quandary? The Role of One‐Electron Resonance Structures in the Bonding of a {Cu3S2}3+ Core

“…Of primary concern to the Alvarez-Hoffmann-Mealli (AHM) argument is the question of whether or not a transmolecular S À S bond exists in 1, a question that has not yet received a definitive answer. [1,3,4] This stirred my enthusiasm, because the gray area between what is to be considered a chemical bond and what is not is one of great interest to me.…”

“…For 1). [3] The merits and shortcomings of each of these two limiting electronic structures, A and C, are amply detailed in the AHM article [1] and will not be recapitulated here. Instead, it should be noted that Alvarez, Hoffmann, and Mealli all presume full delocalization among the three Cu centers and that this delocalization is mediated by the covalency of the CuÀS bonds.…”