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

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.

Classical and non-classical isomers of both neutral and dianionic BC(2)P(2)H(3) species, which are isolobal to Cp(+) and Cp(-), are studied at both B3LYP/6-311++G(d,p) and G3B3 levels of theory. The global minimum structure given by B3LYP/6-311++G(d,p) for BC(2)P(2)H(3) is based on a vinylcyclopropenyl-type structure, whereas BC(2)P(2)H(3)(2-) has a planar aromatic cyclopentadienyl-ion-like structure. However, at the G3B3 level, there are three low-energy isomers for BC(2)P(2)H(3): 1) tricyclopentane, 2) nido and 3) vinylcyclopropenyl-type structures, all within 1.7 kcal mol(-1) of each other. On the contrary, for the dianionic species the cyclic planar structure is still the minimum. In comparison to the isolobal Cp(+) and H(n)C(n)P(5-n)(+) isomers, BC(2)P(2)H(3) shows a competition between pi-delocalised vinylcyclopropenyl- and cluster-type structures (nido and tricyclopentane). Substitution of H on C by tBu, and H on B by Ph, in BC(2)P(2)H(3) increases the energy difference between the low-lying isomers, giving the lowest energy structure as a tricyclopentane type. Similar substitution in BC(2)P(2)H(3)(2-) merely favours different positional isomers of the cyclic planar geometry, as observed in 1) isoelectronic neutral heterodiphospholes EtBu(2)C(2)P(2) (E = S, Se, Te), 2) monoanionic heterophospholyl rings EtBu(2)C(2)P(2) (E = P(-), As(-), Sb(-)) and 3) polyphospholyl rings anions tBu(5-n)C(n)P(5-n) (n = 0-5). The principal factors that affect the stability of three-, four-, and five-membered ring and acyclic geometrical and positional isomers of neutral and dianionic BC(2)P(2)H(3) isomers appear to be: 1) relative bond strengths, 2) availability of electrons for the empty 2p boron orbital and 3) steric effects of the tBu groups in the HBC(2)P(2)tBu(2) systems.

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

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