Parallel disulfido bridges in bi- and poly-nuclear transition metal compounds: Bonding flexibility induced by redox chemistry

We recently studied an elongated S 4 2 - rectangle bridging transition metals such as Ir, Rh, Cu, or Fe and found that its formation is related to the oxidation of parallel S 2 2 - molecules. The removal of two electrons can occur either externally or by an internal metal–ligand redox process that implies depopulation of the high-lying S4 σ∗ MO and population of a lower metal level. Thus, the interaction may be described as metal back-donation. However, sufficiently electropositive metals can more easily reach higher oxidation states and this reverses the direction of the two-electron interaction. In such a case, the system may be viewed as formed by two uncoupled disulfides donating an additional electron pair to the metals. In this paper, we analyze the aspects of the 2 S 2 2 - / S 4 2 - dichotomy through DFT calculations and qualitative MO analysis of the known bimetallic MS4M cores having either triple-decker (TD) or chair-type structures. We correlate the extent of the disulfide coupling with the nature (electronegativity) of the terminal metal fragments (the metals range from Group IV to X) beside their electron configuration. We find that the formation of an S 4 2 - unit is favored by metals which cannot stabilize high oxidation states (e.g. Fe(IV) or Cu(III)), whereas the two S 2 2 - units remain substantially uncoupled with early transition metal in high oxidation states (e.g. Ti(IV)).