X-ray Absorption Spectroscopic and Computational Investigation of a Possible S···S Interaction in the [Cu₃S₂]³⁺ Core

The electronic structure of the [Cu3S2]3+ core of [(LCu)3(S)2]3+ (L = N,N,N′,N′-tetramethyl-2R,3R-cyclohexanediamine) is investigated using a combination of Cu and S K-edge X-ray absorption spectroscopy and calculations at the density functional and multireference second-order perturbation levels of theory. The results show that the [Cu3S2]3+ core is best described as having all copper centers close to but more oxidized than Cu2+, while the charge on the S2 fragment is between that of a sulfide (S2–) and a subsulfide (S23–) species. The [Cu3S2]3+ core thus is different from a previously described, analogous [Cu3O2]3+ core, which has a localized [(Cu3+Cu2+Cu2+)(O2–)2]3+ electronic structure. The difference in electronic structure between the two analogues is attributed to increased covalent overlap between the Cu 3d and S 3p orbitals and the increased radial distribution function of the S 3p orbital (relative to O 2p). These features result in donation of electron density from the S–S σ* to the Cu and result in some bonding interaction between the two S atoms at ∼2.69 Å in [Cu3S2]3+, stabilizing a delocalized S = 1 ground state.