X-Ray absorption spectroscopic and theoretical studies on (L) ₂[Cu₂(S₂)_n]²⁺ complexes: Disulfide versus disulfide(•1-) bonding

Cu K-, Cu L-, and S K-edge X-ray absorption spectroscopic (XAS) data have been combined with density functional theory (DFT) calculations on [{(TMPA)Cu}₂S₂](CIO₄)₂ (1), [{Cu[HB(3,5-Prⁱ₂pz)₃]}₂-(S ₂)] (2), and [{(TMEDA)Cu}₂(S₂) ₂](OTf)₂ (3) to obtain a quantitative description of their ground state wavefunctions. The Cu L-edge intensities give 63 and 37% Cu d-character in the ground state of 1 and 2, respectively, whereas the S K-pre-edge intensities reflect 20 and 48% S character in their ground states, respetively. These data indicate a more than 2-fold increase in the total disulfide bonding character in 2 relative to 1. The increase in the number of Cu-S bonds in 2 (μ-η²:η²η² S₂^2- bridge) compared to 1 ((μ-η¹: η¹ S₂^2- bridge) dominantly determines the large increase in covalency and Cu-disulfide bond strength in 2. Cu K- and L- and S K-pre-edge energy positions directly demonstrate the Cu ^II/(S₂^-)₂ nature of 3. The two disulfide(•1-)'s in 3 undergo strong bonding interactions that destabilize the resultant filled antibonding π* orbitale of the (S₂ ^-)₂ fragment relative to the Cu 3d levels. This leads to an inverted bonding scheme in 3 with dominantly ligand-based holes in its ground state, consistent with its description as a dicopper(ll)-bis-disulfide(•1- ) complex.