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}(2)S-2](CIO4)(2) (1), [{Cu[HB(3,5-Pr(2)(i)pz)(3)]}(2)(S-2)] (2), and [{(TMEDA)Cu}(2)(S-2)(2)](OTf)(2) (3) to obtain a quantitative description of their ground state wavefunctions. The. Cu L-edgeintensities 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 (mu-n(2):n(2) S-2(2-) bridge) compared to 1 ((mu-n(1):n(1) S-2(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/(S-2(-))(2) nature of 3. The two disulfide(center dot 1-)'s in 3 undergo strong bonding interactions that destabilize the resultant filled antibonding pi* orbitals of the (S-2(-))(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(II)bis-disulfide(center dot 1-)complex.