Ambidentate coordination in hydrogen bonded dimethyl sulfoxide, (CH3)2SO⋯H3O+, and in dichlorobis(dimethyl sulfoxide) palladium(ii) and platinum(ii) solid solvates, by vibrational and sulfur K-edge X-ray absorption spectroscopy

Abstract

The strongly hydrogen bonded species (CH₃)₂SO⋯H₃O⁺ formed in concentrated hydrochloric acid displays a new low energy feature in its sulfur K-edge X-ray absorption near edge structure (XANES) spectrum. Density Functional Theory-Transition Potential (DFT-TP) calculations reveal that the strong hydrogen bonding decreases the energy of the transition S(1s) → LUMO, which has antibonding σ*(S–O) character, with about 0.8 eV. Normal coordinate force field analyses of the vibrational spectra show that the SO stretching force constant decreases from 4.72 N cm⁻¹ in neat liquid dimethyl sulfoxide to 3.73 N cm⁻¹ for the hydrogen bonded (CH₃)₂SO⋯H₃O⁺ species. The effects of sulfur coordination on the ambidentate dimethyl sulfoxide molecule were investigated for the trans-Pd((CH₃)₂SO)₂Cl₂, trans-Pd((CD₃)₂SO)₂Cl₂ and cis-Pt((CH₃)₂SO)₂Cl₂ complexes with square planar coordination of the chlorine and sulfur atoms. The XANES spectra again showed shifts toward low energy for the transition S(1 s) → LUMO, now with antibonding σ*(M–Cl, M–S) character, with a larger shift for M = Pt than Pd. DFT-TP calculations indicated that the differences between the XANES spectra of the geometrical cis and trans isomers of the M((CH₃)₂SO)₂Cl₂ complexes are expected to be too small to allow experimental distinction. The vibrational spectra of the palladium(II) and platinum(II) complexes were recorded and complete assignments of the fundamentals were achieved. Even though the M–S bond distances are quite similar the high covalency especially of the Pt–S bonds induces significant increases in the S–O stretching force constants, 6.79 and 7.18 N cm⁻¹, respectively.