Quantum mechanical investigation of the inner sphere reduction mechanism of the [(NSSSN)Co(III)Cl²⁺], [(NSNSN)Co(III)Cl³⁺], and their nitrogen analog [(NNNNN)Co(III)Cl⁴⁺],   cations by iron(II)

Will Smith, Thomas Jackman, Olaseni Sode

University of Tampa

The inner-sphere pathway is an electron transfer mechanism that utilizes a bridging ligand to covalently link oxidant and reductant centers. The reduction of chloro-N-methyl-bis(5-amino-3-thiapentyl)amine cobalt(III) [(NSNSN)Co(III)Cl⁺] and chloro(1,11-diamino-3,6,9-trithiaundecane)cobalt(III) cation [(NSSSN)Co(III)Cl⁺] by Iron(II) via inner-sphere electron transfer have been shown experimentally to occur with similar rate constants and more than 10⁵ times faster than the nitrogen analog (NNNNN) It has been hypothesized that this is due to non-bridging ligand effects. To test this hypothesis, the role of ground state electronic effects by the sulphur-containing ligands on the electron transfer is investigated through the use of quantum chemistry methods. The non-bridging ligand effects were investigated through the geometry and vibrational frequencies of the precursor, transition state, and successor complexes using density functional theory (DFT) and multi-configurational self-consistent field (MCSF) methods to provide molecular orbital based evidence of the results. Rate constants for the electron transfer processes were determined utilizing Marcus theory and compared against experimental values.