The four-coordinate Ni+ complex [PhTt(/Bu)](NiCO)-C-I, where PhTt(/Bu) = phenyltris((tert-buthylthio)methyl)borate (a tridentate thioether donor ligand), serves as a possible model for key Ni-CO reaction intermediates in the acetyl-CoA synthase (ACS) catalytic cycle. Resonance Raman, electronic absorption, magnetic circular dichroism (MCD), variable-temperature variable-field MCD, and electron paramagnetic resonance spectroscopies were utilized in conjunction with density functional theory and semiemperical INDO/S-Cl calculations to investigate the ground and excited states of [PhTt(/Bu)](NiCO)-C-I. These studies reveal extensive Ni+ - CO pi-back-bonding interactions, as evidenced by a low C - O stretching frequency (1995 cm(-1)), a calculated C - O stretching force constant of 15.5 mdyn/Angstrom (as compared to k(CO)(free CO) = 18.7 mdyn/Angstrom), and strong Ni+ - CO charge-transfer absorption intensities. Calculations reveal that this high degree of T-back-bonding is due to the fact that the Ni+ 3d orbitals are in close energetic proximity to the CO pi(*) acceptor orbitals. In the ACS "paramagnetic catalytic cycle", the high degree of pi-back-bonding in the putative Ni+-CO intermediate (the NiFeC species) is not expected to preclude methyl transfer from CH3-CoFeSP.