Spectroscopic and computational studies of a Ni(+)-CO model complex: implications for the acetyl-CoA synthase catalytic mechanism.

The four-coordinate Ni(+) complex [PhTt(t)(Bu)]Ni(I)CO, where PhTt(t)()(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-CI calculations to investigate the ground and excited states of [PhTt(t)()(Bu)]Ni(I)CO. 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/A (as compared to k(CO)(free CO) = 18.7 mdyn/A), and strong Ni(+) --> CO charge-transfer absorption intensities. Calculations reveal that this high degree of pi-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 CH(3)-CoFeSP.