Molecular and Electronic Structures of Homoleptic Six-Coordinate Cobalt(I) Complexes of 2,2′:6′,2"-Terpyridine, 2,2′-Bipyridine, and 1,10-Phenanthroline. An Experimental and Computational Study

The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, 1-9, containing either 2,2':6',2 ''-terpyridine (tpy), 4,4'-di-tert-butyl-2,2'-bipyridine ((t)bpy), or 1,10-phenanthroline (phen) ligands have been determined in three oxidation levels, namely, cobalt(III), cobalt(II), and, for the first time, the corresponding presumed cobalt(I) species. The intraligand bond distances in the complexes [Co-I(tpy(0))(2)](+), [Co-I((t)bpy(0))(3)](+), and [Co-I(phen(0))(3)](+) are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy(0), bpy(0), and phen(0). On this basis, a cobalt(I) oxidation state assignment can be inferred for the monocationic complexes. The trications are clearly low-spin Co-III (S = 0) species, and the dicationic species [Co-II(tpy(0))(2)](2+), [Co-II((t)bpy(0))(3)](2+), and [Co-II(phen(0))(3)](2+) contain high-spin (S = 3/2) Co-II. Notably, the cobalt(I) complexes do not display any structural indication of significant metal-to-ligand (t(2g) -> pi*) pi-back-donation effects. Consistent with this proposal, the temperature-dependent molar magnetic susceptibilities of the three cobalt(I) species have been recorded (3-300 K) and a common S = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [Ni-II(tpy(0))(2)](2+), [Ni-II(bpy(0))(3)](2+), and [Ni-II(phen(0))(3)](2+), which display d -> d bands with very small molar extinction coefficients (epsilon < 60 M-1 cm(-1)), the spectra of the cobalt(I) species exhibit intense bands (epsilon > 10(3) M-1 cm(-1)) in the visible and near-IR regions. Density functional theory (DFT) calculations using the B3LYP functional have validated the experimentally derived electronic structure assignments of the monocations as cobalt(I) complexes with minimal cobalt-to-ligand pi-back-bonding. Similar calculations for the six-coordinate neutral complexes [Co-II(tpy(center dot))(2)] and [Co-II(bpy(center dot))(2)(bpy(0))](0) point to a common S = 3/2 ground state, each possessing a central high-spin Co-II ion and two pi-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [Co-I(tpy(0))(2)][CoI-(CO)(4)] have been explored by variable-temperature variable-magnetic-field magnetic circular dichroism (MCD) spectroscopy. A series of strong signals associated with the paramagnetic monocation exhibit pronounced C-term behavior indicative of the presence of metal-to-ligand charge-transfer bands [in contrast to d-d transitions of the nickel(II) analogue]. Time-dependent DFT calculations have allowed assignment of these transitions as Co(3d) -> pi*(tpy) excitations. Metal-to-ligand charge-transfer states intermixing with the Co(d(8)) multiplets explain the remarkably large (and negative) zero-field-splitting parameter D obtained from SQUID and MCD measurements. Ground-state electron- and spin-density distributions of [Co-I(tpy(0))(2)](+) have been investigated by multireference electronic structure methods: complete active-space self-consistent field (CASSCF) and N-electron perturbation theory to second order (NEVPT2). Both correlated CASSCF/NEVPT2 and spin-unrestricted B3LYP-based DFT calculations show a significant delocalization of the spin density from the Co-I d(xz,yz) orbitals toward the empty pi* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating alpha,beta-spin polarization type (McConnel mechanism I) and is definitely not due to magnetic metal-to-radical coupling. A comparison of these results with those for [Ni-II(tpy(0))(2)](2+) (S = 1) is presented.