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Abstract

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 (^tbpy), 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(tpy⁰)₂]⁺, [Co^I(^tbpy⁰)₃]⁺, and [Co^I(phen⁰)₃]⁺ are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy⁰, bpy⁰, and phen⁰. 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(tpy⁰)₂]²⁺, [Co^II(^tbpy⁰)₃]²⁺, and [Co^II(phen⁰)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 → π*) π-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(tpy⁰)₂]²⁺, [Ni^II(bpy⁰)₃]²⁺, and [Ni^II(phen⁰)₃]²⁺, which display d → d bands with very small molar extinction coefficients (ε < 60 M^–1 cm^–1), the spectra of the cobalt(I) species exhibit intense bands (ε > 103 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 π-back-bonding. Similar calculations for the six-coordinate neutral complexes [Co^II(tpy^•)₂]⁰ and [Co^II(bpy^•)₂(bpy⁰)]⁰ point to a common S = 3/2 ground state, each possessing a central high-spin Co^II ion and two π-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [Co^I(tpy⁰)₂][Co^I−(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) → π*(tpy) excitations. Metal-to-ligand charge-transfer states intermixing with the Co(d⁸) 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(tpy⁰)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 π* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating α,β-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(tpy⁰)₂]²⁺ (S = 1) is presented.