Spin-state energetics and spin-crossover behavior of pseudotetrahedral cobalt(III)-imido complexes. The role of the tripodal supporting ligand.

DFT calculations have underscored the importance of the tripodal supporting ligand in tuning the spin-state energetics of pseudotetrahedral transition metal imido complexes. In particular, we have focused on Co(III)-imido complexes, where our best estimate (OLYP) of the singlet-triplet splitting varies from 0.75 eV for a trisphosphine complex (1) and 0.3 eV for a tris(N-heteroyclic-carbene) complex (2) to essentially 0.0 eV for a hydrotris(pyrazolyl)borate (3) complex. The experimentally studied analogues of 1, 2, and 3 all exhibit S = 0 ground states; however, the experimental analogue of 3 exhibits spin-crossover behavior due to a low-lying S = 1 state. Interestingly, whereas all the pure functionals examined successfully predict nearly equienergetic singlet end triplet states for 3, the hybrid functionals B3LYP and O3LYP exhibit a clear (and incorrect) preference for the S = 2 state. In addition, we have also carried out an exploratory survey of Cr(III), Mn(III), and Fe(III) imido complexes with trisphosphine and hydrotris(pyrazolyl)borate (Tp) supporting ligands. Among the more interesting predictions of this study is that an FeIII(Tp)(imido) species should exhibit a high-spin S = 5/2 ground state, which would be unique for an iron-imido complex.