Valence tautomerism and macrocycle ruffling in nickel(III) porphyrins

Nonlocal density functional calculations with full geometry optimization have been carried out on the low-lying electronic states of oxidized nickel porphyrins. For [Ni-III(P)(Py)(2)](+), the ground state corresponds to a t(g)6(z(2))(1) configuration and the t(2g)(6)(x(2) - y(2))(1) configuration is 0.43 eV higher in energy. In contrast, the ground state of [Ni-III(P) (CN)(2)](-) corresponds to a t(2g)(6)(x(2) - y(2))(1) configuration,the t(2g)(6)6(z(2))(1) configuration being 0.96 eV higher in energy. The results are consistent with EPR spectroscopic results on the TPP analogs of these complexes. For [Ni(P) (Py)(2)](+), the a(2u)- and a(1u)-type Ni(II) porphyrin cation radical states are higher in energy by 0.63 and 1.23 eV, respectively, relative to the t(2g)6(z(2))(1) Ni(III) ground state. The Ni-N-porphyrin distance is significantly shorter in [Ni-III(P)(Py)(2)](+) (196 pm) than in [Ni-III(P)(CN)(2)](-) (206 pm), which is consistent with the ruffled and planar macrocycle conformations, respectively, in the two complexes. (C) 2000 Elsevier Science Inc. All rights reserved.