Electronic structure, molecular structure, and electronic spectra for unligated and ligated Group 4A metal porphines MP and MP(C1)2, M = Si, Ge, Sn, Pb are investigated in detail using a DFT/TDDFT method. All the divalent MP's, including SiP and GeP, are non-planar with the metal atom considerably out of the porphyrin plane. Each MP has a closed-shell (3a1)2 ground state, but the HOMO 3a1 is not a metal-npz orbital as proposed in the literature based on the EHMO results. For PbP the HOMO (3a1) is almost made up of the porphyrin 2a2u orbital, which accounts for the fact that PbP undergoes initial oxidation at the ring. For the second oxidation step of [PbP]+ → [PbP]2+, the electron is removed initially from the HOMO, and then an internal redistribution of electrons takes place, i.e. two electrons transfer from the metal to the ring: Pb11 → PbIV. According to the TDDFT results, the 3a1 → π* transition has the lowest excitation energy; the B1 and B2 bands assigned early in the experimental spectra of SnII and PbII porphyrins are actually the Q and B bands respectively. The early semiempirical EHMO calculations prove not to give a correct description for the electronic structure in the Group 4A metal MP's.
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