GEOMETRY, ELECTRONIC-STRUCTURE, AND COORDINATION ABILITY OF [CU(DIN)(PH3)2]+ (DIN=HN=CHCH=NH) AT THE LOWEST ENERGY TRIPLET METAL-TO-LIGAND CHARGE-TRANSFER EXCITED-STATE - A THEORETICAL-STUDY

The ground state and the lowest triplet metal-to-ligand charge-transfer (MLCT) excited state of [Cu(diN)(PH3)2]+ (diN = HN=CHCH=NH) are investigated with the ab initio MO method, where electron correlation is incorporated by the second-order Moller-Plesset perturbation theory. This complex is calculated to be pseudotetrahedral (pTd) at the ground state but planar (Pl) at the lowest triplet MLCT (3A2) excited state. In the MLCT (3A2) state, one alpha-spin electron is localized on the Cu d orbital and the other a-spin electron is on the diN pi*1 orbital. The electron distribution around the Cu atom in this MLCT excited state resembles very much that in a similar Cu(II) complex, [Cu(diN)(PH3)2]2+. H2O coordination to [Cu(diN)(PH3)2]+ is difficult at the ground state but easily occurs at the 3A2 MLCT excited state. The H2O binding energy at the MLCT excited state is calculated to be 19 kcal/mol. This result produces a theoretical support to a quenching mechanism of McMillin et al. proposed for the triplet MLCT excited state of such Cu(I) complexes as [Cu(NN)2]+ and [Cu(NN)(PR3)2]+ (NN = 1,10-phenanthroline and its derivatives; PR3 = tertiary phosphine).