π* Molecular Orbital Crossing a2(χ)/b1(ψ) in 1,10-Phenanthroline Derivatives.Ab InitioCalculations and EPR/ENDOR Studies of the 4,7-Diaza-1,10-phenanthroline Radical Anion and Its M(CO)4Complexes (M = Cr, Mo, W)

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Abstract
Ab initio, semiempirical, and HMO perturbation calculations were employed to assess the relative positioning of the closely situated low-lying unoccupied π* MOs a2(χ) and b1(ψ) in 1,10-phenanthroline (phen) and its 3,4,7,8-tetramethyl (tmphen) and four symmetrical diaza derivatives (n,m-dap). Compared to a2(χ), the b1(ψ) π MO is distinguished by markedly higher MO coefficients at the chelating nitrogen π centers in 1,10-positions; eventually, a higher Coulomb integral value at those positions will thus always favor the lowering of b1 beyond a2. Using the Coulomb integral parameter at the chelating 1,10-nitrogen π centers as the HMO perturbation variable, the crossing of both energy levels in terms of decreasing preference for the a2(χ) over the b1(ψ) orbital as the lowest unoccupied MO follows the sequence 5,6-dap > 2,9-dap > 4,7-dap > phen > 3,8-dap. The calculations reveal a2(χ) as the LUMO in 5,6-dap for all reasonable perturbation parameters, in agreement with previous observations for ruthenium(II) complexes which reveal a discrepancy between the lowest-lying “redox π* orbital” (a2) and the “optical π* MO” (b1) to which the most intense low-energy MLCT transition occurs. According to the HMO calculations, the situation is more ambiguous for the 4,7-dap analogue, yet EPR/ENDOR studies clearly show that the one-electron-reduced ligand and its tetracarbonylmetal(0) complexes (Cr, Mo, W) have the b1(ψ) orbital singly occupied. Only ab initio calculations with double-ζ basis and inclusion of d polarization functions reproduced correctly the experimentally observed orbital ordering for tmphen (a2 < b1) and for phen and 4,7-dap (b1 < a2).

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