PI-ASTERISK MOLECULAR-ORBITAL CROSSING A(2)(CHI) B(1)(PSI) IN 1,10-PHENANTHROLINE DERIVATIVES - AB-INITIO CALCULATIONS AND EPR/ENDOR STUDIES OF THE 4,7-DIAZA-1,10-PHENANTHROLINE RADICAL-ANION AND ITS M(CO)(4) COMPLEXES (M=CR, MO, W)/

Ab initio, semiempirical, and HMO perturbation calculations were emplo yed to assess the relative positioning of the closely situated low-lyi ng unoccupied pi MOs a(2)(chi) and b(1)(psi) in 1,10-phenanthroline ( phen) and its 3,4,7,8-tetramethyl (tmphen) and four symmetrical diaza derivatives (n,m-dap). Compared to a(2)(chi), the b(1)(psi) pi MO is d istinguished by markedly higher MO coefficients at the chelating nitro gen pi centers in 1,10-positions; eventually, a higher Coulomb integra l value at those positions will thus always favor the lowering of b(1) beyond a(2). Using the Coulomb integral parameter at the chelating 1, 10-nitrogen pi centers as the HMO perturbation variable, the crossing of both energy levels in terms of decreasing preference for the a(2)(c hi) over the b(1)(psi) orbital as the lowest unoccupied MO follows the sequence 5,6-dap > 2,9-dap > 4,7-dap, phen > 3,8-dap. The calculation s reveal a(2)(chi) as the LUMO in 5,6-dap for all reasonable perturbat ion parameters, in agreement with previous observations for ruthenium( II) complexes which reveal a discrepancy between the lowest-lying ''re dox pi orbital'' (a(2)) and the ''optical pi* MO'' (b(1)) to which th e most intense low-energy MLCT transition occurs. According to the HMO calculations, the situation is more ambiguous for the 4,7-dap analogu e, yet EPR/ENDOR studies clearly show that the one-electron-reduced li gand and its tetracarbonylmetal(0) complexes (Cr, Mo, W) have the b(1) (psi) orbital singly occupied. Only ab initio calculations with double -zeta basis and inclusion of d polarization functions reproduced corre ctly the experimentally observed orbital ordering for tmphen (a(2) < b (1)) and for phen and 4,7-dap (b(1) < a(2)).