A theoretical study on structures, bonding energies and aromaticity of two new series of dinuclear phosphametallocenes:: (η5-P5)MM′(η5-P5) and (η5-C5H5)MM′(η5-P5) (M, M′ = Zn, Cd)

The equilibrium geometries, the energies, the harmonic vibrational frequencies, and the nucleus independent chemical shifts (NICSs) of the ground state of (eta(5)-P-5)MM' (eta(5)-P-5) and (eta(5)-C5H5)MM'(eta(5)-P-5) (M, M' = Zn, Cd) are calculated by the hybrid density functional method B3LYP with LANL2DZ basis sets. The analysis of energy and harmonic frequencies on the equilibrium geometries of both series dinuclear deca- and pentaphosphametallocenes shows that all the minima with singlet electronic state have an staggered (9 degrees <= D(E-M-M-E') <= 36 degrees) conformation except for the eclipsed CpCd2(P-5) (C-5 nu), and all the D-5h and the D-5d symmetric conformations are transition states while the energy differences between the most stable minimum and the transition states are very small (<= 0.1 kcal/mol), thus, the rotation of the P-5 ring about M-M bond in all complexes is almost free. The analysis of the NBO, the Laplacian of the electron density, the electrostatic interaction energy, the bonding energy decomposition, and the molecular orbital correlation diagrams for the two series complexes reveals that the properties of all the dinuclear phosphametallocenes investigated are similar to that of the dizinc metallocenes. The M-M (or M-M') bond in the dinuclear phosphametallocenes is a weak a covalent bond, and the magnitude of bonding energy of the M-M (or M-M') bond correlates with the energy difference between the energy of HOMO in M(eta(5)-eta(5)) (or MCp) ((2)A, C-5v) fragment and the energy of HOMO-2 in [MM'(eta(5)-eta(5))(2)] ((1)A, D-5, or D-5h) [or (eta(5)-P-5)MM'Cp ((1)A, C-5, or C-5 nu)] (M, M'= Zn, Cd). The strength of the M-M (or M-M') bond plays a decisive role on the stability of the dinuclear phosphametallocenes. However, the M-(eta(5)-P-5) (or M-Cp) bonding mainly is ionic. Among the different dinuclear phosphametallocenes with the same ligands, the bonding energies of the M-M (or MM') bond and of the M-(eta(5)-P-5) (or M-Cp) bond decrease as M varying from Zn to Cd. Among the different phosphametallocenes with the same metals, the bonding energies of the M-M (or M-M') bond and the M-ligand bond increase with ligand varying from the (q5 -P5) to the Cp. The negative NICSs indicate that all dinuclear phosphametallocenes have aromaticity. The various dissected bond NICS contributions reveal that the NICS contributions of the metal-ligand bond, the metal-metal bond, and the metal lone pair electrons to the overall aromaticity in the Zn-containing dinuclear phosphametallocenes are different from their counterpart contributions in the Cd-containing dinuclear phosphametallocenes, such difference causes that the overall NICSs of the Zn-containing dinuclear phosphametallocenes are more negative than that of the Cd-containing dinuclear phosphametallocenes. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).