Theoretical study on structures and aromaticities of P5- anion, [Ti(η5-P5)]- and sandwich complex [Ti(η5-P5)2]2-

The equilibrium geometries, energies, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICSs) of the ground state Of P-5(-) (D-5h) anion the [Ti (eta(5) - P-5](-) fragment (C-5v), and the sandwich complex [Ti(eta(5)-P-5)(2)](2-) (D-5h and D-5d) are calculated by the three-parameter fit of the exchange-correlation potential suggested by Becke in conjunction with the LYP exchange potential (B3LYP) with basis sets 6-311+G(2d) (for P) and 6-311+G(2df) (for Ti). In each of the three molecules, the P-P and Ti -P bond distances are perfectly equal: five P atoms in block P-5(-) lie in the same plane; the P-P bond distance increases and the Ti-P bond distance decreases with the order P-5(-), [Ti(eta(5)-P-5)(2)](2-) and [Ti(eta(5)-P-5)](-). The binding energy analysis, which is carried out according to the energy change of hypothetic reactions of the three species, predicts that the three species are all very slable, and [Ti (eta(5) -P-5)](-) (C-5v), more stable than P-5(-) and [Ti (eta(5)-P-5)(2)](2-) synthesized in the experiment, could be synthesized. NICS values, computed for the anion and moiety of the three species with GIAO-B3LYP, reveal that the three species all have a larger aromaticity, and NICS (0) of moiety, NICS (1) of moiety, and minimum NICS of the inner side of ring P-5 plane in magnitude increase with the order P-5(-), [Ti (eta(5) -P-5)(2)](2-),and [Ti (eta(5)-p(5))](-). By analysis of the binding energetic and the molecular orbital (MO) and qualitative MO correlation diagram, and the dissection of total NICS, dissected as NICS contributions of various bonds, it is the main reason for P-5(-) (D-5h) having the larger aromaticity that the P-P sigma bonds, and pi bonds have the larger diatropic ring currents in which NICS contribution are negative, especially the P-P sigma bond. However, in [Ti (eta(5) -P-5)](-) (C-5v) and [Ti (eta(5)-P-5)(2)](2-) (D-5h, and D-5d), the reason is the larger and more negative diatropic ring currents in which the NICS contributions of P-P pi bonds and P-5-Ti bonds including pi, delta, and sigma bonds, especially P-5-Ti bonds, are much more negative and canceled the NICS contributions of P ant. Ti core and lone pair electrons.