title: Theoretical description of dihydrogen/hydride and trihydride molybdocene complexes: an insight from static and molecular dynamics simulations.
abstract: In this study ab initio Car-Parrinello molecular dynamics simulations, extended transition state (ETS)-natural orbitals for chemical valence (NOCV) and QTAIM bonding analyses, were performed to characterize the ansa-bridged molybdocene complexes [(C(5)H(4))(2)XMe(2)MoH(3)](+) for X = C, Si, Ge, Sn, Pb, and nonbridged Cp(2)MoH(3)(+) system. The results have shown that the [(C(5)H(4))(2)CMe(2)MoH(H(2))](+) complex exhibits nonclassical dihydrogen/hydride (H(2)/H) conformation (97.6% of time of simulation), contrary to trihydride (H(3)) structure noted for nonbridged Cp(2)MoH(3)(+) (86.9%) and ansa-bridged [(C(5)H(4))(2)SnMe(2)MoH(3)](+) (84.8%), [(C(5) H(4))(2)PbMe(2)MoH(3)](+) (84.9%) systems. Further, [(C(5)H(4))(2)SiMe(2)MoH(3)](+) and [(C(5)H(4))(2)GeMe(2)MoH(3)](+) complexes, appeared to exist in both conformations (H(2)/H--55.4%, H(3)--44.6% for Si-based system and H(2)/H--36.2%, H(3)--63.8 % for germanium congener). It has been proven that the "steric availability" of the metal center, measured by the changes in the Cp-Mo-Cp angle (α), determines the existence of a given conformation--namely, the smaller value of the angle (molybdenum is sterically more accessible) the larger preference for the formation of dihydrogen/hydride structure. ETS-NOCV method allowed to conclude that increase in the Cp-Mo-Cp angle (from α ca. 120° to α ca. 150°) leads to the enhancement of donation from H(2) and back-donation from Mo to the σ*(H-H), what consequently leads to breaking of the H-H bond and formation of the trihydride structure. Systematical increase in the charge depletion from the σ-bonding orbital of H(2) can be related to the reduction of the energy gap between the major orbitals involved in this contribution, namely highest occupied molecular orbital (HOMO) of H(2) with lowest unoccupied molecular orbital (LUMO) of [MoHL](+); ΔE = 0.0868 a.u. [for L =(C(5)H(4))(2)C], ΔE = 0.0827a.u. [for L = (C(5)H(4))(2)Si] ΔE = 0.0638 a.u. [for L = Cp(2)]. Further, the relatively low energetic barrier to hydrogen exchange (ΔE(#) = 3.3 kcal/mol) for carbon-bridged complex, [(C(5)H(4))(2)CMe(2)MoH(c)(H(a)H(b))](+) → [(C(5)H(4)) (2)CMe(2)MoH(a)(H(b)H(c))](+), is related to strengthening of the Mo-H bonds when going from the substrate to the transition state (TS). Notably higher barrier to hydrogen rotation (ΔE(#) = 10.1 kcal/mol) in [(C(5)H(4))(2) CMe(2)MoH(H(2))](+) is due to lowering in the electrostatic stabilization as well as weakening of the donation (H(2) → Mo charge transfer) and practically lack-of back-donation (Mo → H(2)) in the rotated TS.
authors.affiliation: Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, R Ingardena 3, 30-060 Cracow, Poland.
authors.forename: Łukasz
authors.initials: Ł
authors.lastname: Piękoś
authors.forename: Mariusz Paweł
authors.initials: MP
authors.lastname: Mitoraj