Natural bond orbital (NBO) analysis of the metallotropic shifts in cyclopentadienyl(trimethyl)silane, -germane and -stannane

NB0/B3LYP/3-21G, NBO//B3LYP/3-21G* and NBO//B3LYP/LANL2DZ* analysis was used to investigate the aptitude of the alkyl and metallotropic 1,2-shifts in 5-tert-butylcyclopentadiene (1), cyclopentadienyl(trimethyl)silane (2), cyclopentadienyl (trimethyl) germane (3) and cyclopentadienyl(trimethyl)stannane (4). B3LYP/3-21G, B3LYP/3-21G* and B3LYP/LANL2DZ* results show that the NMe3 (M=Si (2), Ge (3) and Sn (4)) migration barrier heights around cyclopentadienyl rings in compounds 1-4 decrease in following order: 4 < 3 < 2 < 1. The NBO analysis of donor-acceptor (bond-antibond) interactions revealed that the hyperconjugation between sigma(Ccyclpentadienyl-M) (M=Si (2), Ge (3) and Sn (4)) and pi*(C=C) of cyclopentadienyl ring facilitate the metal migration around cyclopentadienyl ring. The donor-acceptor interactions resulted in decreasing of occupancy of sigma(Ccyclopentadinyl-M) bonding orbital of C5-M bonds of the idealized Lewis structure in following order: 4 < 3 < 2 < 1, and also increasing occupancy of pi*(cyclopnuidienyl) anti-bonding orbital of C1-C2 and C3-C4 bonds of cyclopentadienyl rings in following order: 4 > 3 > 2 > 1, as calculated by NBO//B3LYP/3-21G, NBO//B3LYP/3-21G* and NBO//B3LYP/LANL2DZ*. The results suggest that in compounds 1-4, the metallotropic shifts is controlled by sigma ->pi* energetic stabilization. The delocalization energies from sigma(C-M) bonding orbital to pi*(C=C) antibonding orbital of cyclopentadienyl ring in compounds 1-4 are 1.45, 7.24, 7.63 and 12.49 kcal.mol(-1), respectively, as calculated by B3LYP/3-21G level of theory. Also, B3LYP/3-21G* results show that the delocalization energies from sigma(C-M) bonding orbital to pi*(C=C) antibonding orbital of cyclopentadienyl ring in compounds 1-4 are 1.45, 6.58, 9.18 and 14.11 kcal.mol(-1). Further, B3LYP/LANL2DZ* results show that the above delocalization energies in compounds 1-4 are 1.26, 6.08, 8.86 and 13.47 kcal.mol(-1), respectively.