Ground-state geometry preferences in (tris(pyrazolyl)borato)- and cyclopentadienylniobium alkyne complexes

Reaction of NbCl3(DME)(RC=CR') with KTp (DME = 1,2-dimethoxyethane, Tp = hydrotris(pyrazolyl)borate) yields TpNbCl(2)(RC=CR') (R = Ph, R' = Me (2a); R = R' = Me (2b), Et (2c), SiMe(3) (2d), Ph (2e)), which has the alkyne in the molecular mirror plane in the solid state (X-ray crystal structure for 2a) and in solution. The barriers to alkyne rotation are low, the highest being measured for complex 2d (52 kJ mol(-1) at 273 K), which contains the bulky SiMe(3) group on the alkyne. In TpCpNb(Cl)(PhC=CMe) (3a), formed by reaction of 2a with NaCp . DME, the alkyne is parallel to the Cp plane, as observed in solution and in the crystal structure. Extended Huckel molecular orbital calculations indicate that in complexes 2 the geometry is mainly governed by steric interactions, whereas in the case of 3a orbital interactions dictate the observed geometry.