Bridging-terminal exchange in ansa-bridged tetrahydroborate niobocene compounds: a density functional study

Density functional calculations on [Nb(eta-C5H5)(2)(eta(2)-BH4)], [Nb{H2Si( eta-C5H4)(2)}(eta(2)-BH4)] and [Nb{H2C(eta-C5H4)(2)} (eta(2)-BH4)], show th e barrier to terminal-bridging hydrogen exchange decrease as the inter-ring angle increases, in agreement with earlier experimental data. The calculat ions suggest the bonding of BH, weakens as the metallocene unit becomes mor e bent, and that there is stronger bonding in the transition state as the d (yz) orbital becomes more accessible. (C) 2000 Elsevier Science S.A. All ri ghts reserved.