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

Density functional calculations on [Nb(η-C5H5)2(η2-BH4)], [Nb{H2Si(η-C5H4)2}(η2-BH4)] and [Nb{H2C(η-C5H4)2}(η2-BH4)], show the barrier to terminal-bridging hydrogen exchange decrease as the inter-ring angle increases, in agreement with earlier experimental data. The calculations suggest the bonding of BH4 weakens as the metallocene unit becomes more bent, and that there is stronger bonding in the transition state as the dyz orbital becomes more accessible.