THEORETICAL-STUDY OF THE EFFECT OF A LEWIS-ACID ON HYDROGEN-EXCHANGE COUPLING IN A TRIHYDRIDE METALLOCENE - THE CP2NBH3.ALH3 SYSTEM

In this paper the effect of a Lewis acid on the quantum exchange coupl ing of polyhydride transition metal complexes is analyzed. Tn particul ar, by means of the density functional methodology (DFT): we have calc ulated the different adducts and transition states that appear in the Cp2NbH3 . AlH3 system. The quantum exchange coupling that arises when two hydrides interconvert has been evaluated through a one-dimensional tunneling model within a basis set formalism. We find that a Lewis ac id of the AlH3 type may increase the exchange couplings through format ion of an adduct where the Lewis acid is only bonded to the outer hydr ide. In this structure, which is not the absolute minimum but chat is close enough in energy to be significantly populated at the experiment al range of temperatures. the two ''free'' hydrides came closer one to each other so that an incipient dihydrogen ligand that favors the exc hange tends to be formed. This result can be explained by the reductio n of the electronic density on niobium induced by the Lewis acid. Our results are in agreement with experimental data for the similar Cp2NbH 3 . AlEt3 system and recent H-1 NMR spectral analysis of the reaction of niobocene complexes with sails of the Lewis acidic cations Cu+, Ag, or Au+, though it is probable that, within the same general model fo und in the present work, the particular structure causing the exchange coupling will depend on each kind of Lewis acid.