In this paper the effect of a Lewis acid on the quantum exchange coupling of polyhydride transition metal complexes is analyzed. Tn particular, by means of the density functional methodology (DFT) : we have calculated 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 acid of the AlH3 type may increase the exchange couplings through formation of an adduct where the Lewis acid is only bonded to the outer hydride. In this structure, which is not the absolute minimum but chat is close enough in energy to be significantly populated at the experimental range of temperatures. the two "free" hydrides came closer one to each other so that an incipient dihydrogen ligand that favors the exchange tends to be formed. This result can be explained by the reduction of the electronic density on niobium induced by the Lewis acid. Our results are in agreement with experimental data for the similar Cp2NbH3 . 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 found in the present work, the particular structure causing the exchange coupling will depend on each kind of Lewis acid.