The reaction between W(N2)5- and N2: the importance of the adsorption barrier.

Theoretical calculations indicate that the reaction of W(N2)5- with N2 to form octahedral W(N2)6- is exothermic, which suggests that W(N2)6- is more stable than W(N2)5-. However, our experiments show that the ion signal of W(N2)6- is much weaker than that of W(N2)5-. A detailed theoretical study reveals that there is a weak interaction barrier for adsorption at a W-N distance of about 2.8 Å along the W(N2)5- + N2 reaction pathway, whereas the corresponding neutral reaction, W(N2)5 + N2, is barrierless. The weak interaction barrier is primarily due to repulsion between the electrons in the frontier molecular orbitals of W(N2)5- and N2 at long range. The studies therefore confirm that the formation of octahedral W(N2)6- is hindered by kinetics rather than by thermodynamics.