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.