The role of electronegativity on the extent of nitridation of group 5 metals as revealed by reactions of tantalum cluster cations with ammonia molecules

Reactions of the free tantalum cation, Ta⁺, and tantalum cluster cations, Ta_n⁺ (n = 2-10), with ammonia are presented. The reaction of the monomer cation, Ta⁺, with two molecules of NH₃ leads to the formation of TaN₂H₂⁺ along with release of two H₂ molecules. The dehydrogenation occurs until the formal oxidation number of the tantalum atom reaches +5. On the other hand, all the tantalum cluster cations, Ta_n⁺, react with two molecules of NH₃ and form Ta_nN₂⁺ with the release of three H₂ molecules. Further exposure to ammonia showed that Ta_nN_mH⁺ and Ta_nN_m⁺ are produced through successive reactions; a pure nitride and three H₂ molecules are formed for every other NH₃ molecule. The nitridation occurred until the formal oxidation number of the tantalum atoms reaches +5 as in the case of TaN₂H₂⁺ in contrast to other group 5 elements, i.e., vanadium and niobium, which have been reported to produce nitrides with lower oxidation states. The present results on small gas-phase metal-nitride clusters show correlation with their bulk properties: tantalum is known to form bulk nitrides in the oxidation states of either +5 (Ta₃N₅) or +3 (TaN), whereas vanadium and niobium form nitrides in the oxidation state of +3 (VN and NbN). Along with DFT calculations, these findings reveal that nitridation is driven by the electron-donating ability of group 5 elements, i.e., electronegativity of the metal plays a key role in determining the composition of the metal nitrides.