Reactions of free tantalum cluster cations, Tan+ (n = 1â€“10), with
ammonia molecules were performed to investigate nitridation mechanism of
tantalum by probing reactions step by step with precise control of the
number of atoms and molecules involved in the reaction. 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, Tan+, react with two
molecules of NH3 and form TanN2+ with the release of three H2 molecules.
Further exposure to ammonia showed that TanNmH+ and TanNm+ 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.