Reactions involving niobium pentachloride and lithium dialkylamides (LiNR2, where R = Me, Et, Prn, Bun) led to considerable reduction of niobium to Nb(IV) as the length of the alkyl chains increased. Nb(NMe2)5 was isolated by sublimation but distillation of the other products gave tetrakis-(dialkylamino)-niobium(IV) compounds. Steric factors are apparently responsible for the low stability of pentakis-(dialkylamino)-niobium(V) compounds. The smaller steric effects of —NMeBun and piperidino groups allowed the isolation of pentakis derivatives. The thermal decomposition of Nb(NMeBun)5 gave mainly Nb(NMeBun)4 with some BunN=Nb(NMeBun)3. Steric effects were also revealed in the replacement of —NMe2 groups in Nb(NMe2)5 by —NEt2 groups and in replacement of —NEt2 groups in Nb(NEt2)4 by piperidino groups.
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Bradley et al. (1962) studied this question.