The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes [(BDI)MeM(NtBu)][X] (BDI = [Ar]NC(CH3)CHC(CH3)N[Ar], Ar = 2,6-iPr2C6H3; M = Nb, Ta; X = MeB(C6F5)3, B(C6F5)4] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but formed phosphine-trapped acyl complexes [(BDI)(R3PC(O)Me)M(NtBu)][B(C6F5)4] (R = Et, Cy) in the presence of a combination of trialkylphosphines and CO. Treatment of the monoalkyl cationic Nb complex with XylNC (Xyl = 2,6-Me2-C6H3) resulted in irreversible formation of the iminoacyl complex [(BDI)(XylNC(Me))Nb(NtBu)][B(C6F5)4], which did not bind phosphines but would add a methide group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe2)Nb(NtBu). Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; ii) protonolysis reactions with Ph3SiOH to form thermally robust cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the cationic Nb methyl complex. [ABSTRACT FROM AUTHOR]