The alkylation of the starting pseudooctahedral dichlorido imido hydridotris(3,5-dimethylpyrazolyl)borate niobium and tantalum(V) compounds [MTp*Cl₂(NtBu)] (M = Nb,Ta; Tp* = BH(3,5-Me₂C₃HN₂)₃) with MgClR in different conditions led to new alkyl chlorido imido derivatives [MTp*ClR(NtBu)] (M = Nb/Ta, R = CH₂CH₃1a/1b, CH₂Ph 2a/2b, CH₂tBu 3a/3b, CH₂SiMe₃4a/4b, CH₂CMe₂Ph 5a/5b), whereas the dimethyl derivatives [MTp*Me₂(NtBu)] (M = Nb 6a, Ta 6b) could be isolated as unitary species when the reaction was carried out using 2 equivalents of the magnesium reagent MgClMe. However, the chlorido methyl [MTp*ClMe(NtBu)] (M = Nb 7a, Ta 7b) complexes were obtained by heating at 50 °C the dichlorido and dimethyl imido complexes mixtures in a 1 : 1 ratio. All of the complexes were studied by multinuclear magnetic resonance spectroscopy and the molecular structures of 1b, 2a/b, 3a/b, 4a and 5a/b were determined by X-ray diffraction methods. In the solid state the complexes 1b, 4a and 5a exhibit only a gauche–anti conformation and the complexes 2a/b, 3a/b and 5b exhibit only a gauche–syn conformation of the alkyl substituents, whereas both conformational states, which do not show mutual exchange in the NMR time scale, were observed for 3a/b in a benzene-d₆ solution. The ¹⁵N chemical shifts of the complexes 1–7 are discussed.