To probe the reactivity of two potential sites at tetravalent thorium and uranium metal centers, we examined the monophosphido methyl complexes, (C5Me5)(2)An(CH3)[P(SiMe3)(Mes)], An = Th, U; Mes = 2,4,6-Me3C6H2. Reaction of these mixed ligand complexes with one, two, and three (or excess) equivalents of (BuNC)-Bu-t was explored. When (C5Me5)(2)An(CH3)[P(SiMe3)(Mes)] is treated with one equivalent of (BuNC)-Bu-t, the iminoacyl products, (C5Me5)(2)An[eta(2)-(BuN)-Bu-t=CCH3][P(SiMe3)(Mes)], are formed. Using three equivalents (or excess) of (BuNC)-Bu-t results in the formation of an alpha-diimine moiety, (C5Me5)(2)An[kappa(2)-(N,N)-N(Bu-t)C=CN(Bu-t)C=N(Bu-t)CH2]. When two equivalents of (BuNC)-Bu-t are added, only the mono-insertion or alpha-diimine was observed in a 3:1 ratio. Density functional theory calculations were carried out to determine the lowest energy pathway in the formation of the alpha-diimine product via the iminoacyl complex.