Mixed amidinato amido complexes [Me3SiNC(Bu-t)NSiMe3]M[N(SiMe3)(2)] (M = Sn 2, Ge 3) were prepared by the reaction of [Me3SiNC(Bu-t)NSiMe3]Li (la) with SnCl2 and GeCl2(dioxane) in ether. The N(SiMe3)(2) ligand in these compounds is derived from the rearrangement of the [Me3SiNC(Bu-t)NSiMe3](-) anion with extrusion of (BuCN)-Bu-t. The susceptibility of [Me3SiNC(Bu-t)NSiMe3](-) to rearrangement appears to be dependent on reaction solvent and on the coordinated metal center. Single-crystal X-ray diffraction studies of 2 and 3 are presented. Replacement of Me for Bu-t in the ligand allowed [Me3SiNC(Me)NSiMe3](2)Sn-II (4) to be isolated, and an X-ray structure of this compound is reported. The isolation of 4 indicates that steric factors also play a role in the stability of [Me3SiNC(Bu-t)NSiMe3](-). Compounds 2 and 3 are outstanding catalysts for the cyclotrimerization of phenyl isocyanates to perhydro-1,3,5-triazine-2,4,6-triones (isocyanurates) at room temperature. In contrast, complex 4 catalytically reacts with phenyl isocyanate to produce isocyanate dimer and trimer in a 52:35 ratio.