The reaction of a kinetically stable triaminosilane RSi(NH2)3 (R = 2,6-iPr2C6H3NSiMe2iPr) (1) with trialkyl group 13 metal compounds MR3 (R = Me, Et; M = Al, Ga, In) leads to Si−NH−M cage molecules, which increase in solubility from aluminum to indium in common weakly-coordinating organic solvents like toluene. These compounds may be regarded as model systems for group 13 metal containing iminosilicates. X-ray studies show a prismatic cage framework which is isostructural and isoelectronic with known heterosiloxanes. Further functionalization of these compounds without cleavage of the cage molecule was achieved by reaction with elemental bromine and iodine.