The reaction of a silsesquioxane trisilanol with methylmagnesium chloride leads to an unprecedented tetranuclear magnesium silsesquioxane complex. 1 in high yield. The crystal structure shows an unusually short Mg-Cl bond, indicative of an electron-deficient magnesium atom; 1 has been used as transmetalation agent for the synthesis of metal silsesquioxane complexes. Transmetalation activity was low, but can easily be followed by multinuclear NMR. Crystal data for 1: C78H142Cl2Mg4O26Si14.6(C4H8O), a = 15.744(1) Angstrom, b = 26.526(2) Angstrom, c = 16.917(1) Angstrom, beta = 113.229(2)degrees, monoclinic, P2(1)/n, , Z = 2.