The magnesiation of tri(tert-butyl)silylphosphane in THF yields tetrameric (tetrahydrofuran-O)magnesium tri(tert-butyl)silylphosphandiide 1. The central moiety is a slightly distorted Mg4P4 cube with tetracoordinate magnesium and phosphorus atoms. The reaction of dibutylmagnesium with (H2PSiBu3)-Bu-t in toluene gives tetramagnesium tetrakis[mu -tri(tert-butyl)silylphosphanide] bis[mu (4)-tri(tert-butyl)silylphosphandiide] 2. The central fragment is a Mg4P2 octahedron with the phosphorus atoms in a trans position. The Mg Mg edges are bridged by the phosphanide substituents. Crystallographic data of 1: C68H148Mg4O5P4Si4, monoclinic, P2(1)/c, a = 13.454(1) Angstrom, b =26.123(1) Angstrom, c = 24.539(2) Angstrom, beta = 96.53(1)degrees, Z = 4; crystallographic data of 2: C72H166Mg4P6Si6, monoclinic, P2(1)/n, a = 13.951(1) Angstrom, b = 14.269(1) Angstrom, c = 24.209(2) Angstrom, beta = 102.415(1)degrees, Z = 2.