The tetrahydrofuran complexes of the alkaline earth metal bis [bis(trimethylsilyl)phosphanides] react with 2 equiv of benzonitrile to the bis[1,3-bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenides] of magnesium (1), calcium (2), strontium (3), and barium (4). Whereas 1 and 2 crystallize as bis(tetrahydrofuran) adducts, the heavier homologous derivatives 3 and 4 precipitate as tris(tetrahydrofuran) complexes. The bidentate trans/trans-isomeric 1,3-bis (trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenide ligands of all the se compounds display very similar spectroscopic data; however, the influence of the alkaline earth metal is observed in the silicon NMR. The nitrogen atoms of these anions have trigonal planar geometry, whereas the phosphorus atoms are coordinated trigonal pyramidal. Crystallographic data : 1, monoclinic, P2(1)/c, a=1505.3(3) pm, b=1259.0(2) pm, c= 2278.4(5) pm, beta=91.91(2)degrees, Z=4, wR2=0.1279; 2, monoclinic, P2(1)/c, a=1522.8(7) pm, b=1279.2(5) pm, c=2314.0(10) pm, beta=92.53(2)degrees, Z=4, wR2=0.1513; 3, triclinic, , a=1144.8(3) pm, b=1579.4(6) pm, c=1670.6(6) pm, alpha=98.02(3)degrees, beta=109.67(3)degrees, gamma=104.18(3)degrees, Z=2, wR2=0.1745; 4, triclinic, , a 1245.2(4) pm, b=1907.1(4) pm, c=2212.3(7) pm, alpha=76.46(2)degrees, beta=76.45(2)degrees, gamma=89.31(2)degrees, Z=4, wR2=0.1409.