Chromium, Molybdenum, and Tungsten Chlorophosphazenes: Molecular Structures of N3P3Cl5[Cr(CO)3(ɳ-C5H5)] and N₃P₃C1₄(C₅H₅)[MO(CO)₃(ɳ-C₅H₅)]

Cyclopentadienylchromium tricarbonyl anion reacts with hexachlorocyclotriphosphazene, (NPCl₂)₃, to form a metallophosphazene of formula N₃P₃Cl₅[Cr(CO)₃(ɳ-C5H5)] (5). The reactions of the analogous molybdenum and tungsten anions with (NPCl₂)₃ are more complex and lead to the formation of metallophosphazenes of formula N₃P₃Cl₄(C₅H₅)[M(CO)₃(ɳ-C5H₅)] (6a, M = Mo; 6b, M = W) in which the chlorine atom geminal to the metal has been replaced by a cyclopentadiene group. A key requirement for the success of these reactions is the use of a tetra-n-butylammonium counterion. The products are among the first chlorophosphazenes to contain metal-phosphorus side-group bonds. The structures of 5 and 6a were examined in detail by spectroscopic and X-ray diffraction techniques. In both compounds the N-P-N bond angle at the metal-bearing phosphorus is unusually narrow, being 112.3 (3)° in 5 and 111.0 (3)° in 6a. Moreover, an alternation of longer and shorter bond lengths is found for the P-N bonds located at increasing distances from the metal. For 5, the phosphazene ring was found to be significantly nonplanar. These features are compatible with appreciable interaction between the metal and the phosphorus-nitrogen ring. Crystal data; crystals of 5 are monoclinic with the space group P2₁/m and with a = 8.334 (3) Å, b = 12.897 (8) Å, c = 8.783 (4) Å, β = 105.44 (3)°, V = 909.9 (15) \³, and Z = 2; crystals of 6a are orthorhombic with the space group Pca2₁ and with a = 16.075 (3) Å, b = 8.832 (3) Å, c = 14.780 (3) Å, V = 2098 (1) ų, and Z = 4.