Crystal and Molecular Structure of a Platinum-Cyclophosphazene Complex: Cis-Dichloro[octa-(methylamino)cyclotetraphosphazene-N,A'']platinum(II)1'2

The compound, PtCl2[N4P4(NHCH3)8], is both an antitumor agent and a model compound for the analogous poly-meric derivative, (PtCl2)x [NP(NHCH3)2]n. The x-ray structure of PtCl2[N4P4(NHCH3)8] has revealed a unique form of bonding in which a square planar platinum atom is bonded to two antipodal skeletal nitrogen atoms of the saddle-shaped cyclotetraphosphazene ring. The orientation of the phosphazene ring relative to the platinum atom cannot be explained solely by σ-coordinate bonding or by a π-type interaction. The crystals are monoclinic with the space group P21/n, and with a = 10.34 (2), b = 15.61 (4), c - 14.32 (4) \, β= 92.36 (18)°, Z = 4. The Pt-N bond distances are 2.026 (16) and 2.081 (15) \ and the Pt-Cl bond distances are 2.304 (5) and 2.300 (6) \. The N-Pt-N angle is 87.0 (6)° and the Cl-Pt-Cl angle is 92.9 (2)°. Within the cyclotetraphosphazene unit the P-N bonds flanking the Pt-N units are longer (1.61-1.64 (2) \) than those at a greater distance from the coordination site (1.57-1.61 (2) \). The skeletal P-N-P bond angle is marginally wider (128.8 (10)°) at the coordination sites than at the uncoordinated locations (122.2-127.8°). The bonding implications of these results are discussed.