SYNTHESIS, UNUSUAL TRIGONAL PRISMATIC GEOMETRY, AND THEORETICAL-STUDYOF THE HOMOLEPTIC TRIS-(2,2'-BIPHOSPHININE) COMPLEXES OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN

The preparation of homoleptic tris-(4,4',5,5'-tetramethyl-2,2'-biphosp hinine) complexes of chromium 2, molybdenum 3, and tungsten 4 [M(tmbp) (3)] is described. These complexes can be obtained by the simple reduc tion of CrCl3. 3THF, MoCl5, and WCl6, respectively, with magnesium in the presence of the ligand. The tungsten complex 4 was alternatively p repared in better yields by the reduction of the [WBr4(MeCN)(2)] compl ex with zinc dust. The structure of 4 has been unambiguously confirmed by an X-ray crystal structure analysis. Surprisingly, whereas an octa hedral geometry (theta = 60 degrees) would have been expected for a d( 6) ML6, complex 4 presents a nearly trigonal prismatic structure (thet a = 15 degrees). Two data reveal that a significant electronic transfe r from the metal to the ligand occurs: short P-W bond distances (2.35- 2.36 Angstrom) and a shortened C-C bridge (1.442(4) Angstrom) between two phosphinine subunits. Ab initio calculations at the DFT(B3LYP) lev el have been undertaken to rationalize this deviation from the ideal o ctahedral geometry. The geometry of [W(P2C2H4)(3)], which was chosen a s a model for 4, and that of the d(0) ML6 tris-(dithiolene) complex [W (S2C2H4)(3)], which is known to be trigonal prismatic, have been calcu lated. In both cases the calculated geometries are in good agreement w ith the corresponding observed data. On the basis of these results it is suggested that [W(P2C2H4)(3)] can be most conveniently described as a fully oxidized d(0) complex in which the phosphorus P lone pairs ar e conjugated to the C=C double bond and not bonded to the metal center .