The preparation of homoleptic tris-(4,4',5,5'-tetramethyl-2,2'-biphosphinine) complexes of chromium 2, molybdenum 3, and tungsten 4 [M(tmbp)(3)] is described. These complexes can be obtained by the simple reduction of CrCl3. 3THF, MoCl5, and WCl6, respectively, with magnesium in the presence of the ligand. The tungsten complex 4 was alternatively prepared in better yields by the reduction of the [WBr4(MeCN)(2)] complex with zinc dust. The structure of 4 has been unambiguously confirmed by an X-ray crystal structure analysis. Surprisingly, whereas an octahedral geometry (theta = 60 degrees) would have been expected for a d(6) ML6, complex 4 presents a nearly trigonal prismatic structure (theta = 15 degrees). Two data reveal that a significant electronic transfer 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) level have been undertaken to rationalize this deviation from the ideal octahedral geometry. The geometry of [W(P2C2H4)(3)], which was chosen as 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 calculated. In both cases the calculated geometries are in good agreement with 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 are conjugated to the C=C double bond and not bonded to the metal center.