Synthesis, Unusual Trigonal Prismatic Geometry, and Theoretical Study of the Homoleptic Tris-(2,2‘-biphosphinine) Complexes of Chromium, Molybdenum, and Tungsten

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 (θ = 60°) would have been expected for a
d6 ML6, complex 4 presents a nearly
trigonal
prismatic structure (θ = 15°). Two data reveal that a
significant electronic transfer from the metal to the
ligand
occurs:  short P−W bond distances (2.35−2.36 Å) and a shortened
C−C bridge (1.442(4) Å) 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 d0 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 d0 complex in which the phosphorus P lone
pairs are conjugated to the CC double bond and not
bonded to the metal center.