2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 291
adopting the gauche configuration whereas in [W2⊲OSiPh3⊳2⊲NMe2⊳4] (W W,
2.295 ˚A) the anti configuration occurs. 300 The structure of optically active ⊲C⊳
[W2⊲OC10H19⊳6] (W W, 2.338 ˚A) containing mentholato ligands has also been
determined. 301 In the mixed ligand symmetrical dimer [W2⊲OBu t ⊳4⊲O3SCF3⊳2⊲PMe3⊳2]
(W W, 2.421 ˚A) the eclipsed conformation is found with the tert-butoxo
ligands trans to one another. 302 The symmetrical dimer [W2⊲hftb⊳4⊲NMe2⊳2] has
the staggered anti conformation. 321 The benzyltungsten isopropoxide diketonate
[⊲PhCH2⊳⊲tmhd⊳⊲Pr i O⊳W W⊲OPr i ⊳⊲tmhd⊳⊲CH2Ph⊳] (W W, 2.328 ˚A) also has a
nonbridged metal–metal triple bond with the ligands in a staggered conformation. 303 In
the symmetrical carbonyl complex [W2⊲tftb⊳6⊲CO⊳2] (W W, 2.450 ˚A) the two halves
of the molecule are eclipsed and the carbonyls are in the syn configuration. 304 Studies
on the chemical reactivity of the ditungsten species [⊲RO⊳3W W⊲OR⊳3] have produced
a multitude of interesting new dinuclear species containing a variety of different
bridging ligands.
In [W2⊲ -OMe⊳2⊲OMe⊳8] the edge-shared bi-octahedral structure (Fig. 4.2) was
found. 306 The diamagnetism and W–W distance (2.790 ˚A) indicated the presence
of a single W–W bond. A similar structure occurs in the W(V) chloride ethoxide
[W2⊲ -OEt⊳2⊲OEt⊳4Cl4] (W–W, 2.715 ˚A) in which the chlorines are cis and the
terminal ethoxo ligands trans to one another. 307 Interestingly the W(IV) complexes
[W2⊲ -OR⊳2⊲OR⊳2⊲ROH⊳2Cl4] (RD Me, Et; WDW, 2.481, 2.483 ˚A) have nearly the
same structure with an ROH ligand replacing a terminal alkoxo group on each
tungsten. Hydrogen bonding ROH....OR occurs between syn-alcohol and alkoxo
ligands across the metal–metal bond. 308 Exchange reactions with other alcohols R 0 OH
produced a series of mixed ligand complexes [W2⊲ -OR⊳2⊲OR 0 ⊳2⊲R 0 OH⊳2Cl4](RD Et,
R 0 D Pr i ;RD Et, R 0 D Et2CH) and [W2⊲ -OPr i ⊳2⊲OPr i ⊳2⊲Pr i OH⊳2Cl4] all having the
same overall edge-shared bi-octahedral structure. 309
Reaction of [W2⊲OEt⊳4Cl4⊲EtOH⊳2] with ketones R2CO gave rise to novel W(V)
dinuclear complexes [W2⊲ -OEt⊳2⊲OCR2CR2O⊳2Cl4] containing the bridging diolate
ligands in place of the terminal ethoxo and ethanol ligands (Fig. 4.46). 310 On the other
hand ˛-diketones R 0 CO.COR 0 reacted with ⊲RO⊳3WW⊲OR⊳3 to give centrosymmetrical
dinuclear molecules [W2⊲ -OR⊳2⊲OR⊳4⊲OCR 0 DCR 0 O⊳2] (RD Pr i ;R 0 D Me, p-tolyl;
W–W, 2.745, 2.750 ˚A) in which the alkylene diolate ligands are chelated to tungsten(V)
(Fig. 4.47). 311 In the W(VI) complex [W2⊲ -OBu t ⊳2⊲OBu t ⊳4⊲CNMe2⊳2] themetalis
five-coordinated (TBP) with a carbyne CNMe2 and a bridging Bu t O occupying axial
positions. The bridging Bu t O in the equatorial position is shorter (W–O, 1.95 ˚A) than
Figure 4.46 Structure of [W2⊲ -
OEt⊳2⊲OC2Me4O⊳2Cl4] (H atoms
omitted).