2 Homometallic Alkoxides

Metal Oxo-alkoxides 427
metals bonded to terminal oxo ligands trans to the 5-oxo ligand. The four -OMe
ligands bridge the apical Mo to the basal metal atoms which are linked to each other
by -oxo ligands. The basal Mo atoms also have terminal oxo ligands (trans to the
bridging -OMe oxygens) which are linked to the sodium atom. Each Mo is thus in a
distorted octahedral configuration.
5.6.3 Tungsten
Like molybdenum its congener tungsten is represented by many oxo-alkoxides
with various nuclearities. Among the mononuclear species the W(VI) anion
[W⊲O⊳2⊲OCMe2CMe2O⊳⊲CH2Bu t ⊳] exhibits trigonal pyramidal five-coordination
with the terminal oxo ligands occupying equatorial positions with the neopentyl
group in an axial site. 71 Two neutral octahedral complexes [W⊲O⊳⊲OBu t ⊳4⊲thf⊳] 164
and [W⊲O⊳⊲OSiMe2Bu t ⊳4⊲py⊳] 41 have been reported. In the mixed ligand neutral oxoalkoxide-aryloxide
[W⊲O⊳⊲OBu t ⊳2⊲OC10H5Me.C10H5MeO⊳] the oxo ligand occupies
an equatorial site in a distorted TBP structure. 165
The binuclear W(VI) complex [W2⊲O⊳2⊲ -O⊳2⊲OSiMe2Bu t ⊳4⊲py⊳2] has a ⊲ -O⊳2
edge-shared bioctahedral structure with the terminal oxo-ligands trans to the pyridines
whereas the W(IV) compound [W2⊲ -O⊳⊲ -OBu t ⊳⊲OSiMe2Bu t ⊳5⊲py⊳2] (W–W,
2.488 ˚A) has an unsymmetrical structure with octahedral and TBP metals bridged
by -oxo and -tert-butoxo ligands. 41 By contrast the W(VI) phenylimido complex
[⊲PhN⊳⊲Bu t O⊳2W⊲ -O⊳⊲ -OBu t ⊳2W⊲NPh⊳⊲OBu t ⊳2] has a confacial bi-octahedral
structure with a -oxo and two -OBu t bridges 40 and the phenylimido diolato complex
[⊲PhN⊳⊲OC2Me4O⊳⊲HOC2Me4O⊳W⊲ -O⊳W⊲NPh⊳⊲OC2Me4O⊳⊲OC2Me4OH⊳] has an
apex sharing bioctahedral structure. 166
The dimeric molecules [W2⊲O⊳2⊲ -OR⊳2⊲OR⊳6] (RD Me, c-C6H11) haveterminal
oxo ligands trans to the bridging alkoxo groups in an edge-shared bioctahedral
structure. 49
AnunsymmetricalbinuclearW(V) molecule wasfoundin[⊲CF3CO2⊳2⊲Bu t CH2O⊳2W⊲ -
OCH2Bu t ⊳⊲ -CMe2⊳W⊲O⊳⊲OCH2Bu t ⊳⊲py⊳] which had one octahedral and one TBP
metal. 36 In the related molecule [⊲Bu t CH2O⊳3W⊲ -OCH2Bu t ⊳2⊲ -CHC3H5-c⊳-
W⊲O⊳⊲OCH2Bu t ⊳⊲py⊳] (W–W, 2.659 ˚A) both metals achieve octahedral coordination
through the triple-bridged confacial bi-octahedral structure. 37
More examples of -oxo apex-sharing bi-octahedral structures are given by the diolato
complexes [⊲HOC2Me4O⊳⊲OC2Me4O⊳⊲O⊳W⊲ -O⊳W⊲O⊳⊲OC2Me4O⊳⊲OC2Me4OH⊳],
[⊲HOC7H12O⊳⊲O2C7H12-c⊳⊲O⊳W⊲ -O⊳W⊲O⊳⊲O2C7H12-c⊳⊲OC7H12OH⊳], and
[⊲HOC8H14O⊳⊲O2C8H14-c⊳⊲O⊳W⊲ -O⊳W⊲O⊳⊲O2C8H14-c⊳⊲OC8H14OH⊳]. 73 The novel
W(IV) binuclear anion [⊲Bu t O⊳3W⊲ -O⊳⊲ -H⊳W⊲OBu t 3 ⊳] (W–W, 2.445 ˚A) contains
five-coordinated tungsten (TBP). 26
The structure of [W3⊲ 3-O⊳⊲ 3-OPr i ⊳⊲ -OPr i ⊳3⊲OPr i ⊳6] has been reported 168 and
it is isomorphous with [Mo3⊲ 3-O⊳⊲ 3-OPr i ⊳⊲ -OPr i ⊳3⊲OPr i ⊳6]. 34 The trinuclear
compounds [W3⊲ 3-O⊳⊲ 3-OPr i ⊳⊲ -OPr i ⊳3⊲OPr i ⊳4R2] (RD CH2Ph, Ph) have related
structures. 169 A variation on these trinuclear structures was found in the W(IV) complex
[W3⊲ -O⊳2⊲OBu t ⊳8] which has an isosceles triangle of metal atoms (W–W, 2.45 ˚A,
2.93 ˚A) with tert-butoxo bridges across the two longer sides and no capping ligand.
Each tungsten atom has a different coordination with respect to oxygen (three, four
and five). 170