2 Homometallic Alkoxides

Metal Oxo-alkoxides 425
latter complex the third metal is cis-bonded to two terminal oxo ligands and a terminal
methoxo group.
It is noteworthy that one of the earliest examples of a tetranuclear molybdenum oxoalkoxide,
the mixed ligand species [Mo4⊲O⊳4⊲ 3-O⊳2⊲ -OPr n ⊳4⊲OPr n ⊳2Cl4], has a core
framework Mo4O12Cl4 analogous to the M4O16 core of the titanium tetra-alkoxides.
The presence of metal–metal bonding was indicated by Mo–Mo, 2.669 ˚A intwopairs
of metals. 155 A similar structure was found in [Mo4⊲O⊳4⊲ 3-O⊳2⊲ -O⊳2⊲ -OPr i ⊳2⊲py⊳4]
(Mo–Mo, 2.600 ˚A). 156 However, the n-propoxide reported earlier 155 was reformulated
as an Mo(V) complex [Mo4⊲O⊳4⊲ -O⊳2⊲ -OPr n ⊳4⊲Pr n OH⊳2Cl4]. 157 The capping
nature of the pyrazolylborato ligand (HB⊲pz⊳3] generated a tetranuclear species
[Mo4⊲O⊳4⊲ -O⊳4⊲ -OMe⊳2⊲MeOH⊳2fHB⊲pz⊳3g2] having a zig-zag chain of metal
atoms. 157 Using the hmmp ligand the neutral centrosymmetric tetranuclear complex
[Mo4⊲O⊳4⊲hmmp⊳2⊲OEt⊳2] was obtained. 158 By contrast the tetranuclear anion
[Mo4⊲ 3-O⊳⊲ -OCH2Bu t ⊳4⊲OCH2Bu t ⊳7] has a “butterfly” configuration of the Mo4
unit (Mo–Mo, 2.422 ˚A, 2.484 ˚A, 2.608 ˚A) with one Mo3 triangle being capped by the
3-oxo ligand. One wingtip Mo is octahedrally coordinated but the other metals are
only four-coordinated to oxygen donor ligands. 159
The hexanuclear molecule [Mo6⊲O⊳6⊲ -O⊳4⊲ -OPr i ⊳6⊲OPr i ⊳6] contains a remarkable
serpentine chain of six coplanar Mo atoms linked by bridging oxo or isopropoxo ligands.
The structure is centrosymmetrical (Fig. 5.18) with the exterior octahedrally coordinated
Mo(VI) atoms edge-sharing ⊲ -OPr i ⊳2 with adjacent five-coordinated (square pyramidal)
Mo(V) atoms which in turn are ⊲ -oxo⊳2 bridged and metal–metal bonded (Mo–Mo,
2.585 ˚A) to the inner Mo(V) atoms. 34,160
The interesting 6-oxo centred hexanuclear species [Mo6⊲ 6-O⊳⊲ -OEt⊳12⊲OEt⊳6]
has a distorted octahedron of Mo atoms joined by -OEt bridges along all twelve
edges of the octahedron. Each metal also has one terminal ethoxo ligand. 161 The
average oxidation state is 3.33 [2xMo⊲IV⊳ C 4xMo⊲III⊳] and it is the metal–metal
bonding which causes the distortion of the Mo6 core. The structure can be viewed
as two trinuclear [Mo3⊲ 3-O⊳⊲ -OEt⊳6⊲OEt⊳3] clusters each containing eight electrons
for Mo–Mo bonding (2.611 ˚A) and sharing the 3-oxo and three -OEt ligands with
Figure 5.18 Structure of [Mo6⊲O⊳6⊲ -
O⊳4⊲ -OPr i ⊳6⊲OPr i ⊳6] (ž D Mo; H
atoms omitted).