2 Homometallic Alkoxides

282 Alkoxo and Aryloxo Derivatives of Metals
(Me2pz D 3,5-dimethylpyrazolylato; R D R 0 D Pr i and R D Et, R 0 D Pr i ) containing
linear Mo–N–O angles. 218 A similar structure was found in the compound
[Mo⊲OPr i ⊳2⊲NO⊳fHB⊲impz⊳3g] [HB⊲impz⊳3 D isopropylmethylpyrazol-1-yl]. 219 It is
noteworthy in these formally Mo(II) complexes that the Mo–O bond distances of
the alkoxo–Mo groups are relatively short. The formally Mo(VI) nitrido complexes
[Mo⊲ -N⊳⊲OR⊳3]n (R D Pr i , Bu t ) have infinitely linear structures due to bridging
involving the Mo N ! Mo system. The long dative N ! Mo bond (2.88 ˚A, cf.
1.66 ˚A for Mo N) suggests a weak interaction and the complex is fully dissociated
in benzene solution. 220 Interestingly, methylation of the tert-butoxo complex in
the presence of 4-Bu t pyridine gave rise to the tetranuclear nitrido-bridged species
[Mo4⊲ -N⊳4⊲OBu t ⊳4Me8⊲NC5H4-4Bu t ⊳2]. 221 The eight-membered Mo4⊲ -N⊳4 ring has
alternating short (1.664 ˚A, 1.689 ˚A) and long (2.147 ˚A, 2.209 ˚A) Mo–N bonds with
two five-coordinated and two six-coordinated Mo atoms. Reaction of MoN⊲OBu t ⊳3
with ethyleneglycol and pinacol gave the tris-chelated [Mo⊲OC2H4O⊳3] and
[Mo⊲OCMe2CMe2O⊳3] complexes with Mo(VI) in distorted octahedral coordination. 222
The metal–metal triple bonded diamagnetic dimers Mo2⊲OR⊳6 are unique in having
no bridging alkoxo ligands (Fig. 4.8). The structure of Mo2⊲OCH2Bu t ⊳6 showed that
the Mo2O6 system had virtual D3d symmetry with ethane-like geometry. 223 With
less sterically demanding groups (OMe, OEt) polymeric compounds [Mo⊲OR⊳3]n are
formed whereas the bulkier ligands (OPr i ,OBu t , OSiMe3, OSiEt3) all give dinuclear
Mo Mo species Mo2⊲OR⊳6. On the other hand the Mo(IV) tetraisopropoxide forms
[Mo2⊲ -OPr i ⊳2⊲OPr i ⊳6] with unsymmetrical isopropoxo bridges and five-coordinated
molybdenum. 224 The Mo2O8 system is composed of two MoO5 trigonal bipyramids
joined along a common axial–equatorial edge. The two terminal ligands in equatorial
sites have shorter Mo–O bond lengths (1.872 ˚A, 1.884 ˚A) than the one in the axial
position (1.976 ˚A).
The dinuclear species Mo2⊲OR⊳6 form adducts with a variety of Lewis bases
[Mo2⊲OR⊳6L2] (L D NH3, MeNH2, Me2NH, Me3N, Me2PhP) and the structure of
[Mo2⊲OSiMe3⊳6⊲Me2NH⊳2] has been determined. 225 Reversible CO2 insertion also
occurs with Mo2⊲OR⊳6 and the structure of [Mo2⊲O2COBu t ⊳2⊲OBu t ⊳4] shows that the
O2COBu t groups bridge across the two Mo atoms, and the tert-butoxo groups are all
in terminal positions (Fig. 4.38). 226
Figure 4.38 Structure of [Mo2⊲ -
O2COBu t ⊳2⊲OBu t ⊳4]( D O;ž D Mo;
H atoms omitted).