2 Homometallic Alkoxides

426 Alkoxo and Aryloxo Derivatives of Metals
the other cluster giving the central 6-oxo ligand. The Mo6O19 core is of course
reminiscent of the transition metal isopolyanions.
Another structural motif was exhibited by the structure of the mixed ligand hexanuclear
molecule [Mo6⊲O⊳6⊲ 3-O⊳2⊲ -OEt⊳6⊲ -Cl⊳2Cl6] with a cyclic array of six
Mo(V) atoms in a chair conformation. The six valence electrons are paired off in three
Mo–Mo bonds (2.665 ˚A, 2.672 ˚A, 2.741 ˚A) alternating around the ring. The 3-oxo
ligands are in planar three-coordination (T-shaped). 162
The octanuclear Mo(VI) complex anion [Mo8⊲O⊳16⊲ 4-O⊳2⊲ 3-O⊳4⊲ -O⊳2⊲ -
OMe⊳2⊲OMe⊳2] 4 has an Mo8O28 framework similar to that found in various
substituted octamolybdates. 52
Another octanuclear Mo(V) complex [Mo8⊲O⊳8⊲ -O⊳8⊲ -OMe⊳8⊲PMe3⊳4] contains
a puckered ring of eight Mo atoms. The molecule comprises a tetramer of the
[Mo2⊲O⊳2⊲ -O⊳2⊲ -OMe⊳2⊲PMe3⊳] moiety which involves Mo–Mo bonds (2.567 ˚A)
between the Mo⊲ -O⊳2Mo pairs. Four of the metals which have coordinated PMe3
ligands are octahedrally coordinated and the other four are five-coordinated (square
pyramidal). 163
In the octanuclear Mo(VI) anion [Mo8⊲O⊳14⊲ -O⊳6⊲ 3-OMe⊳2⊲OMe⊳2f⊲OCH2⊳3CMeg2] 2
the centrosymmetric species may be viewed as two [Mo4⊲O⊳7⊲ -O⊳2⊲ 3-
OMe⊳3⊲OMe⊳f⊲OCH2⊳3CMeg] units joined by two ( -O) bridges (Fig. 5.19). 58
A most remarkable mixed ligand heterometallic oxide-alkoxide anion
[Ag 2 Mo10⊲O⊳8⊲ 5-O⊳2⊲ -O⊳16⊲ -OMe⊳8⊲NO⊳2] 4 has been characterized. 167a The
centrosymmetrical anions consist of two Mo5⊲O⊳4⊲ 5-O⊳⊲ -O⊳8⊲ -OMe⊳4⊲NO⊳ units
connected by four Mo⊲ -O⊳Ag⊲ -O⊳Mo bridges involving square planar AgO 4 with a
short Ag....Ag distance (2.873 ˚A). All of the molybdenums are in distorted octahedral
configurations.
The pentanuclear Mo5O13⊲OMe⊳4⊲NO⊳ moiety was earlier found in the salts
of the heterometallic alkali metal complexes ⊲NBu4⊳2[Mo5⊲O⊳4⊲ 5-O⊳⊲ -O⊳8⊲ -
OMe⊳4⊲NO⊳⊲Na,MeOH⊳], K2[Mo5⊲O⊳4⊲ 5-O⊳⊲ -O⊳8⊲ -OMe⊳4⊲NO⊳fNa⊲H2O⊳⊲MeOH⊳g],
⊲NMe4⊳2[Mo5⊲O⊳4⊲ 5-O⊳⊲ -O⊳8⊲ -OMe⊳4⊲NO⊳⊲Na,H2O⊳]and[NBu4⊳2[Mo5⊲O⊳4⊲ 5-
O⊳⊲ -O⊳8⊲ -OMe⊳4⊲Na,DMF⊳]. 167b Each complex has a square pyramid of Mo atoms
with the apical metal bonded to a linear NO ligand trans to the 5-oxo and the basal
Figure 5.19 Structure of [Mo8⊲O⊳14⊲ -
O⊳6⊲ 3-OMe⊳2⊲OMe⊳2f⊲OCH2⊳3CMeg2] 2
(ž D Mo; H atoms omitted).