2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 285
1,2-Mo2⊲OBu t ⊳2⊲CH2SiMe3⊳4 is ethane-like ⊲C2h symmetry). 235 Addition of dialkyl
cyanamides R2NCN to Mo2⊲OR⊳6 compounds produced [Mo2⊲OR⊳6⊲ -NCNR2⊳] complexes
and the crystal structure [Mo2⊲OR⊳6⊲ -OCH2Bu t ⊳⊲ -NCNMe2⊳⊲OCH2Bu t ⊳5]
was determined. One Mo atom is bonded to two terminal Bu t CH2O ligands, the 2 -CN
portion of the cyanamide and one bridging Bu t CH2O ligand. The second Mo is bonded
to three terminal Bu t CH2O ligands, the bridging nitrogen of the cyanamide and the
bridging Bu t CH2O ligand. The terminal Bu t CH2O trans to the bridging cyanamide
nitrogen has a longer Mo–O bond length (1.954 ˚A) than the other terminal neopentoxide
groups (av. 1.87 ˚A) whilst the alkoxo bridge is unsymmetrical (Mo–O, 1.999,
2.146 ˚A). 236
In the dimer [Mo2⊲ - 1 , 1 -O2CPh⊳2⊲OBu t ⊳4] the Mo Mo unit (d, 2.236 ˚A) is
spanned by a pair of cis bridging benzoato ligands with terminal Bu t O ligands
trans to each of the benzoate oxygens. 237 By contrast in the acetylacetonate
[Mo2⊲OBu t ⊳4⊲acac⊳2] (Mo Mo, 2.237 ˚A) the diketonato groups are chelating
in preference to bridging. 238 Addition of tert-butyl isonitrile gave the edgebridged
bi-octahedral molecule [Mo2⊲ -OCH2Bu t ⊳2⊲OCH2Bu t ⊳2⊲CNBu t ⊳2⊲acac⊳2]. 239
Alkoxo-ligands in Mo2⊲OR⊳6 can be replaced by thiolato groups as in
[Mo2⊲OPr i ⊳2⊲SAr⊳4] (Ar D 2,4,6-trimethylphenyl) where the M M bond distance
(2.230 ˚A) is typical for this class of compound. 240 The arylselenato complex
[Mo2⊲OPr i ⊳2⊲SeAr⊳4] (Mo Mo, 2.219 ˚A) is similar. 299 In the novel bromo complex
[Mo2⊲ -OCH2Bu t ⊳2⊲ -Br⊳⊲OCH2Bu t ⊳4Br(py)] each Mo is in a distorted octahedral
environment and the Mo–Mo distance (2.534 ˚A) implies metal–metal double
bonding. 241
Reaction of Mo2⊲OBu t ⊳6 with diphenyldiazomethane gave the mononuclear
five-coordinated (TBP) [Mo⊲OBu t ⊳4⊲NNCPh2⊳] where the equatorial Mo–O bond
lengths (av. 1.89 ˚A) are shorter than the axial Mo–O bond (1.944 ˚A). With
Mo2⊲OPr i ⊳6 in the presence of pyridine the binuclear confacial bi-octahedral
complex [Mo2⊲ -OPr i ⊳3⊲NNCPh2⊳2⊲py⊳] with Mo–Mo D 2.66 ˚A indicative of a single
metal–metal bond. 242 One Mo atom is coordinated to an NNCPh2 ligand, two terminal
Pr i O ligands and three bridging Pr i O ligands whilst the other Mo is coordinated to one
NNCPh2 ligand, one terminal Pr i O, one pyridine, and three bridging Pr i O ligands.
The bridges are distinctly unsymmetrical. The binuclear phenylisocyanate insertion
product [Mo2⊲OPr i ⊳4fNPhC⊲O⊳⊲OPr i ⊳g2] hastwocis bridging bidentate ligands and
four terminal isopropoxo ligands with a Mo Mo (2.221 ˚A). 243
The quadruply metal–metal bonded dimers [Mo2⊲OPr i ⊳4⊲py⊳4], [Mo2⊲OCH2Bu t ⊳4
⊲PMe3⊳4], [Mo2⊲OCH2Bu t ⊳4⊲Me2NH⊳4], and [Mo2⊲OPr i ⊳4⊲Pr i OH⊳4] are of special
interest (Mo Mo; 2.195, 2.209, 2.133, 2.110 ˚A, respectively) in having no bridging
ligands. 244 Generally in the [Mo2⊲OR⊳4L4] complexes the OR groups are cis to the L
ligands and the two halves of the molecule are in the eclipsed conformation (Fig. 4.41).
When L D Pr i OH or Me2NH there is hydrogen bonding RO....HOR(HNMe2) across
the metal–metal bond which is thereby shortened. The same situation occurs in
[Mo2⊲OC5H9-c⊳4⊲c-C5H9OH⊳4] although it was not possible to distinguish c-C5H9O
ligands from c-C5H9OH in this structure. 245
The unsymmetrical Mo Mo (2.235 ˚A) complex [Mo2⊲OPr i ⊳4⊲CH2Ph⊳2⊲PMe3⊳]
has one molybdenum bonded to benzyl groups and one PMe3 ligand. 246,247
With the bidentate phosphine ⊲Me2P⊳2CH2 (dmpm) the symmetrical dimer