2 Homometallic Alkoxides

238 Alkoxo and Aryloxo Derivatives of Metals
together with lattice molecules of MeOH. Thus the compound is properly formulated
as f[Mg 4 ⊲OMe⊳6⊲MeOH⊳10] 2C [⊲MeO⊳2H] 2 [Mg 4 ⊲OMe⊳8⊲MeOH⊳8]8MeOHg. In
the neutral cubane structure each Mg is octahedrally coordinated by three 3-bridging
methoxides and three terminal ligands comprising one methoxo group and two methanol
molecules. In the di-cation each Mg also has three bridging methoxo groups but there
are only two methoxo groups with ten methanol molecules in the terminal positions.
From the Mg–O bond distances given in Table 4.2 (p. 241) it is evident that there is
little difference between bridging and terminal bond lengths although there is a large
variation in individual values. There is also extensive hydrogen bonding involving both
coordinated and lattice methanol molecules.
2.2.3 Calcium
Calcium, strontium, and barium dialkoxides containing less sterically demanding
alkoxo ligands are also insoluble and nonvolatile, and very bulky or
functionalized alkoxo groups are required to solubilize these derivatives. Only
three crystal structures of calcium alkoxides have been reported. The complex
[Ca9⊲OC2H4OMe⊳18⊲HOC2H4OMe⊳2] 43 involves four different bonding modes for the
alkoxo ligand with 6 seven-coordinated and 3 six-coordinated calciums (Fig. 4.12).
By using the very bulky ligand OR (R D CPh2⊲CH2C6H4Cl)) the dimeric molecule
[Ca⊲ -OR)(OR)(thf)]2 was obtained with calcium exhibiting a distorted tetrahedral
configuration. 44 With the triphenylsiloxide ligand and ammonia as a supporting ligand
the dimeric molecule [Ca2⊲ -OSiPh3⊳3⊲OSiPh3⊳⊲NH3⊳4] is formed. 45 The structure is
unsymmetrical (Fig. 4.13) with one octahedrally coordinated Ca having one siloxide
and two NH3 ligands in terminal positions with three facially bridging siloxides
connected to a five-coordinated Ca containing two terminal NH3 molecules. The
terminal bonded siloxide ligand has the shortest Ca–O bond length and the widest
Ca � OSi bond angle whilst the five-coordinated Ca has shorter Ca–O bond lengths
than the six-coordinated Ca and smaller Ca � OSi angles than the six-coordinated Ca
(Table 4.2). See also [Ca2f -OC⊲CH2Pr i ⊳2Bu t g4]. 444
2.2.4 Strontium
The only strontium alkoxo crystal structures reported so far all involve siloxy ligands.
For example the triphenylsiloxide of strontium forms a mononuclear complex with
THF and 15-crown-5 as supporting ligands in [Sr⊲OSiPh3⊳2(15-crown-5)(thf)]. 46 The
Sr atom is eight-coordinated to five oxygens from the 15-crown-5 ligand, two from
the siloxides and one from the THF ligand. Barium forms an analogous complex.
In the mixed ligand complex [Sr3⊲ 3-OSiPh3⊳2⊲ -OSiPh3⊳⊲tmhd⊳3] (tmhdD 2,2,6,6tetramethylheptane-3,5-dionate)
a trinuclear molecule is formed. 47 One Sr atom is
in a distorted octahedral configuration and the other two are five-coordinated in
distorted trigonal-bipyramidal (TBP) configurations. The three metals are in a triangular
arrangement capped by two 3-OSiPh3 ligands and bridged by one -OSiPh3 and
two -tmhd ligands in the metal plane. One tmhd is chelating and the other two
are chelating and bridging. The two five-coordinated strontiums are involved in
agostic interactions with neighbouring carbon atoms. Another trinuclear species was
obtained with the tetraphenyl disiloxanediolate ligand in conjunction with tetraglyme