2 Homometallic Alkoxides

234 Alkoxo and Aryloxo Derivatives of Metals
formed. 25 Bond length data on these and other alkoxo lithium complexes are presented
in Table 4.1. Also included are some structurally characterized complexes involving
LiOBR2 and LiOSiPh3 molecules.
2.1.2 Sodium
Sodium methoxide is isostructural with lithium methoxide 33 whereas the tert-butoxide
has a novel structure containing both hexameric and nonameric molecules. 34 The triphenylsiloxide
[Na4⊲OSiPh3⊳4⊲H2O⊳3] has a cubane Na4O4 framework but three of the
four Na atoms are terminally bonded by water molecules. 35
2.1.3 Potassium, Rubidium, and Caesium
The methoxides of potassium, rubidium, and caesium are isostructural, with a layer
structure different from the lithium and sodium analogues. 36 The K, Rb, and Cs
atoms are five-coordinated in a tetragonal pyramidal configuration. The tert-butoxides
of K, Rb, and Cs all form tetranuclear molecules with the cubane structure 37
as do the trimethyl siloxides [MOSiMe3]4. 38 However, it was shown that the
solvates [MOBu t .Bu t OH] (M D K, Rb) crystallize in one-dimensional chain structures
(Fig. 4.10) in which the metals are in distorted tetrahedral configurations and
strong hydrogen bonds link the asymmetric MOBu t .Bu t OH units. 39 Some phenylsiloxo
complexes of potassium have also been structurally characterized. In [K(18-crown-
6)(OSiPh3⊳] there are dimers due to bridging involving the ether oxygens giving the
potassiums an eight-coordination. 30 With dimethoxyethane (DME) as supplementary
ligand the novel complex [K8⊲OSiPh3⊳8⊲MeOC2H4OMe⊳3] was obtained in which two
K4O4 cubane units are linked by a DME molecule (Fig. 4.11). 30 The other DME
molecules each chelate one potassium whilst one Ph3SiO group in each K4O4 unit is
also involved in ipso-C....K bonding (C–K D 3.078 ˚A) with one of its phenyl groups.
Each potassium is in a different coordination environment with coordination numbers
of 3, 4, and 5. In the complex [KOSiPh2⊲OSiPh2⊳O.SiPh2OH]2 the dimeric molecule
is formed by the silanolate bridges and the potassiums each achieve three-coordination
by bonding with the terminal SiOH oxygen of the hexaphenyl trisiloxane ligand which
thereby forms an eight-membered ring. 40 Interestingly the shortest ipso-C....K bonds
present are 3.157(5) ˚A and 3.185(5) ˚A.
The directional nature of the M–O bonds in the alkali metal alkoxides together with
the solubility of these compounds in organic solvents and the absence of electrical
conductivity suggests that they are significantly covalent in nature.
2.2 Beryllium, Magnesium, and Alkaline Earth Alkoxides
2.2.1 Beryllium
Beryllium dialkoxides are generally insoluble nonvolatile compounds, but the use
of bulky alkoxo groups imparts solubility and volatility. Thus the tert-butoxide is
trimeric 42 and the triethylcarbinol derivative [Be(OCEt 3⊳2] is dimeric, 3 but to date no
crystal structure has been reported.