Chemistry Reference
In-Depth Information
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
LiOBR
2
and LiOSiPh
3
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 tri-
phenylsiloxide [Na
4
OSiPh
3
4
H
2
O
3
] has a cubane Na
4
O
4
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 [MOSiMe
3
]
4
.
38
However, it was shown that the
solvates [MOBu
t
.
Bu
t
OH] (M D K, Rb) crystallize in one-dimensional chain struc-
tures (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)(OSiPh
3
] 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 [K
8
OSiPh
3
8
MeOC
2
H
4
OMe
3
] was obtained in which two
K
4
O
4
cubane units are linked by a DME molecule (Fig. 4.11).
30
The other DME
molecules each chelate one potassium whilst one Ph
3
SiO group in each K
4
O
4
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 [KOSiPh
2
OSiPh
2
O
.
SiPh
2
OH]
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.
