2 Homometallic Alkoxides

Homometallic Alkoxides 51
First homoleptic dimers of tungsten(V) (Eq. 2.166) and rhenium(V) (Eq. 2.167) have
been prepared 429 according to the following reactions:
WF6 C Me3SiOMe ! WF2⊲OMe⊳4
ReF6
⊲i⊳ Si⊲OMe⊳4
!
CH3CN, 40°C
⊲i⊳ Li
!
THF W2Fx⊲OMe⊳10 x
⊲x D 1–3⊳
⊲ii⊳ Mg⊲OMe⊳2⊲HOMe⊳2
! Re2⊲OMe⊳10
THF
⊲ii⊳ NaOMe
!
THF
W2⊲OMe⊳10
⊲2.166⊳
(2.167)
An unusual exchange reaction takes place when the organoactinide hydrides
[AnH2⊲ 5 -C5Me5⊳2]2 are treated with trimethyl phosphite. 430 A methoxo group is
transferred to the metal atom while phosphorus is hydrogenated to give a PH bridge:
5[AnH2⊲ 5 -C5Me5⊳2]2 C 4P⊲OMe⊳3
toluene
! fAn⊲OMe⊳⊲
25°C
5 -C5Me5⊳2g2PH C 3An⊲OMe⊳2⊲ 5 -C5Me5⊳2 C 3H2 ⊲2.168⊳
In contrast to the expected elimination of an ester in the reaction of Sn⊲OBut⊳4 with
Sn⊲OAc⊳4 as well as with Me3SnOAc in a refluxing hydrocarbon solvent, only ligand
exchanges of the types shown below have been demonstrated to occur in coordinating
solvents such as pyridine: 431
3Sn⊲OBu t ⊳4 C 3Sn⊲OAc⊳4
pyridine
! 2Sn⊲OBu t ⊳⊲OAc⊳3 C 2Sn⊲OBu t ⊳2⊲OAc⊳2 C 2Sn⊲OBu t ⊳3⊲OAc⊳ ⊲2.169⊳
Sn⊲OBu t ⊳4 C Me3SnOAc pyridine
! Sn⊲OBu t ⊳3⊲OAc⊳⊲py⊳ C Me3Sn⊲OBu t ⊳ ⊲2.170⊳
The X-ray crystallographically characterized 431a derivative of zinc, Zn2⊲OAc⊳3(OMe),
has been prepared by the reaction shown in Eq. (2.170a):
2Zn⊲OAc⊳2 C Bu n 2 Sn⊲OMe⊳2
3 PHYSICAL PROPERTIES
3.1 Introduction and Structural Characteristics
! Zn2⊲OAc⊳3⊲OMe⊳ C Bu n 2 Sn⊲OAc⊳⊲OMe⊳
⊲2.170a⊳
The physical properties of the metal alkoxides [M(OR)x]n are largely influenced by
the size and shape of the alkyl (R) group as well as by the valency (x), atomic
radius, stereochemistry, and coordination number of the metal. Owing to the high
electronegativity of oxygen (3.5 on Pauling scale), the M–OR bonds (in alkoxides
of metallic elements) would be expected to possess significant ionic character. Thus
metal–oxygen bonds (M υC —O υ —C) in metal alkoxides could be expected to possess
around 65% ionic character for metals with electronegativity values of the order of
1.5–1.3 (aluminium, titanium, and zirconium) to about 80% ionic character for more