2 Homometallic Alkoxides

⊲T⊳, ⊲U⊳ C 2Al⊲OPr i ⊳3
⊲T⊳, ⊲U⊳ C 2Nb⊲OPr i ⊳5
Heterometallic Alkoxides 201
benzene
! [fAl⊲OPr i ⊳2g2fM 0 ⊲tea⊳2g] C 2Pr i OH "
(V), (W) Soluble, monomeric solids
benzene
! [fNb⊲OPr i ⊳4g2fM 0 ⊲tea⊳2g] C 2Pr i OH "
(X), (Y) Soluble, monomeric solids
⊲3.114⊳
⊲3.115⊳
All the products (V), (W), (X), and (Y) are white crystalline solids, soluble in hydrocarbon
solvents, out of which the titanium–niobium product fNb⊲OPr i ⊳4g2fTi⊲tea⊳2g
could be distilled 175 unchanged at 205 Ž C/0.05 mm.
A survey of the literature has revealed 176 that the triethanolaminate lanthanum derivative,
La⊲H2tea⊳⊲Htea⊳ reacts with 3 moles of Nb⊲OPr i ⊳5 to give a heterobimetallic
derivative (Eqs 3.116 and 3.117), which has been characterized by X-ray crystallography:
La⊲OPr i ⊳3 C 2H3tea
La⊲H2tea⊳⊲Htea⊳ C 3Nb⊲OPr i ⊳5
room temp.
! La⊲H2tea⊳⊲Htea⊳ C 3Pr
Insoluble
i OH " ⊲3.116⊳
dissolution
!
r.t.
fLa⊲tea⊳2gfNb⊲OPr i ⊳4g3
Soluble
C3Pr i OH " ⊲3.117⊳
The heterobimetallic La–Nb product is readily soluble in hydrocarbons and sublimes
at 180–250 Ž C under 0.1 mm pressure.
3 PROPERTIES OF HETEROMETALLIC ALKOXIDES
3.1 Introduction
A close parallelism in the properties of homo- and heterometal alkoxides has been
elucidated by Mehrotra and Singh. 47 The role of mainly steric (coupled with inductive)
factors was established in the homometal species since the early 1950s by a systematic
study of the oligomerization and volatility of the alkoxides of metals (groupwise) in a
number of publications from the research school of Bradley. 177 Two interesting publications
emphasizing the similarity of the neopentyloxides of titanium and zirconium 178 to
their secondary amyloxides, followed by a similar conclusion for aluminium 179 tended
to indicate the more significant role of steric factors. Bradley 180 made a highly significant
contribution in 1958 by his conclusion that metals in homometal alkoxides generally
tend to attain the higher (preferred) coordination number by the lowest possible
degree of oligomerization through alkoxo( 2-/ 3-) bridges between similar metals. A
similar generalization was arrived at by Caulton et al. 181 that the ratio of m/⊲a C b⊳ in
closed polyhedral structures of heterobimetallic alkoxides, [MaM 0 b ⊲OR⊳m] tends to be
typically low. All the M and M 0 metal atoms are enclosed in a closed polyhedron by
2- and 3-OR-type bridging to yield a compact structural unit; the heterometal atoms
try to fit themselves in convenient geometries: the metal atoms in [KZr2⊲OPr i ⊳9] 64
and [BaZr2⊲OPr i ⊳10] 131 arrange themselves in a triangular pattern whereas the metal
atoms of [K2Zr2⊲OPr i ⊳10] 131 and [K4Zr2O⊲OPr i ⊳10] 64 are arranged respectively in tetrahedral
and octahedral geometries. The last example, [K4Zr2O⊲OPr i ⊳10], depicts the