2 Homometallic Alkoxides

206 Alkoxo and Aryloxo Derivatives of Metals
a thermodynamically favoured heterometal system appears to be governed by parameters
like size and electronegativity of the component metals as well as the ramification
of the alkyl group and the presence of electronegative (e.g. fluorine) or coordinating
substituents (e.g. alkoxy, amino, etc.) on the alkyl group(s) involved.
Since the basic information available from physico-chemical techniques (e.g. conductivity,
spectroscopy, etc.) is essentially similar to that available from the corresponding
homometal systems, it will not be repeated here (see Chapter 2, Section 3), but applications
are illustrated for a few typical systems in Section 3.3. Further, although more
definitive information is available from crystal structure elucidations, this will not be
dealt with in this chapter, in view of the separate treatment in Chapter 4.
3.3 Properties of Tetra-alkoxoaluminate and Hexa alkoxoniobate
(-tantalate) Derivatives
As the above three ligands generally function in a bidentate manner, their heterometallic
coordination alkoxides resemble one another in forming derivatives of the type
R
M O Al(OR) 2 and
O
R
R
M O Nb(OR) 4
O
R (Ta)
the main difference being that the ligated aluminium atoms are tetrahedral whereas
the niobium (tantalum) atoms are octahedral. These are, therefore, being dealt with
together.
3.3.1 Tetraalkoxoaluminate Derivatives
Of the tetra-alkoxoaluminates of various alkali metals (Li, Na, K, Rb, Cs), the potassium
derivative was found in 1976 205 to show the maximum solubility and volatility.
This solubility behaviour was ascribed to the suitability of the size of potassium
enabling the formation of a di-isopropanolate adduct,
Pr i O
Pr i O
Pr i
Al O K
O Pr i
on the lines of the well-known suggestion by Sidgwick and Brewer in 1925 about the
formation of an adduct
Ph
C
HC
C
Me
O
O K
Pr i
O
O
Pr i
OH2
H
OH 2
H