Bimetallic alkoxides, homoleptic

Chelation of a central metal atom by ligands (represented in general by L) like A1(0R)4 , Nb(OR)6 , Ta(OR>6, and Zr2(OR>9 gives rise to a large number of homoleptic bimetallic alkoxides, which can be represented by ML c (where x is the principal valency and 2x is the coordination number of M) this coordination number would be higher (3x or 4x) when the alkoxometallate ligand is tri- or tetradentate e.g. (before six-coordinate) nickel in Ni (/r-OMe)3Al(OMe) 2 (Section 3.3.1). 

In addition to the homoleptic bimetallic derivatives like [M Al(OPr )4 j] hetero-leptic tri- and tetrametalUc derivatives with the general formula, [M Al(OIV)4 a Nb(OPr )6 6 Zr2(OPr )9 cX c a c] (where X is a ligand of the type Cl, OR, acac, etc.) have also been synthesized in the laboratories of Mehrotra " since 1985. Many of these have been shown to volatilize unchanged under reduced pressure. These stable derivatives have thus added a novel dimension to heterometallic coordination systems. The prominent effect of steric factors on the stability and structural features of some bimetallic species has been observed in a comparison of similar pairs (i) [NaZr2(OPr )9] and [ NaZr(OBu )s 2] (ii) [Ni (/4-OMe)3Al(OMe) 2] and [Ni (/4-OPr )(/i-OBu )Al(OBu )2 2]." Some other novel types of heterometallic alkoxide/glycolate/aminoalkoxide derivatives have also been synthesized." ... 

The alkoxides can be arranged in four general groups homoleptic alkoxides of the Ln(OR)3 type, mixed organo/alkoxides RxLn(OR )3.x, halide/alkoxides Ln(OR)xHal3 j and similar compounds including oxo/alkoxides and bimetallic complexes Ln[M(OR)j3. 

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