Mixed carboxylate ligands

Zirconium oxalates exist as compounds, double compounds, and mixed oxalato complexes (165,195,225—226). When the carboxylate ligand is a longer alkyl chain, the materials often are called zirconium soaps. 

With N,0 mixed donor ligands several complexes have been reported with ligands such as 4,5-dichloro-2-cyano-3,6-dione-l,4-cyclohexen-l-ol,1445 isonicotinic acid,1446 p-aminobenzoic acid,1447 alanine, histidine or histamine derivatives,1448-1450 [N(0)C(CN)2]-,1451 pyridine-carboxylate derivatives, 1452 1454 [N(pph20)2] (263),1455 bis(sulfonyl)amide derivatives,1456,1457 tris(pyridyl)-... 

Multinuclear mixed-metal complexes can be formed with carboxylate ligands. 

NMR studies of the mixed carboxylates containing acetates and trichloroacetates have indicated that ligand scrambling reactions occur in solution although all of the species were isolated, and isomerization possibilities occur, no discrete isomers were identified.329 Gas... 

Planar M30 units occur in the basic carboxylates of such metals as iron, ruthenium, manganese, vanadium and chromium the chromium compound has been known since 1919. The metal atoms are also linked by pairs of carboxylate ligands (often acetates), and have terminal ligands (generally pyridine or water). Mixed metal units, e.g. Fe2CrO,124 and mixed oxidation states, e.g. Ci CifO,1 ... 

Oxoalkoxocomplexes are oligomers of varying molecular complexity. The extent and conditions of distillation allow to control the nature of the species in solution, thus influencing the film-formation process [1368], This process of in situ modification ofmetal alkoxide solution by carboxylate ligand may have certain advantages with respect to the chemical uniformity as compared to the techniques based on simple mixing of a titanium alkoxide with alkaline-earth carboxylates. 

Reactions of different Re(V) starting materials with 2-(diphenylphos-phanyl)benzoic acid (DPPBA) gave only monosubstituted species, containing the [ReO(PCOO)]2+ unit, even when an excess of ligand was used. This unit also allowed the synthesis of mixed 3+2 complexes with tridentate Schiff bases (Scheme 3) [33,35]. X-ray structural analysis of some of the complexes (15-19) confirmed the bidentate coordination mode of the monoanionic phosphino-carboxylate ligand, and a distorted octahedral coordination geometry around the metal [35]. 

In the search for model systems, hundreds of new compounds have been made, those with carboxylate ligands being the most common. However, other ligands employed have been bipyridine, pyrazolylborates, triazacyclononanes, and Schiff bases of various sorts. These have been obtained in oxidation states II-V with both unitary and mixed valencies. The structures vary from quite simple to extremely complicated. Some of the core types are shown in Fig. 17-D-2. 

This last compound, treated as a mixed metal-carbon cluster Ic4(( ()) iT R (R = the one-electron carboxylate ligand COOMe) contains six skeletal bond pairs to hold together its five skeietai atoms (one carbon atom and four metal atoms). It is therefore a closo cluster, with the expected trigonal bipyramidal shape defined by its skeietai carbon and metai atoms. 

Very often, ligands contain more than one potential donor group. Where these are not identical, we have a mixed-donor ligand. These are extremely common, indeed the dominant class classic examples are the amino acids [H2N—CH(R)—COOH], which present both N (amine) and O (carboxylate) donors. Where there are choices of donor groups available to a metal ion, it is hardly surprising that some preference may exist - we shall return to a discussion of this aspect in Chapter 3.5. 

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