Square-planar complexes, in solution

Despite this attention, however, major areas of uncertainty persist. Perhaps the most fundamental of the unanswered questions is the nature of square-planar complexes in solution. It has yet to be established if they are solvated at the metal center (and if so, how, since these complexes can undergo both nucleophilic and electrophilic attack). A number of structural studies reported in the section on 5-coordination (Section 5.7) highlight these uncertainties. 

Despite the richness of this field of study, however, a number of anomalies and some major uncertainties remain. One curious feature, which is probably no more than an historical accident, is that though most studies of nucleophilic ligand substitution have been carried out on complexes of platinum(II) and palladium(II) and few on complexes of rhodium(I) and iridium(I), the reverse distribution is apparent for studies of oxidative additions. The scope for rectifying this imbalance is vast. On the other hand, a fundamental and persistent uncertainty in this field of study concerns the very nature of square-planar compounds in solution. We address this problem in some detail. 

Addition of halide ions to aqueous copper(II) solutions can give a variety of halo-complexes for example [CuCl4] (yellow square-planar, but in crystals with large cations becomes a flattened tetrahedron) [CuClj] (red, units linked together in crystals to give tetrahedral or distorted octahedral coordination around each copper). 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

As seen, the square planar complex exhibits in MeCN solution a reversible one-electron oxidation (Eo1 = -0.12 V vs. Fc/Fc+ AEp = 70 mV, at 0.05 V s-1), which suggests the maintaining of the square planar CuN4 coordination. In fact, as illustrated in Figure 126, the Cu(III) derivative [Cu(Hmal)] is square planar.182... 

SlOO proteins, calcium binding, 46 451-456 Spruhtrocken process, 4 26 Square-planar complexes, 4 157-164 octahedral, compared, 4 162-174 in solution, 34 270-271 Square-planar iridium complexes, 44 295, 297 Square-planar nickel macrocyclic complexes equilibrium with octahedral species, 44 116-118... 

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

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