Platinum penta-coordinate complex

In general, penta-coordinated complexes are rather rare. So far as nickel, palladium, and platinum are concerned, it seems that such complexes may be formed as intermediates when square-planar complexes accept further ligands in their tendency to become hexa-coordinated. The... 

Complex C (Scheme 21) seems to shows stable penta-coordination for apparently very different reasons. The compound is cationic, but the counterion is a non-coordinating tetraarylborate. Interestingly, neither the dichloromethane solvent nor the diethylether present in the reaction mixture seem to coordinate to the open site in solution. The compound was crystallized from a tetrahydrofuran/pentane mixture as the tetrahy-drofuran (THF) solvate, but in the crystal structure, the THF is remote from the open site at platinum. The open site is shielded somewhat by the methyl groups of the protonated TpMe2 ligand, but it does not appear completely inaccessible. A reasonable explanation for the... 

When the apparently penta-coordinated diarsine complexes just described are dissolved in solvents more polar than nitrobenzene, they tend to dissociate into halide ions and bivalent cations, thus becoming 2 1 electrolytes (119). The effect is most marked with the platinum compounds. It has been shown that solvation effects might be less with platinum than with palladium, and so, other things in the equilibria being equal, it can also be concluded that the bonding of further ligands by a square-planar complex is much weaker with platinum than with palladium. Square-planar nickel complexes are of course the most ready to take up further ligands. 

To prepare complexes with different phosphites it may be important to use either THF as reaction solvent, or if an alcohol solvent is desired, the alcohol having the same alkyl substituents as the phosphite must be used in order to avoid problems with transesterification of the phosphite ligand. The equilibrium between tetra- and penta-coordination (equation 410) is controlled by steric effects. Using similar synthetic procedures the alkyl diphenylphosphinite complexes of platinum [Pt PPh2(OR) 4](BPh4)2 (R = Me, Et) can also be prepared. Unexpectedly the reaction between Pt(l-a,4,5- j-CiHi3)(cod) and P(OR)3 (R = Me, Et, Ph) gives the hydride complex [PtH P(OR)3 4] . ... 

Organorhodium compounds are found in oxidation states ranging fi-om +6 to -3. However, the most conunon oxidation states are +1 and +3. Rh(I) species exist as both a tetra-coordinated, square planar complex as well as a penta-coordinated trigonal bipyramidal complex. Rhodium, like platinum group metals, exhibits reversible oxidation states making a broad range of organic-catalytic transformations possible. 

Among the complexes which may function in this way are pentacyano-cobaltate ion, iron pentacarbonyl, the platinum-tin complex, and iridium and rhodium carbonyl phosphines. It has been suggested that with tristriphenylphosphine Rh(I) chloride, a dihydride is formed and that concerted addition of the two hydrogen atoms to the coordinated olefin occurs (16). There are few examples of the homogeneous reduction of other functional groups besides C=C, C=C, and C=C—C=C penta-cyanocobaltate incidentally is specific in reducing diolefins to monoolefins. 

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