Palladium Penta-coordinate complexes

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... 

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. 

Because substitution chemistry at square-planar palladium is dominated by associative processes [48], coordination of the alkene in 22.2 would undoubtedly initially generate penta-coordinate intermediate 22.6. Complex 22.6 could then either evolve to square-planar complex 22.5 by a series of pseudorotations and eventual expulsion of the halide ligand or undergo... 

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