Coordination number stereochemical nonrigidity

Stereochemical nonrigidity, especially if it is fluxional, seems likely to be consistently characteristic of complexes with coordination numbers of 7 or greater. All 7-coordinate complexes so far investigated by nmr techniques have shown ligand-atom equivalence even though there is no plausible structure for a 7-coordinate complex that would give static or instantaneous equivalence. 

The basic steps of the mechanism of hydroformylation catalyzed by the combination of Rh and PPhj are similar to those of hydroformylation catalyzed by HCo(CO). However, the number of possible intermediates is much larger because many combinations of the number of phosphines and geometric orientation are possible in each intermediate complex. For example, phosphines in a five-coordinate complex can occupy the apical or equatorial positions, and these complexes are typically stereochemically nonrigid. Similarly, two phosphines in a four-coordinate intermediate can be located cis or trans to each other. A five-coordinate hydridorhodium-alkene complex containing diequatorial phosphines might be expected to convert to a four-coordinate Rh alkyl complex containing trans phosphines, but it could also lead to an alkyl complex containing cis phosphines. 

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