Five-coordinate species fluxionality

Figure 22-7 Simplified catalytic cycle for hydroformylation using rhodium complexes. Note that the configurations of complexes are not known with certainty and that five-coordinate species are fluxional. Rhodium can be added as Rh(acac)(CO)(PPh3), HRh(CO)(PPh3)3, or similar complexes. The solvent in QH4 or CHjCH=CH2 hydroformylation is the aldehyde trimer which is in equilibrium with aldehyde.

P, but no exchange between Pt-P and " Pt-P, eliminating phosphine exchange as the mechanism. Intermolecular transfer of SnCl2 was implicated in the process, and Scheme 1, involving a five-coordinate SnClj adduct of a type reported in Volume 6 of this series, accounted for all of the observations. The five-coordinate species was required to be fluxional, presumably by the Berry or turnstile route. 

Five-coordinate species represent a rare class of coordination compounds. This rather unusual coordination number is a delicate balance between electronic factors which favor disproportion into four- and sbc-coordinate species and directional covalent bonds which help to stabilize them. The two limiting geometries (trigonal bipyramidal and square pyramidal) are therefore similar in energy and easily interconverted. As a result, five-coordinate species can exist as fluxional molecules, where their ligands can exchange places rapidly on the NMR time scale. 

The NMR spectroscopic examples discussed so far have assumed that, with the exception of free rotation about single bonds, the molecule or ion is static in solution. For the majority of organic and inorganic species, this assumption is valid, but the possibUily of stereochemical non-rigidity (fluxionality) on the NMR spectroscopic timescale must be considered. Five-coordinate species such as Fe(CO)5, 4.10, PF5,4.11, and BrF5,4.12, constitute one group of compounds for which the activation barrier for dynamic behaviour in solution is relatively low, and exchange of substituent positions is facile. 

Neutral monochelate complex [RhCl(P 0)(PniO)] (P 0 = t -(P) coordinated Cy2PCH2CH20CH3 PoO = t - (0,P) chelated) was reported by Lindner et al. [9] and the complex exhibits fluxional behavior as demonstrated by variable temperature P H NMR spectroscopy. The observed dynamic phenomena are reversible and proceed with an associative mechanism involving a five-coordinated species as shown below (Scheme 3) ... 

Figure 11.3.1. Simplified catalytic cycle for hydroformylation using rhodium complexes. The five coordinated species are fluxional. The solvent is C2H4 or CH3CH=CH2, see F. A. Cotton and G. Wilkinson, ref. 6. Reproduced by permission of John Wiley Sons. Inc. Copyright 1999.

Under hydrogen RhCl (PPh3)3 solution rapidly becomes yellow and and 3IP NMR studies show that an octahedral dihydride is first formed but, due to the strong trans effect of H, this rapidly dissociates at room temperature to give a fluxional five-coordinate rhodium(III) species. 

Ligand exchange reactions in octahedral carbonyl complexes often involve five-coordinate intermediates (see Chapter 1) and these species may be fluxional on the time-scale of the reaction. Square-pyramidal [W(CO)4(PPhs)] formed in the reaction of [W(CO)4(tmpa)l (tmpa= VWW -tetramethyl-l,3-diaminopropane) with PPhg is fluxional, but the most stable isomer probably contains an equatorially bonded... 

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