Five-coordinate molecules fluxionality

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

This rather surprising result is explained by recognizing that the 16-electron intermediate is five coordinate and could equilibrate the cis and trans carbonyls. A fluxional five-coordinate intermediate see Fluxional Molecule) would allow for enrichment in the axial position without dissociation of the axial CO. Thus, the stereochemistry of CO dissociation is consistent with expectations. 

Zerovalent complexes of Pd and Pt are Pd( -alkene)(Bu -DAB) and Pt(cod)(R-DAB). Divalent Pd" and Pt" compounds are abundant. Key examples are five-coordinate PtCl2(R-DAB)(>j -alkene) and planar PtCl2(R-DAB). H and C NMR data showed that in the ground state the C=C skeleton of the alkene in PtCl2(R-DAB)( 7 -alkene) is in the trigonal plane. When a phosphine is in the plane of the alkene the compound, i.e. PtCl2(PR3)(R-DAB) contains monoden-tate R-DAB. This molecule is fluxional and the five-coordinate geometry is then an intermediate in the cr-N ct-N fluxional process. 

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. 

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