Of course, there is a catch. Let us consider typical 5-coordinate compound, #PCl_5#, and #PF_5# are suitable, and have to a first approx. trigonal bipyramidal #D_(3h)# symmetry. #PF_5# is especially suitable given that both atoms are #100%# #I=1/2#, and #""^19F{""^1H}#, and #""^31P{""^1H}# #"NMR spectroscopies"# are VERY convenient means to monitor their solution behaviour.
For #PF_5# there are CLEARLY 2 fluorine environments, axial and equatorial. Under normal conditions, however, only one #F# environment is observed in the spectrum (as a doublet due to #""^31P,""^19F# coupling). There exists a fairly soft mechanism of intramolecular exchange thru so-called #"pseudorotation"# or #"turnstile rotation"#, which exchanges the axial ligands for equatorial ligands and vice versa. At lower temperatures, this exchange can be frozen out, and the two enviroments observed in the solution spectrum.
Five coordinate molecules are thus highly conformationally mobile. This is not the case with four- and six-coordinate molecules.......
