Does a Sterically Bulky Group Occupy the Equatorial Site in Trigonal Bipyramidal Phosphorus?

The sterically bulky tert-butyl group occupies an apical position in trigonal bipyramidal phosphorus in the compound {CH2(6-t-Bu-4-Me-C6H2O)2}P(t-Bu)(1,2-O2C6Cl4) in contrast to the occupation of an equatorial position by the small methyl group in {CH2(6-t-Bu-4-Me-C6H2O)2}P(Me)(1,2-O2C6Cl4); this observation contradicts the familiar “apicophilicity rules” for trigonal bipyramidal phosphorus. Low-temperature solution
31P NMR spectra of {CH2(6-t-Bu-4-Me-C6H2O)2}P(R)(1,2-O2C6Cl4) (R = Me, Et, and n-Bu) show the presence of more than two isomers.