Pentacovalent phosphorus compounds normally have trigonal-bipyramidal structures. In such structures the more electro negative substituents show a strong preference for the apical positions. With these facts in mind, suggest an explanation for the following data obtained by nuclear magnetic resonance studies. \(\mathrm{CH}_{3} \mathrm{PF}_{4}: \quad\) one kind of methyl one kind of fluorine \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{PF}_{3}:\) one kind of methyl two kinds of fluorine in the ratio \(2: 1\) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{PF}_{2}:\) one kind of methyl two kinds of fluorine

In pentacovalent phosphorus compounds with trigonal-bipyramidal structures, the more electronegative substituents prefer the apical positions. For \(\mathrm{CH}_{3} \mathrm{PF}_{4}\), the four fluorine atoms occupy apical positions and one methyl group is in the equatorial position, resulting in one kind of methyl and fluorine environment. In \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{PF}_{3}\), one fluorine atom occupies an apical position and two occupy equatorial positions, with two equatorial methyl groups, resulting in two kinds of fluorine in a 2:1 ratio and one kind of methyl environment. For \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{PF}_{2}\), the two fluorine atoms occupy apical positions, with three symmetry-equivalent equatorial methyl groups, resulting in one kind of methyl environment and two kinds of fluorine environments.