Question

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

Short answer

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.

Step-by-step solution

Understand Trigonal Bipyramidal Structures

In a trigonal bipyramidal structure, there are five positions for the atoms around the central atom. Three of these positions form an equatorial plane in the shape of an equilateral triangle, while the remaining two positions are above (apical) and below (apical) the central atom.

Examine the First Compound: \(\mathrm{CH}_{3} \mathrm{PF}_{4}\)

In this compound, there is one methyl group and four fluorine atoms bound to the central phosphorus atom. Since fluorine is more electronegative than carbon, the four fluorine atoms will prefer the apical positions and the single methyl group will be placed in the equatorial position. Thus, there is only one kind of methyl and one kind of fluorine environment in this molecule.

Examine the Second Compound: \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{PF}_{3}\)

In this compound, there are two methyl groups and three fluorine atoms bound to the central phosphorus atom. One fluorine atom will occupy an apical position, while the other two fluorine atoms will be in equatorial positions due to the limited availability of apical positions. The two different fluorine environments are in the ratio of 2:1, representing two equatorial and one apical fluorine atom. The methyl groups will occupy the remaining two equatorial positions, resulting in one kind of methyl environment.

Examine the Third Compound: \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{PF}_{2}\)

In this compound, there are three methyl groups and two fluorine atoms bound to the central phosphorus atom. The two fluorine atoms will occupy the apical positions. The three methyl groups will occupy the remaining equatorial positions, which are symmetry-equivalent, resulting in one kind of methyl environment. The two kinds of fluorine in this molecule refer to the two distinct apical positions. In conclusion, the given nuclear magnetic resonance data can be explained by considering the trigonal bipyramidal structures of these pentacovalent phosphorus compounds and the preferences of the more electronegative substituents for the apical positions.

Key concepts

Pentacovalent Phosphorus Compounds

Phosphorus is a versatile element that can exhibit various covalency states, one of which is pentacovalency. In pentacovalent phosphorus compounds, phosphorus forms five covalent bonds, often exhibiting a trigonal bipyramidal structure.
This is a geometric arrangement that optimizes the electron density around the central atom. In this structure, phosphorus is at the center, with five substituents positioned such that three form an equatorial plane and two are positioned axially (apical positions).
The equatorial plane is more spread out and offers less steric hindrance compared to apical positions.

• The axial (apical) positions are preferred by smaller or more electronegative atoms due to electronic and steric reasons.
• The equatorial positions can accommodate larger groups more easily.

This is why, in these compounds, electronegative atoms such as fluorine often occupy the apical positions, influencing the property and reactivity of these molecules significantly.

Electronegativity in Molecular Structure

Electronegativity plays a crucial role in determining the arrangement of substituents in a molecular structure. It is a measure of an atom's ability to attract and hold onto electrons. In the context of trigonal bipyramidal structures, electronegative elements are strategically placed to minimize repulsions and stabilize the structure.
In pentacovalent phosphorus compounds, such as (CH₃PF₄), (CH₃)₂PF₃, and (CH₃)₃PF₂, fluorine being highly electronegative, tends to occupy the apical positions.

• This positioning minimizes the repulsion between the high electron density of the fluorine atoms and the central phosphorus.
• Methyl groups, which are bulkier and less electronegative, generally prefer the equatorial positions.

The selective placement of atoms based on electronegativity ensures that the structure is as stable as possible while reducing steric hindrance and electron-electron repulsion.

Nuclear Magnetic Resonance (NMR) Analysis

Nuclear Magnetic Resonance (NMR) is a powerful analytical tool used to determine the structure and dynamics of molecules. NMR is particularly useful in elucidating the geometry of molecules like pentacovalent phosphorus compounds. It measures the local magnetic fields around atomic nuclei, which differ based on the nuclear environment.
For the compounds in question, NMR data reveal distinct environments for substituents.

• In (CH₃PF₄), NMR shows only one kind of fluorine because all fluorine atoms occupy equivalent apical positions.
• In (CH₃)₂PF₃, the two types of fluorine environments, in a 2:1 ratio, indicate that fluorine atoms occupy both equatorial and apical positions.
• Finally, in (CH₃)₃PF₂, two distinct fluorine signals come from their unique environment in the apical positions.

Through NMR, insights into the arrangement and symmetry within these molecules are gained, highlighting the influence of electronegativity on molecular structure stability.