How do the P-Ci single bond lengths in PCl5, PCl4+, and PCl6- generally compare? 4. Identify the type of bonding the central atom undergoes in PCl5 and PCl6-.
The Correct Answer and Explanation is:
Answer:
- Comparison of P–Cl single bond lengths:
Generally, the P–Cl bond lengths increase in the order:
PCl₄⁺ < PCl₅ < PCl₆⁻ - Type of bonding in the central atom:
- In PCl₅, the central phosphorus atom undergoes sp³d hybridization.
- In PCl₆⁻, the central phosphorus atom undergoes sp³d² hybridization.
Explanation
The bond lengths and hybridization of molecules such as PCl₅, PCl₄⁺, and PCl₆⁻ can be understood by examining their molecular structures, electron domains, and the effects of electron repulsion.
In PCl₅, phosphorus forms five covalent bonds with chlorine atoms and has no lone pairs. The molecule adopts a trigonal bipyramidal geometry, which results from sp³d hybridization. There are two distinct types of P–Cl bonds: axial and equatorial. Due to greater electron repulsion, axial bonds are usually longer than equatorial bonds. This mixed bond character leads to an intermediate average bond length in PCl₅.
In PCl₄⁺, one chloride ion is removed from PCl₅, reducing the electron count around phosphorus. This leads to a tetrahedral geometry with sp³ hybridization. With only four bonding pairs and no lone pairs, the electron repulsion is reduced, resulting in shorter P–Cl bond lengths compared to PCl₅.
On the other hand, in PCl₆⁻, one extra electron is added to PCl₅, making a six-coordinate species. The molecule adopts an octahedral geometry due to sp³d² hybridization. The added electron increases electron-electron repulsion, and the greater number of bonds around the central atom leads to longer P–Cl bonds than in PCl₅.
Therefore, as the formal positive charge increases (as in PCl₄⁺), the P–Cl bonds shorten due to increased effective nuclear attraction. Conversely, with a negative charge (as in PCl₆⁻), added electron density weakens the bonding interaction slightly, leading to bond lengthening.
In summary, P–Cl bond lengths increase in the order PCl₄⁺ < PCl₅ < PCl₆⁻, and the central atom’s hybridization changes from sp³ in PCl₄⁺, to sp³d in PCl₅, to sp³d² in PCl₆⁻.
