Phosphorus pentachloride (PCl5) exhibits different structures depending on its physical state. In the gas phase and in solution, PCl5 adopts a trigonal bipyramidal geometry. However, in the solid state, its crystal structure is tetragonal.
Molecular Geometry in Gas Phase and Solution
In its gaseous and liquid states, as well as when dissolved in non-polar solvents, PCl5 forms discrete molecules with a trigonal bipyramidal arrangement. This molecular geometry can be described as:
- Central Atom: A phosphorus (P) atom is at the center.
- Equatorial Positions: Three chlorine (Cl) atoms lie in a plane around the phosphorus atom, forming a triangular base. These are known as equatorial chlorine atoms.
- Axial Positions: Two other chlorine atoms are positioned along an axis perpendicular to the equatorial plane, one above and one below the phosphorus atom. These are the axial chlorine atoms.
This symmetrical arrangement results in a D3h coordination geometry and a zero dipole moment (0 D), indicating that the molecule is nonpolar overall due to the cancellation of bond dipoles.
Crystal Structure in the Solid State
When PCl5 is in its solid form, it undergoes a transformation and exists as an ionic compound. Its crystal structure is tetragonal, consisting of discrete cations and anions:
- Tetrachloro-phosphonium Cation ([PCl4]+): This ion has a tetrahedral geometry.
- Hexachloro-phosphate Anion ([PCl6]-): This ion has an octahedral geometry.
These ions pack together in a highly ordered, repeating pattern that defines the tetragonal crystal lattice.
Key Structural Properties of PCl5
The table below summarizes the key structural characteristics of Phosphorus Pentachloride:
| Property | Description |
|---|---|
| Formula | PCl5 |
| Phase-Dependent | Yes (Gas/Solution vs. Solid) |
| Coordination Geometry (Gas/Solution) | Trigonal Bipyramidal (D3h) |
| Crystal Structure (Solid) | Tetragonal |
| Dipole Moment | 0 D (Gas/Solution) |
| Ionic Composition (Solid) | [PCl4]+ (tetrahedral) and [PCl6]- (octahedral) |
For further details on PCl5, you can refer to its dedicated page on Wikipedia.
