Understanding Isostructural Species

Isostructural species are molecules or ions that have the same molecular geometry or shape, even if their atoms or charges differ. To determine this, we analyze their electron pair geometry (based on VSEPR theory) and molecular structure.

Step 1: Analyze Each Pair

Pair 1: PCl₄⁺ and SiCl₄

• PCl₄⁺: Central P has 4 bond pairs, no lone pairs. Tetrahedral geometry.
• SiCl₄: Central Si has 4 bond pairs, no lone pairs. Tetrahedral geometry.
• Both are isostructural.

Pair 2: PF₅ and BrF₅

• PF₅: Central P has 5 bond pairs, no lone pairs. Trigonal bipyramidal geometry.
• BrF₅: Central Br has 5 bond pairs and 1 lone pair. Square pyramidal geometry.
• Different geometries → not isostructural.

Pair 3: CO₃^2- and NO₃^-

• CO₃^2-: Central C has 3 bond pairs, no lone pairs. Trigonal planar geometry.
• NO₃^-: Central N has 3 bond pairs, no lone pairs. Trigonal planar geometry.
• Both are isostructural.

Pair 4: AlF₆^3- and SF₆

• AlF₆^3-: Central Al has 6 bond pairs, no lone pairs. Octahedral geometry.
• SF₆: Central S has 6 bond pairs, no lone pairs. Octahedral geometry.
• Both are isostructural.

Step 2: Final Answer

The pair that is not isostructural is PF₅ and BrF₅.

Related Topics & Formulae

VSEPR Theory (Valence Shell Electron Pair Repulsion): Predicts molecular geometry based on electron pair repulsion. Key formula for steric number (SN):

$\mathrm{SN}=\text{number of bond pairs}+\text{number of lone pairs}$

Common geometries:

• SN=4, no lone pairs: Tetrahedral
• SN=5, no lone pairs: Trigonal bipyramidal
• SN=6, no lone pairs: Octahedral
• SN=5, 1 lone pair: Square pyramidal
• SN=4, 1 lone pair: Trigonal pyramidal

Isostructuralism: Depends on identical electron pair arrangement and molecular shape, regardless of atom type or charge.