The number of molecules/ion/s having trigonal bipyramidal shape is .... PF₅$PF_5$, BrF₅$BrF_5$, PCl₅$PCl_5$, [PtCl₄]²⁻$[PtCl_4]^{2-}$, BF₃$BF_3$, Fe(CO)₅$Fe(CO)_5$

Core Logic

Using VSEPR theory to find the hybridization and shape:

• PF₅$PF_5$: P has 5 valence electrons, forms 5 single bonds with F. Steric number = 5 (sp3d). 0 lone pairs. Shape = Trigonal bipyramidal.
• BrF₅$BrF_5$: Br has 7 valence electrons, forms 5 single bonds, 1 lone pair. Steric number = 6 (sp3d2). Shape = Square pyramidal.
• PCl₅$PCl_5$: P has 5 valence electrons, 5 bonds, 0 lone pairs. Steric number = 5 (sp3d). Shape = Trigonal bipyramidal.
• [PtCl₄]²⁻$[PtCl_4]^{2-}$: Pt²⁺$Pt^{2+}$ is a d⁸$d^8$ system. With Cl^-$Cl^-$ (but 4d/5d transition metals always form low spin square planar complexes), it's dsp²$dsp^2$ hybridized. Shape = Square planar.
• BF₃$BF_3$: B has 3 valence electrons, 3 bonds, 0 lone pairs. Steric number = 3 (sp2). Shape = Trigonal planar.

6. Fe(CO)₅$Fe(CO)_5$: Fe (d6s2 arrow$\rightarrow$ d8 under strong field CO$CO$). Carbonyls strongly prefer 5-coordinate trigonal bipyramidal geometry for d⁸$d^8$ (dsp³$dsp^3$ hybridization). Shape = Trigonal bipyramidal.

Step 1: Count Trigonal Bipyramidal Molecules

Molecules with trigonal bipyramidal shape:

• PF₅$PF_5$
• PCl₅$PCl_5$
• Fe(CO)₅$Fe(CO)_5$

Total count = 3.

Pattern Recognition

Steric Number = 5 with 0 lone pairs ALWAYS yields Trigonal Bipyramidal geometry. Watch out for BrF₅$BrF_5$ which has 5 bonds but 1 lone pair (SN = 6, Square Pyramidal).

Chapter Mix

Class 11 Chemistry: Chemical Bonding and Molecular Structure Class 12 Chemistry: Coordination Compounds