Q.29 Which one of the following statements is CORRECT?
(A) BF3 is a stronger Lewis acid than BI3 .
(B) CO and CN– are good π-accepting ligands.
(C) cis-Diamminedichloroplatinum (II) has zero dipole moment.
(D) Central atom in BCl 3 is sp3 hybridized.

BF3 is a weaker Lewis acid than BI3 due to stronger pπ-pπ back bonding in BF3, which reduces boron’s electron deficiency. The correct statement is option (B), as both CO and CN⁻ effectively accept electrons into their π* orbitals, acting as good π-accepting ligands in metal complexes.

Option Analysis

Option (A): BF3 vs BI3 Lewis acidity
BF3 exhibits significant back bonding with fluorine’s small size and high electronegativity, donating electron density to boron’s empty p-orbital and decreasing its ability to accept electron pairs. BI3 shows minimal back bonding due to iodine’s larger size and poor orbital overlap, making boron more electron-deficient and thus a stronger Lewis acid. The order is BI3 > BBr3 > BCl3 > BF3.

Option (B): CO and CN⁻ as π-acceptors
CO accepts electrons into low-lying π* orbitals from metal d-orbitals, forming strong synergic bonds. CN⁻, isoelectronic with CO, also functions as a good π-acceptor despite stronger σ-donation, commonly stabilizing low-spin metal complexes. Both qualify as effective π-acceptors.

Option (C): Cis-[Pt(NH₃)₂Cl₂] dipole moment
This square planar complex has adjacent NH₃ (less electronegative) and Cl (more electronegative) ligands, creating an asymmetric charge distribution. Computational studies confirm a substantial dipole moment around 11.8 D, not zero.

Option (D): BCl₃ hybridization
Boron in BCl₃ forms three σ-bonds with no lone pairs, using sp² hybrid orbitals (steric number 3) for trigonal planar geometry with 120° bond angles. sp³ hybridization applies to tetrahedral arrangements.

In inorganic chemistry exams like CSIR NET, questions on BF3 BI3 Lewis acid strength, BCl3 hybridization, and ligand properties test core concepts of bonding and molecular geometry. BF3 BI3 Lewis acid strength follows the trend BI3 > BCl3 > BF3 due to back bonding effects, while cis-Diamminedichloroplatinum(II) exhibits a non-zero dipole moment in its square planar cis form.

Lewis Acid Strength: BF3 vs BI3

Boron trihalides act as Lewis acids via boron’s electron deficiency (6 electrons in valence shell). In BF3, fluorine’s compact 2p orbitals enable strong pπ-pπ back donation to boron’s empty p-orbital, reducing acidity. Larger iodides in BI3 hinder overlap, preserving boron’s electron hunger—making BI3 the strongest.

• BF3: Weakest due to optimal back bonding

• BCl3: Moderate acidity

• BI3: Strongest Lewis acid

BCl3 Hybridization Details

BCl3 adopts trigonal planar geometry with boron at the center forming three B-Cl σ-bonds. sp² hybridization mixes one 2s and two 2p orbitals, yielding three equivalent sp² orbitals at 120° angles—no lone pairs confirm steric number 3.

π-Acceptor Ligands: CO and CN⁻

These ligands donate σ-electrons to metals while accepting via π* orbitals. CO’s low-energy π* excels in back bonding; CN⁻ mirrors this despite σ-donor strength, both stabilizing d⁸ metals like in carbonyl or cyano complexes.

Cis-Diamminedichloroplatinum(II) Dipole

Unlike symmetric trans isomer (zero dipole), cis-[Pt(NH₃)₂Cl₂] places dissimilar ligands (NH₃ vs Cl) adjacently in square planar arrangement, yielding ~11.8 D dipole—key to its anticancer reactivity.