Analyzing Properties of (CF3)3B⋅CO

Let's evaluate each statement regarding the properties of the complex (CF3)3B⋅CO.

Statement A: CO Stretching Frequency

The statement claims the CO stretching frequency in IR is less than 2143 cm$^{-1}$. The stretching frequency of free CO gas is 2143 cm$^{-1}$. When CO coordinates to a Lewis acid or a metal center, its stretching frequency changes. In metal carbonyls with significant metal-to-CO π backbonding, the C≡O bond is weakened, and the frequency decreases.

In (CF3)3B⋅CO, Boron acts as a Lewis acid, accepting electron density from the carbon's lone pair (σ donation). (CF3)3B is a strong Lewis acid due to the electron-withdrawing CF3 groups. This σ donation from CO to Boron removes electron density from the CO molecule. Unlike transition metals, Boron does not have accessible d orbitals for π backbonding to the CO π* orbitals. The σ donation primarily affects the C≡O bond. Studies on related boron carbonyls like BH$_3$CO show that σ donation from CO to the Lewis acidic boron tends to strengthen the C≡O bond compared to free CO, leading to a higher stretching frequency (e.g., BH$_3$CO is around 2164 cm$^{-1}$). Given that (CF3)3B is a stronger Lewis acid, the C≡O bond in (CF3)3B⋅CO is expected to be stronger than in free CO, resulting in a stretching frequency greater than 2143 cm$^{-1}$.

Therefore, statement A is likely incorrect.

Statement B: 19F NMR Spectrum

The statement says the $^{19}$F NMR spectrum shows one singlet resonance only. The molecule (CF3)3B⋅CO has three CF3 groups. If these three CF3 groups are chemically equivalent due to molecular symmetry, all nine fluorine atoms would be equivalent, leading to a single signal in the $^{19}$F NMR spectrum (before considering coupling).

However, Boron has two common isotopes with nuclear spin: $^{11}$B (80.1%, I = 3/2) and $^{10}$B (19.9%, I = 3). Fluorine nuclei ($^{19}$F, I = 1/2) can couple with these Boron nuclei. Coupling with $^{11}$B (I=3/2) would split the $^{19}$F signal into 2I+1 = 2(3/2)+1 = 4 peaks (a quartet). Coupling with $^{10}$B (I=3) would split the $^{19}$F signal into 2I+1 = 2(3)+1 = 7 peaks (a septet). Therefore, even if the CF3 groups are equivalent, the observed $^{19}$F NMR signal is expected to be a complex multiplet due to coupling with Boron isotopes, not a singlet.

Thus, the statement that the spectrum shows "one singlet resonance only" is incorrect.

Statement C: Point Group of (CF3)3B⋅CO

The statement says the point group of (CF3)3B⋅CO is C$_3v$. The molecule has a central Boron atom bonded to three CF$_3$ groups and one CO ligand. The arrangement around the Boron atom is approximately tetrahedral. If the B-CO bond is aligned along a principal axis, and the three CF$_3$ groups are symmetrically arranged around this axis, the molecule would possess a 3-fold rotation axis (C$_3$) along the B-CO bond and vertical mirror planes (σ$_v$) containing this axis and passing through the CF$_3$ groups (assuming a staggered or symmetrical orientation of the CF bonds within the CF$_3$ groups relative to the overall C$_3$ axis). This symmetry corresponds to the C$_3v$ point group. This point group is consistent with the structure of analogous molecules like BH$_3$CO.

Therefore, statement C is likely correct.

Statement D: Reaction with KF

The statement says (CF3)3B⋅CO reacts with KF to form K[(CF3)3BC(O)F]. (CF3)3B⋅CO is a complex formed between a strong Lewis acid ((CF3)3B) and a Lewis base (CO). KF is a source of fluoride ions (F$^-$), which are strong nucleophiles and Lewis bases. Nucleophilic attack on coordinated carbonyl ligands is a known reaction, especially when the carbonyl is bonded to an electron-withdrawing species (like a Lewis acid or a metal center with poor backbonding). The fluoride ion can attack the electrophilic carbon atom of the coordinated CO ligand.

The reaction would be: (CF$_3$)$_3$B⋅CO + F$^-$ → [(CF$_3$)$_3$BC(O)F]$^-$. This reaction forms a new complex where the fluoride has added to the carbonyl carbon, creating an acyl fluoride-like structure coordinated to the (CF3)3B moiety. The resulting anion, [(CF$_3$)$_3$BC(O)F]$^-$, would form a salt with the potassium cation, K$^+$[(CF$_3$)$_3$BC(O)F]$^-$. This is consistent with the product described.

Therefore, statement D is likely correct.

Conclusion

Based on the analysis, statements C and D are correct, while statements A and B are incorrect.

Statement A is incorrect because the CO stretching frequency is expected to be greater than 2143 cm$^{-1}$ due to σ donation to the strong Lewis acid. Statement B is incorrect because coupling with Boron isotopes results in a multiplet, not a singlet, in the $^{19}$F NMR spectrum. Statement C is correct as the molecule is expected to have C$_3v$ symmetry. Statement D is correct as nucleophilic attack by F$^-$ on the coordinated CO carbon is a plausible reaction forming the described product.

The correct statements are C and D only.