Complexes of general formula, fac-$\left[\mathrm{Mo}(\mathrm{CO})_{3}(\text{phosphite})_{3}\right]$ have the C-O stretching bands given below. Phosphines: $\mathrm{PF}_{3}(\mathrm{A}) ; \mathrm{PCl}_{3}(\mathrm{B}) ; \mathrm{P}(\mathrm{Cl})\mathrm{Ph}_{2}(\mathrm{C}) ; \mathrm{PMe}_{3}(\mathrm{D})$ $\bar{v}(\mathrm{CO}), \mathrm{cm}^{-1}: 2090(\mathrm{i}) ; 2040(\mathrm{ii}) ; 1977(\mathrm{iii}) ; 1945(\mathrm{iv})$ The correct combination of the phosphine and the stretching frequency is,
(a)(A-i), (B-ii), (C-iii), (D-iv)
(b)(A-ii), (B-i), (C-iv), (D-iii)
(c)(A-iv), (B-iii), (C-ii), (D-i)
(d)(A-iii), (B-iv), (C-i), (D-ii)
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
ν(CO) falls as the phosphorus ligand becomes a better σ-donor/poorer π-acceptor: PF₃ 2090, PCl₃ 2040, PClPh₂ 1977, PMe₃ 1945 cm⁻¹ — A-i, B-ii, C-iii, D-iv (a).