Chemistry - Coordination Chemistry Question with Solution | TestHub

CO stretching frequency (${\nu }_{CO}$) decreases when M → CO $\pi$ back bonding increases. Strong $\pi$-acceptor ligands on metal increase electron density on Mo, enhancing back bonding and lowering ${\nu }_{CO}$.

Ligand strength ($\pi$-acceptor ability):

$PP{h}_3<P{h}_{2}PCl<C{l}_{3}P<F_{3}P$

So, complexes with weaker $\pi$-acceptor ligands (more back bonding) will have lower ${\nu }_{CO}$ than the reference.

Thus, compared to the given complex, species A, B, D show stronger back bonding, leading to a lower CO stretching frequency.

Final Answer: A, B, D