Question

Predict the relative order of the carbonyl stretching frequencies, ν(CO), for the three compounds (MeC5H4)Mn(CO)3, [(MeC5H4)Mn(CO)2(NO)](PF6), and [(MeC5H4)Mn(CO)(PPh3)(NO)](PF6) and briefly rationalize your answer.

Answer 1

Relative order of the CO stretching frequency as follows :

[(MeC5H4)Mn(CO)2(NO)](PF6) >  (MeC5H4)Mn(CO)3 > [(MeC5H4)Mn(CO)(PPh3)(NO)](PF6)

As the strength of π acceptor ligand increases, electron charge density on metal decreases as all the electron density withdrawn by the π acceptor ligand.

Strength of the π acceptor ligand trend is being followed as :

NO+ > CO > PPh3.

As the electron density decreases on the metal, CO stretching frequency increases. As less electron charge will be transferred from the metal to the LUMO of the CO.