Question

Explain the trend in CO stretches below.

ν(CO) = 2143 cm-1 in CO

ν(CO) = 2090 cm-1 in [Mn(CO)6]+

ν(CO) = 2000 cm-1 in [Cr(CO)6]

ν(CO) = 1860 cm-1 in [V(CO)6]-

Answer 1

Its very simple to understand If a complex has a positive charge on it, which means that metal is electron-deficient, and there will be back bond donation of the electron density through CO to metal. Therefore the bond order of CO will increase because the oxygen will donate its electron to Carbon. Hence the frequency of CO will be higher.

Whereas if a complex has a negative charge, then the metal is electron-rich, and there will be a back donation of electron density from metal to CO. Hence, the bond order of CO will be lower, and the stretching frequency will be lower.

Now from these two statements, we can compare the frequencies of the given complexes.