The isoelectronic [Pd(CO)4] 2+ complex mentioned in the last column of the paper has a significantly
higher CO IR stretching frequency (2248 cm−1 ) compared to [Rh(CO)4]+(2138 cm−1). Is this expected or not? Discuss and compare the two complexes.
It is expected ;
Conjugation with a double bond or benzene ring lowers the stretching frequency.The stretching frequency of the conjugated double bond is also lowered (blue notation) and may be enhanced in intensity.