(a) The IR spectrum of naturally occurring CO shows an absorption at 2170 cm⁻¹ assigned to the molecular vibrational mode. If the sample is enriched in ¹³C, what change do you expect to see in the IR spectrum?

(b) The IR spectra of salts of [Fe(CO)6]²⁺ and [Fe(CO)4]²⁻, respectively show absorptions at 2204 and 1788 cm⁻¹ assigned to carbonyl vibrational modes. What do these tell you about the strengths of the carbon–oxygen bonds in the complexes compared to that in free CO?

(c) [Fe(CO)4]²⁻ and Fe(CO)₅ have T_d and D_3h symmetries respectively. What do you expect to observe in the room temperature ¹³C NMR spectra of Na₂[Fe(CO)₄] and Fe(CO)₅? Rationalize your answer.