The ¹H NMR spectrum of  $$[(C_5H_5)_2Fe(CO)_2]$$ ​exhibits two peaks of equal intensity at room temperature, but four resonances of relative intensities 5:2:2:1 at lower temperature. The hapticities of C₅H₅^- are

Correct option is A

Proton nuclear magnetic resonance or ¹H NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen nuclei within the molecules of a substance, in order to determine the structure of its molecules.

Proton NMR spectra are characterized by chemical shifts in the range of +14 to -4ppm and by spin-spin coupling between protons. The integration curve for each proton reflects the abundance of the individual protons.

This is because the five equivalent protons of the 

ring will show one singlet in ¹H NMR spectra.

While, the 

ring will also show one singlet in ¹H NMR spectra, although the protons are not all equivalent.

Ring whizzing

The compound 

exhibits the phenomenon of "ring whizzing". At 30 °C, the ¹H NMR spectrum shows only two peaks, one typical 

​and the other assigned 

​The singlet assigned to the 

​ligand splits at low temperatures owing to the slow hopping of the Fe center from carbon to carbon in the 

ligand. Two mechanisms have been proposed, with the consensus favoring the 1,2 shift pathway.

By the ring whizzer mechanism, the five ring positions of the monohapto ring of 

interchange via 1,2-metal shift so rapidly at room temperature that the NMR spectrometer can detect only the average signal for the ring.

At lower temperature, this process is slower and the different resonances for the protons of 

become apparent at lower temperatures, the peak at 4.5ppm 

remains constant, but the other peak at 5.7ppm  for 

spreads and then splits into four peaks of intensity ratio (5:2:2:1).