${}^{1}\mathrm{H}$ NMR spectrum of $[\eta^{5}\text{-}\mathrm{C}_{5}\mathrm{H}_{5}\mathrm{Rh})(\mathrm{C}_{2}\mathrm{H}_{4})_{2}]$ at $-20^{\circ}\mathrm{C}$ shows a typical AA' XX' pattern in the olefinic region. On increasing the temperature to $-70^{\circ}\mathrm{C}$, the separate lines collapse into a single line which is due to
(a)free rotation of the ethylene ligand about the metal-olefin bond
(b)interamolecular exchange between the ethylene ligands
(c)intermolecular exchange between the ethylene ligands
(d)change in hapticity of the cyclopentadienyl ligand
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
Warming allows the ethylene ligands to rotate about the Rh–olefin axis, exchanging the inner and outer protons of each C₂H₄ and collapsing the AA′XX′ pattern into a singlet.