Cp ligands are usually considered to be innocent in metallodrugs. However, we observed rapid sequential deuteration of all 15 Cp methyl protons in RhIII complexes [(η5-Cp)Rh(N,N)Cl]PF6 (Fig. 1), where Cp is pentamethylcyclopentadienyl anion and N,N = bipyridyl in aqueous media at ambient temperature. We have discovered that this sequential deuteration proceeds via a substituted fulvene intermediate. This unusual reactivity might contribute towards the anticancer activity of the complexes. We aim to correlate of this unusual reactivity with the cancer cell cytotoxicity. This proposed study will investigate the dynamics of the sequential deuteration using neutron scattering (on TOSCA) to elucidate the distribution of the incorporated deuterium within the Cp* ring at various levels of deuteration.