Hence, isolated gas-phase TM(CO)n models and the chemisorption of CO on metal surfaces are often investigated to shed light on their catalytic potencies. The fact that the dominant binding mode of CO with electropositive metals is via the less electronegative C rather than the more electronegative O is at first glance surprising. This unique characteristic of CO arises from the two-electron s lone pair present on the C, making it the strongest ligand in the spectrochemical series. Often, M•••CO interactions are further supported by the superior π-acceptor ability of CO that induces metal-to-ligand π back-donation. However, the electron flow through the π-frame (or to π* orbitals of CO) simultaneously weakens the C-O bond. To date, a majority of transition metal CO studies are focused on the first-and second-row TMs, with less attention on the heavier third-row TMs. For example, according to the Cambridge Structural Database (CSD, 2021.1.0) there are 164 first-row (TM = Sc-V) and 185 second-row (TM = Y-Nb) crystal structures available with the TM-CºO bonding configuration, whereas the number of complexes for the corresponding search for third-row structures (TM = La-Ta) is only 97. Among the third-row TMs, one of the least studied systems is Hf, of which only 9 hits are obtained from the CSD for a Hf-CºO search. While this analysis does not include a review of the solution or gas phase chemistry studies for third row structures, it nevertheless gives a sense of the proportion of studies that have been dedicated to third-row complexes.