Is a strong axial crystal-field the only essential condition for a large magnetic anisotropy barrier? The case of non-Kramers Ho( iii ) versus Tb( iii )

= ±8) in the case of Ho(iii) complex, where the relaxation is found to proceed via the first excited state. However, the equatorial field disrupts the stabilization of ground pseudo doublets in Tb(iii), inducing QTM and inhibiting SIM behaviour. Further insights into the decisive role played by the secondary coordination sphere and hydrogen-bonding in the SIM characteristics of these two non-Kramers ions were also gained. This combined experimental and computational approach highlights that although strong axiality holds the key for designing high temperature SIMs based on oblate non-Kramers ions, the strength of the equatorial ligand field also cannot be ignored.