Herein, we present monometallic Ln(III) complexes [L3Ln(NO3)3] [where Ln = Ce (1) and La (2)] assembled from a simple reaction of the respective lanthanide nitrate hydrate and a bulky phosphonic diamide tBuPO(NHiPr)2 ligand (L), where complex 1 behaves as a single-ion single-electron magnet under a small applied magnetic field. The Ce(III) ion occupies a nine-coordinate distorted muffin-like coordination environment. The combination of direct and Raman process dominates the relaxation dynamics in 1 under the applied dc field. The low-temperature measurements performed with oriented crystals on a micro-SQUID setup exhibits strong tunnelling at zero-field, consistent with the theoretical results where strong mixing of the ground state with higher excited mJ levels is detected and also throws additional insights on the relaxation dynamics of 1. Ab initio calculations have been performed to understand the origin of anisotropy and models have been proposed for future directions.