AbstractA series of heterometallic [Ti^IV₇M^III] rings are reported which contain a single paramagnetic metal centre, either Fe^III or Cr^III. The structures contain an octagon of metals bridged by oxides within the ring and pivalate ligands outside the ring. The structures encapsulate a single cation, either a secondary ammonium cation or a cesium. Multi-frequency EPR spectroscopy was used to measure the zero-field splitting parameters for the paramagnetic metal sites. The Fe^III sites have rhombic anisotropy, with D around − 0.18 cm⁻¹ and λ (= D/E) approximately 0.25. The Cr^III have a more axial anisotropy, with D around − 0.5 cm⁻¹ and λ around 0.05. Attempts to find a magneto-structural correlation are vitiated by the difficulty of refining the challenging crystal structures.