| Summary: | The conditions for achieving the maximal unquenched orbital angular momentum <i>L</i> = 3 and the highest magnetic anisotropy in mononuclear 3d complexes with axial coordination symmetry are examined in terms of the ligand field theory. It is shown that, apart from the known linear two-coordinate 3d<sup>7</sup> complex Co<sup>II</sup>(C(SiMe<sub>2</sub>ONaph)<sub>3</sub>)<sub>2</sub> characterized by record magnetic anisotropy and single-molecule magnet (SMM) performance (with the largest known spin-reversal barrier <i>U</i><sub>eff</sub> = 450 cm<sup>−1</sup>), the maximal orbital angular momentum <i>L</i> = 3 can also be obtained in linear two-coordinate 3d<sup>2</sup> complexes (V<sup>3+</sup>, Cr<sup>4+</sup>) and in trigonal-prismatic 3d<sup>3</sup> (Cr<sup>3+</sup>, Mn<sup>4+</sup>) and 3d<sup>8</sup> (Co<sup>+</sup>, Ni<sup>2+</sup>) complexes. A comparative assessment of the SMM performance of the 3d<sup>2</sup>, 3d<sup>3</sup> and 3d<sup>8</sup> complexes indicates that they are unlikely to compete with the record linear complex Co<sup>II</sup>(C(SiMe<sub>2</sub>ONaph)<sub>3</sub>)<sub>2</sub>, whose magnetic anisotropy is close to the physical limit for a 3d metal.
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