Three Single‐Ion Magnet Candidates from Erbium(III) Squarate Complexes in Relation with a Square‐Antiprismatic Coordination Structure

Mononuclear [Er(sq)(Hsq)(H 2 O) 6 ]•(H 2 O) (sq = squarate, C 4 O 4 2− ) and heteroleptically bridged 3D polymer [Er 2 (sq) 2 (ox)(H 2 O) 4 ] n (ox = oxalate, C 2 O 4 2− ) were newly prepared. The Arrhenius plot for the alternating‐current magnetic susceptibility data clarified the effective energy barriers for magnetization reversal, U eff / k B = 42.0(7) K at a direct‐current bias field of 1000 Oe for the former and 21.7(8) K at 800 Oe for the latter. They are greater than that of known 2D [Er 2 (sq) 3 (H 2 O) 8 ] n . All of the erbium(III) coordination structures investigated here can be described approximately as a square antiprism with a slight variation in axial compression within a D 4d symmetry framework. To elucidate a suitable degree of D 4d compression, the complete‐active‐space self‐consistent‐field calculation was carried out on the discrete neutral molecule [Er(sq)(Hsq)(H 2 O) 6 ] and its modifications after anisotropically upto 10% elongation or compression along the principal axis. The calculated energy gap between the ground and the first excited states in the Stark sublevels is 151.5 cm −1 for the original [Er(sq)(Hsq)(H 2 O) 6 ] molecule, and the gap became wider with more compressed as well as elongated ErO 8 structures. The gap is highly sensitive to the compressed/elongated modification, justifying the variety and diversity of the single‐ion magnet characteristics observed.