Ab initio calculations performed in two three-coordinate complexes [Mn{N(SiMe3)2}3] (1) and [K(18-crown-6)(Et2O)2][Cr{N(SiMe3)2}3] (2) reveal record-high magnetic anisotropy with the D values -64 cm -1 and -15 cm -1 respectively, enlisting d 4 ion back in the race for single-ion magnets. For the first time, a detailed spin-vibrational analysis was performed in 1 and 2 that suggests a dominant under barrier relaxation due to flexible coordination sphere around the metal ion offering design clues for low coordinate transition metal SIMs. ASSOCIATED CONTENTThe following files are available free of charge.

<p><i>Ab initio</i> calculations performed in two three-coordinate complexes [Mn{N(SiMe₃)₂}₃] (1) and [K(18-crown-6)(Et₂O)₂][Cr{N(SiMe₃)₂}₃] (2) reveal record-high magnetic anisotropy with the <i>D</i> values -64 cm^-1 and -15 cm^-1 respectively, enlisting d⁴ion back in the race for single-ion magnets. For the first time, a detailed spin-vibrational analysis was performed in <b>1</b> and <b>2</b> that suggest a dominant under barrier relaxation due to flexible coordination sphere around the metal ion offering design clues for low coordinate transition metal SIMs.<b> </b></p>

<p><i>Ab initio</i> calculations performed in two three-coordinate complexes [Mn{N(SiMe₃)₂}₃] (1) and [K(18-crown-6)(Et₂O)₂][Cr{N(SiMe₃)₂}₃] (2) reveal record-high magnetic anisotropy with the <i>D</i> values -64 cm^-1 and -15 cm^-1 respectively, enlisting d⁴ion back in the race for single-ion magnets. For the first time, a detailed spin-vibrational analysis was performed in <b>1</b> and <b>2</b> that suggest a dominant under barrier relaxation due to flexible coordination sphere around the metal ion offering design clues for low coordinate transition metal SIMs.<b> </b></p>