Stabilizing an easy axis of magnetization in an octahedral Co(II) complex is an extremely challenging task, which is evident from reports that more than 90% possess easy-plane anisotropy. Here, we report a six-coordinate complex, [Co(L 1 ) 4 (Cl) 2 ] (1, L 1 = thiourea [H 2 N-CS-NH 2 ]), that exhibits a D value of −63(10) cm −1 . The presence of an easy axis of magnetization associated with 1 is experimentally proven by detailed magnetic studies and polarized neutron diffraction studies, and the experimental observations are well corroborated by theoretical calculations. From the combined experimental and theoretical investigations (on 1 and many model systems), we unveil the parameters that control stabilization of negative D in a thermodynamically favorable and air-stable Co(II) ion in the common distorted octahedral geometry. This study paves the way for overcoming the current impediments to alleviate the easy axis of magnetization using rational ligand choice. A synthesized {CoS 4 Cl 2 } complex reveals design principles for stable negative D The SH parameters were confirmed by high-field pulse magnetization measurements The sign of D is determined unambiguously by polarized neutron diffraction Magneto-structural correlations developed by ab initio methods unveil the origin of D Stabilizing the easy axis of magnetization of octahedral Co(II) complexes that are thermodynamically and air stable is an uphill process. Tripathi et al. report a complex that not only overcomes the impediments in stabilizing negative D but also disclose the parameters that control the sign and magnitude of D.