Ligand sphere-enhanced magnetic anisotropy in cobalt(II) and manganese(II) 2,6-bis-(benzimidazol-2′-yl)-pyridine dichloride

Temperature variation of the effective magnetic moment for a pentacoordinate [Co(bzimpy)Cl2] complex shows a pronounced deviation from linearity below 120 K. Quantitative analysis based on the diagonalisation of the spin Hamiltonian matrices gave the set of magnetic parameters: gx=2.501, gy=2.622, gz=1.476, D/hc=71.7 cm−1, E/hc=1.4 cm−1, αTIP=0.98×10−9 m3 mol−1 and zJ/hc=−0.184 cm−1 (SQUID data). The extreme value of the axial zero-field splitting parameter D shows that the magnetic anisotropy determined by the non-spherical distribution of the spin density is enhanced by the low symmetry of the pentacoordinate chromophore. For the [Mn(bzimpy)Cl2]·1/2 MeOH analogue, only a moderate ZFS is found (D/hc=−2.7 cm−1).