Geometrical control of the magnetic anisotropy in six coordinate cobalt complexes.

The geometry of cobalt(ii) ions in the axially distorted octahedral cation in [Co(MeCN)6](BF4)2 (1) was compared to the trigonal prismatic cation in [CoTppy]PF6 (2) which revealed significant differences in magnetic anisotropy. Combined experimental and ab initio CASSCF/NEVPT2 calculations support the observed zero field SMM behaviour for 2, with easy axis anisotropy, attributed to the rigidity of the trigonal prismatic ligand. Strong transverse anisotropy for 1 leads to significant quantum tunnelling processes.