The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni(II) and Co(II) complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters D of [Ni(H(2) DAPBH)(H(2) O)(2) ](NO(3) )(2) ⋅2 H(2) O (1) and [Co(H(2) DAPBH)(H(2) O)(NO(3) )](NO(3) ) [2; H(2) DAPBH=2,6-diacetylpyridine bis- (benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni(II) complex indicates stabilization of the highest M(S) value of the S=1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co(II) indicates stabilization of the M(S) =±1/2 sublevels of the S=3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni(II) complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d(xy) and ${{\rm{d}}_{x^2 - y^2 } }$ orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co(II) , all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.