The magnetic anisotropy of two pentacoordinate trigonal bipyramidal (C_3v symmetry) Co(II) complexes, [Co(Me₆tren)Cl]ClO₄ (1) and [Co(Me₆tren)Br]Br (2), was investigated and analysed by magnetic studies, high field multifrequency electron paramagnetic resonance (EPR) and ab initio calculations. Negative D parameters expressing an Ising-type anisotropy (easy axis of magnetization) were found experimentally for both complexes. Calculations led to D values very close to the experimental ones, which allows a robust rationalisation of the magnetic anisotropy in these complexes. The wavefunctions of the ground and the first four excited states reveal that they are strongly multideterminantal i.e. linear combinations of several determinants. The most important contribution to the spin orbit coupling between the ground and lowest excited states stabilizes the largest M_S = ±3/2 components of the S = 3/2 state and therefore brings a large negative contribution to D. The analysis of the difference between the magnitudes of the anisotropy of the two complexes led to the conclusion that a large Ising anisotropy is preferred when weak σ-donating ligands are in the equatorial plane and strong π-donating ones are in axial positions; thus providing an efficient tool to chemists to predict the magnetic anisotropy in these types of complexes. The investigation of the magnetic behaviour of a single crystal of 1 by micro-SQUID shows, as expected, the presence of an easy axis of magnetization. The magnetic behaviour is consistent with quantum tunnelling of the magnetization mediated by intermolecular three-dimensional antiferromagnetic exchange interactions. Upon dilution of the Co(II) molecules in the isostructural Zn(II) compound, a blocking of the magnetization below 2 K is demonstrated; it results in an opening of the magnetization hysteresis loop in zero applied magnetic field.