A family of four-coordinate Fell complexes formed with N,N'-chelating amido-pyridine ligands was synthesized, and their magnetic properties were investigated. These distorted tetrahedral complexes exhibit significant magnetic anisotropy with zero-field splitting parameter D ranging between -17 and -12 cm(-1). Ab initio calculations enabled identification of the structural factors that control the nature of the magnetic anisotropy and the rationalization of the variation of D in these complexes. It is shown that a reduced N Fe N angle involving the chelating nitrogen atoms of the ligands is at the origin of the negative D value and that the torsion between the two N Fe N planes imposed by steric hindrances further increases the vertical bar D vertical bar value. Field-induced slow relaxation of magnetization was observed for the three compounds, and a single-molecule magnet behavior with an energy barrier for magnetization flipping (U-eff) of 27 cm(-1) could be evidenced for one of them.