Achieving an accurate control on the final structure of Metal-Organic Frameworks (MOFs) is mandatory to obtain target physical properties. Here we describe how the combination of a metalloligand design strategy and a post-synthetic method is a versatile and powerful approach to success on this extremely difficult task. In a first stage, a novel oxamato-based tetranuclear cobalt(III) complex with a tetrahedron-shape geometry is used, for the first time, as metalloligand toward cal-cium(II) cations to lead a diamagnetic Ca(II)-Co(III) three-dimensional (3D) MOF (1). In a second stage, in a single-crystal to single-crystal manner the calcium(II) ions are replaced by terbium (III), dysprosium(III), holmium(III) and erbium(III) ones to yield four isostructural novel Ln(III)-Co(III) [Ln = Tb (2), Dy (3), Ho (4) and Er (5)] 3D MOFs. The direct-current (dc) magnetic properties for 2-5 reveal behaviors as expected for the ground terms of the magnetic isolated rare-earth ions [7F6 (TbIII), 6H15/2 (DyIII), 5I8 (HoIII) and 4I15/2 (ErIII)]. The analysis of the χMT data indicates that the lowest MJ value is the ground state, that is MJ = 0 (2 and 4) and 1/2 (3 and 5). Incipient frequency-dependent alternating current magnetic susceptibility signals are observed for the Kramers' ions (3 and 5) under an external applied magnetics field, supporting the presence of slow magnetic relaxation typical of single-molecule magnets.