The synthesis, structures, and magnetic properties of six families of cobalt-lanthanide mixed-metal phosphonate complexes are reported in this Article. These six families can be divided into two structural types: grids, where the metal centers lie in a single plane, and cages. The grids include [4 X 3] {Co(8)Ln(4)}, [3 X 3] {Co(4)Ln(6)}, and [2 x 2] {Co(4)Ln(2)} families and a [4 x 4] {Co(8)Ln(8)} family where the central 2 X 2 square is rotated with respect to the external square. The cages include {Co(6)Ln(8)} and {Co(8)Ln(2)} families. Magnetic studies have been performed for these compounds, and for each family, the maximum magnetocaloric effect (MCE) has been observed for the Ln = Gd derivative, with a smaller MCE for the compounds containing magnetically anisotropic 4f-ions. The resulting entropy changes of the gadolinium derivatives are (for 3 K and 7 T) 11.8 J kg(-1) K-1 for {Co8Gd2}; 20.0 J kg(-1) K-1 for {Co4Gd2}; 21.1 J kg(-1) K-1 for {Co8Gd4}; 21.4 J kg(-1) K-1 for {Co8Gd8}; 23.6 J kg(-1) K-1 for {Co4Gd6}; and 28.6 J kg(-1) K-1 for {Co6Gd8}, from which we can see these values are proportional to the percentage of the gadolinium in the core.