Four new heterometallic coordination polymers have been successfully synthesized, namely, {[Ho-2(HCAM)(6) Mn-3(H2O)(12)]center dot 17.5H(2)O}(n) (1), {[Er-2(HCAM)(6)Mn-3(H2O)(12)]center dot 17.5H(2)O}(n) (2), {[Ho-2(HCAM)(6)Zn-3(H2O)(12)]center dot 26H(2)O}n (3), and {[Er-2(HCAM)(6)Zn-3(H2O)(12)]center dot 26H(2)O}(n) (4) (H(3)CAM = chelidamic acid). X-ray crystallographic studies reveal that coordination polymers 1-4 are isostructural and crystallized in the rhombohedral crystal system, space group R. These compounds comprise a 2D honeycomb-type framework. A 2D water sheet is first found in 3 and 4, which exhibits a novel topological motif. The magnetic results for 1-4 show that ferromagnetic interactions take place between the Ho3+/Er3+ and Mn2+ ions within 1 and 2.