Here we report the synthesis and structural characterization of heterotrimetallic (Pt(2)(II)pd(2)(II)Mn(2)(II))(n) coordination polymers that show different dimensionalities and electrocatalytic activities depending on chloride and bromide employed in the system. The reaction of the Pt(2)(II)pd(2)(II) tetranuclear complex [Pd2Pt2(NH3)(4)(D-pen)(4)] (1), bearing free carboxylate groups, with Mn2+ in the presence of chloride produced the (Pt(2)(II)pd(2)(II)Mn(2)(II))(n )coordination polymer [Mn2Cl2(H2O)(6)(1)]Cl-2 (2(CI)), in which the Pt(2)(II)pd(2)(II) units of 1 are linked by [MnCl(H2O)(3)](+) moieties in a 1:2 ratio to form a 2D sheetlike structure. The corresponding reaction in the presence of bromide also gave the (Pt(2)(II)pd(2)(II)Mn(2)(II))n coordination polymer [Mn-2(H2O)(6)(1)]Br-4(3(Br)), but 3(Br) adopted a 3D network structure via a 1:1 linkage of the Pt(2)(II)pd(2)(II) units with [Mn-2(H2O)(6)](4+) moieties. These complexes showed appreciably different electrocatalytic activities for H-2 evolution.