A now three-dimensional Mn-II complex, [Mn-3(HCO2)(2)(L)(2)(OCH3)(2)], 1 (L = nicotinate N-oxide), was synthesized by solvothermal reaction and magnetically characterized. Complex I exhibits an unprecedented 3,6-connected 5-nodal net topology with Schlafli notation (4(2);6)4(4(3))2(4(6);6(6);8(3);10}2{4(8);6(6);8) and was assigned as a homospin ferrimagnet. Interestingly, when 1 was placed in air for ca. one month, the methoxy anions in 1 were gradually exchanged by hydroxyl anions and 1 underwent a single-crystal-to-single-crystal structural transition to a new but similar complex, [Mn-3(HCO2)(2)(L)(2)(OH)2]center dot 4H(2)O, 2. The anion exchange and water molecules filling the channels of 2 affect the magnetic behavior at low temperature compared to that of 1.