AbstractA copolymer (PC‐1) was synthesized from a mixture of 1,2‐and 1,3‐isomers of tricarbonyl‐(methylvinylcyclopentadienyl)manganese (1) and styrene (St) crosslinked with 1,4‐divinylbenzene, and a membrane (PM–1) consisting of the1/St copolymer. A polymer‐supported dinitrogen complex (PC‐4) was prepared from the tetrahydrofuran (THF) complex ofPC‐1and molecular nitrogen in a solid‐gas phase reaction under moderate conditions. The dinitrogen complex membrane (PM–4) was readily obtained directly fromPM‐1by a “one‐step reaction” on UV light irradiation under normal pressure of nitrogen at room temperature. A prominent feature of polymer‐supported dinitrogen complexes,PC‐4andPM‐4, is their high stability as compared with the corresponding low molecular weight analogues. The coordinated nitrogen can be substituted by carbon monoxide in aPC‐4/THF mixture. The mechanism of nitrogen co‐ordination toPC‐1andPM‐1was investigated by