A first-principles density functional theory study of the recently reported IrN2 and OsN2 as well as the as yet not synthesized RhN2 transition metal nitrides is presented. It is found that although IrN2 has a slight preference for the arsenopyrite structure, OsN2 prefers the marcasite structure and RhN2 has no clear preference between these two structures. The electronic structure of these nitrides is discussed on the basis of a simple model for the octahedral MN4 chains present in both the marcasite and arsenopyrite structures. It is shown that the distortion from the marcasite to arsenopyrite structure in these nitrides is not a Peierls distortion, contrary to what is often claimed. Both metal-metal interactions along the MN4 chains and interchain interactions through the dinitrogen units compete in these compounds. IrN2 is found to be a semiconductor whereas OsN2 and RuN2 should be three-dimensional metals. It is predicted that RhN2 can exist in two metallic forms with the marcasite and arsenopyrite structures, which should exhibit different anisotropies. © 2008 The Royal Society of Chemistry.