reported the first crystallographic evidence for side-on coordination of N2 to a single metal center in a photoinduced metastable state. When treated with UV light, the transition metal-dinitrogen complex, [Os(NH3)5(N2)]2+ in solid states can be converted into a metastable state of [Os(NH3)5(η2-N2)]2+, where the vibration of dinitrogen has shifted from 2025 to 1831 cm−1. Some other examples are considered to exist in the transition states of intramolecular linkage isomerizations. Armor and Taube has reported these isomerizations using 15N-labelled dinitrogen as ligands. Bridging, side-on In a second mode of bridging, bimetallic complexes are known wherein the N-N vector is perpendicular to the M-M vector, which can be considered as side-on fashion. One example is [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2). The dimetallic complex can react with H2 to achieve the artificial nitrogen fixation by reducing N2. A related ditantalum tetrahydride complex could also reduce N2. Reactivity Cleavage to nitrides When metal nitrido complexes are produced from N2, the intermediacy of a dinitrogen complex is assumed. Some Mo(III) complexes also cleave N2: 2Mo(NR2)3 + N2 → (R2N)3Mo-N2-Mo(NR2)3 (R2N)3Mo-N2-Mo(NR2)3 → 2N≡Mo(NR2)3 Attack by electrophiles Some electron-rich metal dinitrogen complexes are susceptible to attack by electrophiles on nitrogen. When the electrophile is a proton, the reaction is of interest in the context of abiological nitrogen fixation. Some metal-dintrogen complexes even catalyze the hydrogenation of N2 to ammonia in a cycle that involves N-protonation of a reduced M-N2 complex. See also Abiological nitrogen fixation Transition metal nitrido complex References Coordination complexes Nitrogen compounds