Transition metal–dinitrogen complexes are coordination compounds in which molecular nitrogen, N₂, acts as a ligand and coordinates to a transition metal through its nitrogen atoms. Although N₂ is a very stable molecule because of its strong N≡N triple bond, transition metals can coordinate it through donation of electron density from the nitrogen molecule to the metal and, more importantly, through back-donation from filled metal d-orbitals into the antibonding π* orbitals of N₂. Such complexes are important because coordination can activate nitrogen towards further chemical reactions. A well-known example is trans-[Mo(N₂)₂(dppe)₂], where dppe represents 1,2-bis(diphenylphosphino)ethane. It is a typical transition-metal dinitrogen complex containing terminally coordinated N₂ ligands. It can be prepared by reduction of a molybdenum(III) complex in an atmosphere of nitrogen. A suitable representation of the preparation is: [ \mathrm{MoCl_3(dppe)_2 + 3Na + 2N_2 \rightarrow trans-[Mo(N_2)_2(dppe)_2] + 3NaCl} ] The reaction is carried out under an atmosphere of nitrogen so that N₂ can coordinate to the reduced molybdenum centre. The bonding of dinitrogen to a metal may be described by two important components. First, the nitrogen