The reactions of laser-ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end-on bonded NNBeCO and side-on bonded (η2-N2)BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end-on bonded complex has a triplet ground state while the side-on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge- and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s02p2 excited configuration and that the metal-ligand bonds can be described in terms of N2→Be←CO σ donation and concomitant N2←Be→CO π backdonation. The results demonstrate that the activation of N2 with the N−N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.