BRIDGE BONDING OF N2 TO DINUCLEAR TRANSITION-METAL SYSTEMS

Ab initio quantum chemical calculations have been performed on two different structures of M2N2 for first- and second-row transition metal systems involving titanium, yttrium, and zirconium. In both structures N2 is bridging the metal atoms, either in a perpendicular side-on coordination mode, or in a parallel end-on coordination mode leading to a linear M-N-N-M. Both types of structures have been observed experimentally for ligated complexes of the type (LnM)2N2. Comparisons between experiment and theory show that the geometries of the M2N2 unit in the experimentally observed complexes correspond closely to the calculated geometries for different states of the naked M2N2 systems. On the basis of these geometric similarities, the electronic structure of the ligated complexes is analyzed and discussed. Furthermore, it is found that of the first- and second-row transition metals, only the leftmost metals of the second row can form the type Of M2N2 Complex where the N-N distance corresponds to a single bond. This type of complex has the N2 molecule perpendicularly coordinated, and it has been experimentally observed for the case of zirconium. The calculations predict that a similar complex should be possible to make for yttrium.