End-on unidentate coordination

Dinitrogen is isoelectronic with CO and the bonding mechanism is also synergic, but its a -h 7c)-bonding capacity seems to be weaker than that of CO. Thus, metal complexes of N2 generally carry one or two terminal N2 ligands, in contrast to the prevalence of polycarbonyl complexes. As is seen in Table II, the N—distances observed in complexes of the end-on unidentate N2 are not appreciably different from that of free dinitrogen (1.0976 A). 

The first tris(dinitrogen) complex isolated is mer-Mo(N2)3 PPh-(n-Pr)2 3 depicted in Fig. 26. This complex was prepared by the reaction 

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