Thus, a mixture of Fe(HMDS)2 with one equivalent of 1a resulted in two azide stretching bands at 2099 cm -1 , corresponding to free 1a, and at 2118 cm -1 , attributed to adduct 2 (Fig. ). The 19 cm -1 blue-shift upon coordination indicates no π-backbonding of the coordinated azide, in line with the cobalt system of Betley et al. It is worth noting that α-nitrogen coordination of organic azides to first-row transition metals has been detected only very rarely and only with cobalt. The observation of signals for free and coordinated azide supports the presence of an equilibrium. Further evidence of an equilibrium was obtained by changing the equivalents of 1a used for the IR measurement (Fig. ). The use of only half an equivalent of azide resulted in a higher ratio of coordinated vs. free azide, while two equivalents of azide decreased this ratio.