N3-Ligdnd constant, PL/V ( & 0.05 V cstimated error) + 1.46 1.40 0.00 -0.07 -0.10 -0.35 -0.38 -0.40 -0.4:) -0.58 -0.59 -0.77 -0.78 -0.88 -1.00 -1.16 -1.15 -1.17-1.19 -1.22 -1.26 -1.55measured and values which we have deduced by intcrpolation from data from other series of Complexes (see below). These ligand constants refer t o L as a twoelectron donor ligand which must be added t o the 16electron moiety to give the closed-shell 18-electron complexes [M,L], x bonding being ignored. Considered in this way the nitride ligand in [M,N] is formally N+, the nitrosyl NO+, and the hydride H-. From Table 1 it can be seen that charged ligands occupy the extremes of the scale. The weakest Q donors with strong x-acceptor

The first dinitrogen complex, [Ru(NH3)5(N2)]2+, was identified in 1965 by Allen and Senoff,1 who were attempting to obtain [Ru(NH3)6]2+ using a method of preparation which had been published some 20 years earlier.2 This makes it very likely that it was synthesized unknowingly considerably earlier than 1965, but the discovery was certainly greeted with considerable acclaim. It had been foreseen in many circles, in part because of the isoelectronic nature of carbon monoxide and dinitrogen, and in part because it was known that microorganisms could convert dinitrogen to ammonia, and that this transformation is metal-dependent (molybdenum-and/or vanadium-dependent).3It was a simple matter to postulate initial metal-dinitrogen binding in these biological systems.4The discovery of [Ru(NH3)5(N2)]2+, together with the announcement of some less-well-characterized systems for fixing nitrogen uncovered by Vol'pin and his coworkers in Moscow,5 encouraged a huge amount of research into dinitrogen complexes and their reactions. The fears of some of us (who were privileged to be paid to do this kind of research) that [Ru(NH3)5(N2)]2+ would provide a facile route from dinitrogen to ammonia and force us to seek alternative employment were confounded. Despite the steady stream of new dinitrogen complexes,6 it was not until 1972 that the first reaction of coordinated dinitrogen to give a well-defined complex product was announced.7 This was Em acetylation and was followed shortly by the discovery of the protonation of coordinated dinitrogen to give hydrazido(2-) complexes and the subsequent conversion G. Jeffery Leigh was born in London, England, In 1934. He performed his undergraduate and graduate work at King's College, University of London, and In 1959 became a lecturer In the University of Manchester. He took a sabbatical year In Munich with E. 0. Fischer In 1963, and this turned his Interest to transition metal chemistry. In 1965 he joined the recently established Nitrogen Fixation Unit at the University of Sussex, where he has been ever since, currently as Deputy Head of Laboratory. His principal research Interest Is chemical and biological nitrogen fixation, but he also has interests In organometalllc chemistry.

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The trihydride complex [ FeH,(dmpe),] + (dmpe = Me,PCH,CH,PMe,) is best represented as [ FeH (H,) -(dmpe),] +, with the H-H separation in the H, being ca. 0.81 A. The H, can be replaced by N, , CO, MeCN, MeNC and C2H4. The dinitrogen complex [FeH( N, ) (dmpe),] + may be deprotonated to yield the solutionstable, trigonal-bipyramidal species [Fe(N,) (dmpe),], which can be reprotonated by HCI to yield [FeCI,-(dmpe),], N, , H, and NH,. The crystal structures of [FeH(H,)(dmpe),]BPh, and [FeH(N,)(dmpe),]BPh, have been determined.

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