Real time infrared spectroscopic probe of the reactions of Fe(CO)(3) and Fe(CO)(4) with N-2 in the gas phase

Time-resolved infrared absorption spectroscopy has been used to study the g as-phase reactions of Fe(CO)(3), Fe(CO)(3)N-2, and Fe(CO)(4) with N-2, wher e Fe(CO)(3) is generated by 308 nm laser photolysis of Fe(CO)(5). The heret ofore unknown complex Fe(CO)(3)(N-2)(2) forms by addition of N-2 to Fe(CO)( 3)N-2 with a rate constant of (5.4 +/-. 1.8) x 10(-16) cc molecule(-1) s(-1 ). This rate constant is much smaller than is typical for the addition of s mall ligands to coordinately unsaturated metal carbonyls, and data are cons istent with this reaction being activated. The bond dissociation energy (BD E) for the loss of a N-2 ligand from Fe(CO)(4)N-2 is 17.6 +/- 1.8 kcal mol( -1). The activation energy for the loss of N-2 from Fe(CO)(3)(N-2)(2) is 14 .1 +/- 5.2 kcal mol(-1). The kinetics of the system are consistent with a m odel that involves equilibria between Fe(CO)(3), Fe(CO)(3)N-2, and Fe(CO)(3 )(N-2)(2) as well as reactions of coordinatively unsaturated species with F e(CO)(5). Using this kinetic model, an upper limit for the BDE for the Fe-N -2 bond in Fe(CO)(3)N-2 has been estimated and the BDE for the Fe-N-2 bond in Fe(CO)(3)(N-2)(2) has been determined under the assumption that one of t he relevant reactions has a minimal activation energy.