GAS-PHASE REACTIONS OF CR(CO)5(-), FE(CO)4(-), AND NI(CO)3(-) WITH ORGANIC ELECTROPHILES

Rate constants and product distributions for reactions of the title anions with 15 organic electrophiles as measured by Fourier transform ion cyclotron resonance techniques are reported. The electrophiles are all aromatic or olefinic compounds with electronegative substituents. The reaction efficiencies defined as the overall rate constant divided by the collision rate (k/k(c)) vary from unity to unmeasurable (upper limit 10(-3) or less). For each anion reactivity drops rapidly as the electron affinity (EA) of the electrophile drops below some critical range values. The critical range (eV) is 0.62-0.91, 1.01-1.29, and 1.29-1.44 for Ni(CO)3-, Cr(CO)5-, and Fe(CO)4-, respectively. Eight cases were examined where the EA of the electrophile was below the critical range, and no reactivity was observed for any of those cases. Twenty-four cases were examined where the EA of the electrophile is above the critical range, and in all of those cases reaction was efficient (k/k(c) > 0.10). Reactivity was observed for the four cases where the EA of the electrophile was in the critical range, but the efficiency was lower (k/k(c) < 0.05). In most cases where any reaction was observed displacement of one or more CO ligands was the dominant process. Exothermic charge transfer competes with ligand substitution in the Ni(CO)3- reactions, but only with tetracyanoethylene is charge transfer a significant product for Cr(CO)5- and Fe(CO)4-. These results are interpreted in terms of a mechanism for ligand substitution involving incipient charge transfer within a collision complex to produce a 16-electron metal center which then undergoes substitution. The ligand substitution results are compared to previous results on the reactions of the title ions both in the gas phase and in solution. Products of subsequent reactions of the initial products are described. It is suggested that oxidative addition to the metal of carbon-halogen and in one case carbon-carbon bonds (i.e., the decarbonylation of benzophenone) plays a role in this chemistry. One sequence of reactions is described that appears to involve formation of a carbon-carbon bond (i.e., the coupling of two 4-nitrophenyl groups in reactions of 1-bromo-4-nitrobenzene).