Generation and spectroscopic characterization of new 18+δ electron complexes. Relationship between the stability of 18+δ electron organometallic complexes and their ligand reduction potentials

The relationship between the electrochemical reduction potential of a ligand and the ability of that ligand to form a kinetically inert 18+δ complex in a reaction with a 17-electron radical was investigated. (18+δ complexes are 19-electron adducts in which the unpaired electron is primarily located on a ligand orbital.) To probe the relationship, a series of 18+δ complexes was generated by irradiating the Cp′₂Mo₂(CO)₆, Cp₂Fe₂(CO)₄ and Co₂(CO)₈ dimers in the presence of a series of bidentate phosphorus ligands. (Irradiation of the dimers formed 17-electron metal radicals by photolysis of the metal-metal bonds.) These experiments showed that bidentate phosphorus ligands with reduction potentials more positive than −1 volt (versus SCE) formed long-lived 18+δ complexes (in THF or CH₂Cl₂ solutions at 23 °C), while ligands with potentials more negative than −1 V formed reactive 18+δ complexes. The inability to detect 18+δ complexes in the latter case is attributed to kinetic factors: the 18+δ complexes are powerful reductants and they readily initiate a chain disproportionation of the dimers by electron transfer. Analogous experiments with bidentate nitrogen ligands did not produce any detectable 18+δ complexes. In this case, the undetectability of the 18+δ complexes is probably thermodynamic in origin: the hard nitrogen ligands and soft metal centers form adducts that are unstable with respect to metal-nitrogen bond cleavage. 18+δ complexes are the subject of increasing interest, especially as models for their more reactive 19-electron-complex counterparts. These results provide some guidelines for the design of 18+δ complexes that can be synthesized, isolated and characterized for such studies.