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

• Published in: Inorganica Chimica Acta Vol. 240; no. 1; pp. 405 - 412
• Main Authors: Meyer, Reinhard, Schut, David M., Keana, Keith J., Tyler, David R.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.1995
• Subjects: Carbonyl complexes; Dinuclear complexes; Electron transfer; Transition metal complexes; Electron transfer; Carbonyl complexes; Transition metal complexes; Dinuclear complexes
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/0020-1693(95)04561-9

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′ 2Mo 2(CO) 6, Cp 2Fe 2(CO) 4 and Co 2(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 CH 2Cl 2 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.