ELECTROCHEMISTRY AS A DIAGNOSTIC-TOOL TO DISCRIMINATE BETWEEN CLASSICAL M-(H)2 AND NONCLASSICAL M-(H2) STRUCTURES WITHIN A FAMILY OF DIHYDRIDE AND DIHYDROGEN METAL-COMPLEXES

The redox properties of a family of dihydride and dihydrogen complexes of iron, cobalt, rhodium, and iridium have been studied in detail and compared with those of the corresponding monohydrido derivatives. All of the complexes contain as stabilizing coligands either P(CH2CH2PPh2)3 (PP3) or N(CH2CH2PPh2)3 (NP3). The novel dihydride [(PP3)Fe(H)2] and its isotopomers [(PP3)Fe(H)(D)] and [(PP3)Fe(D)2] have been fully characterized by IR and 1H and 31P NMR techniques. The complexes [(PP3)Co(H2)]PF6 and [(PP3)Rh(H2)]BF4, which exhibit the nonclassical dihydrogen structure, undergo in tetrahydrofuran irreversible one-electron oxidation. As a result, the deprotonation of the H2 ligand occurs and the starting η2-H2 compounds are converted to the corresponding monohydrides [(PP3)MH]+ (M = Co, Rh). In contrast, the classical dihydrides [(L)Ir(H)2]BPh4 (L = PP3, NP3) and [(NP3)Rh(H)2]BPh4 show no redox activity within the potential window of tetrahydrofuran. The dihydride [(PP3)Fe(H)2)] can be oxidized to give the unstable species [(PP3)Fe(H)2]+ and [(PP3)Fe(H)2]2+, but no deprotonation reaction occurs. The cis-(hydride)(dihydrogen) complex [(PP3)Fe(H)(H2)]BPh4 undergoes one-electron reduction in tetrahydrofuran to give the neutral dihydride. Rare examples of paramagnetic monohydrides of cobalt(II), rhodium(II), iron(I), and iron(II) have been synthesized chemically or electrochemically and characterized by X-band ESR spectroscopy. © 1990, American Chemical Society. All rights reserved.