Paramagnetic transition-metal carbonyls and cyanides. Part 10. Electron addition to various carbonyl(η5-cyclopentadienyl)metal halides; an electron spin resonance study

Abstract

Exposure of [Mo(cp)(CO)₃I], [W(cp)(CO)₃I], [Fe(cp)(CO)₂Cl], [Fe(cp)(CO)₂1], and [Mn(cp)(CO)₃](cp =η⁵-cyclopentadienyl) either as the pure compounds or as dilute solutions in solvents that facilitate electron capture, to ⁶⁰Co g;-rays at 77 K, gave the corresponding anions. All the halides had e.s.r. spectra dominated by large hyperfine coupling to the halogen nuclei, characteristic of electron capture into the metal-halogen σ* orbitals. This is expected for the iron complexes, which are formally 3d⁶, but surprising for the Mo and W complexes which are formally 3d⁴. No hyperfine features from coupling to metal nuclei were observed for the iron or tungsten complexes, but well defined features from ^95/97Mo nuclei were observed in spectra of irradiated single crystals of [Mo(cp)(CO)₃1]. Analysis of this coupling together with the g-tensor components supports the σ* assignment, suggesting that the 3d_z² orbital is the major metal orbital involved. On annealing, the molybdenum complex anion was converted into another complex, exhibiting smaller hyperfine coupling to iodine which is thought to be the relaxed structure for [Mo(cp)(CO)₃l]^– with a π* semi-occupied molecular orbital (s.o.m.o.) rather than the σ*(s.o.m.o.). In contrast, [Fe(cp)(CO)₂C1]^– and the iodide gave an axially symmetric centre with no halogen hyperfine coupling, thought to be [Fe(cp)(CO)₂]. This was formed together with a more stable protonated species, possibly [FeH (cp)(CO)₂]⁺, which exhibited well defined proton hyperfine coupling (²H coupling when CD₃OD was the solvent). Possible structures for this species are considered. The manganese carbonyl complexes, studied for comparative purposes, gave well defined anions, but we were unable to obtain clear-cut results for the species [Mn(cp)(CO)2]^–, isoelectronic with the corresponding iron derivative.