Electronic structures of some .eta.3-allyl transition-metal complexes

Abstract

In recent years there has been much interest in the electronic structures of transition-metal carbonyl i r complexes of hydrocarbons in these laborat~ries~-~ and in others6 A primary thrust of the work in these laboratories has been to demonstrate that the ultraviolet photoelectron spectra (UPS) of the stable complexes of intriguing labile transients such as cyclobutadiene can be used in estimating the T ionization energies of the In fact, recent UPS studies by Heilbronner and co-workers' on tetra-tert-butylcyclobutadiene have supported our predictions for the i r electronic structure of the parent cyclobutadiene. This note will address UPS of several transition-metal complexes of and the electronic structure of another interesting transient species-the allyl radical. Results and DiscussionThe low-ionization-energy regions of the photoelectron spectra of four q3-allyl transition-metal carbonyl complexes are shown in Figure 1. The resolution in the region above 12 eV for these spectra was not sufficient to render definitive assignments for the ionization bands corresponding to removal of electrons from a-bonding orbitals. However, assignment of the bands in the region below 12 eV for these complexes would appear to be straightforward.The spectrum of q3-allylmanganese tetracarbonyl, A, exhibits four resolved band components below 12 eV. The I1and I2 components at 8.05 and 8.48 eV, respectively, must correspond to ionization of orbitals primarily confined to the transition metal Mn; ample precedent for such an assignment exists in the literatures2" Thus, the I, and I4 components at 9.03 and 11.13 eV can be assigned to ionization of the perturbed i r orbitals on the allyl ligand. Using a very elaborate photoelectron spectrometer, Houle and Beauchamp were able to obtain the spectrum of the allyl radical.8 They report a(1) (aa 9 io 11 1216 7 a o io 11 12 IOGIZAIIOS ESERGY (eV) Figure 1. Photoelectron spectra of some $-allyl transition-metal complexes. The excitation source was the H e I resonance line.first ionization potential of 8.13 eV for this radical which, of course, must refer to ionization of the highest occupied x orbital which is primarily nonbonding in character. Thus, it is evident that this i r orbital is perturbed significantly (stabilized by ca. 0.9 eV) by the bonding interaction with the manganese tetracarbonyl moiety. In fact, the 0.9-eV perturbation is almost exactly the same as that (0.89 f 0.07 eV)u observed for the highest occupied x orbital of a number of hydrocarbons when these systems are coordinated to an iron tricarbonyl moiety. While this may seem fortuitous, a similar observation has been reported for the molybdenum, chromium, and iron carbonyl complexes of norb~rnadiene.~ It is interesting that such a divergent series of transition-metal carbonyl complexes has similar perturbation energy for the x HOMO; however, there would seem to be no theoretical justification for this experimental observation. Because of overlapping bands due to decomposition products of 3-butenyl nitrite used to genera...

Organic Matrix and Solution Photochemical Studies of (Allyl)dicarbonyl(cyclopentadienyl)molybdenum Compounds

“…The metal-based orbitals 1aЈ and 2aЈ are effectively non-bonding and are the highest occupied molecular orbitals, while the LUMO is the antibonding combination of the n allyl orbital and the a" (a" Ϫ n). The MO analysis is supported by ultraviolet photoelectron spectral studies, [17] in which the six observed bands were assigned to: I 1 and I 2 (metal d orbitals, 1aЈ and 2aЈ), I 3 (allyl π HOMO, n ϩ a"), I 4 and I 5 (cyclopentadienyl orbitals) and I 6 (second allyl π orbital, π ϩ s ϩ p z ). Figure 6.…”

Electronic structure of conformational isomers of (.eta.3-allyl)ruthenium complexes

“…The low ionization energy regions of the UPS of the six complexes studied in this work are shown in Figure 1. The band structures above 12 eV in the spectra were much too complex for definitive interpretation. The vertical ionization energies measured from the UPS in Figure 1 are presented in Table I.…”

“…It is impossible to assess the exact difference of this through-space interaction for the two sets of isomers because of extensive band overlapping in the UPS of D and E. Also "through-bond" interactions between the C3H5 and C5HS moieties may be some different for the two sets of isomers, as might be inferred from the appearance of the "metal-orbital" regions of the UPS. In any case we are convinced that the UPS band corresponding to the x-allyl HOMO occurs at higher ionization energy than do all of the metal UPS bands in contrast to the apparent situation for bis(T?3-allyl)metal complexes.7•8 The current UPS interpretations for A, B, D, and E are entirely consistent with those for the other allyl complexes studied previously in these laboratories. 12 The interpretation of the UPS of the iron complex F is more complicated. Clearly the bands at 6.90,7.28, and 7.87…”

The low-lying cationic hole-states of η3-allyl tetracarbonyl complexes of Mn and Re

“…The low-energy photoelectron spectra of 1 and 2 have been recorded by Worley and co-workers [17] ; a definite assignment for the ionization energies, however, has not been given. The photoelectron spectra of various other ~3-complexes with polycarbonyl fragments have been published by Oskam and co-workers [18].…”