Photoelectron studies of metal carbonyls. Part 2.—The valence region photoelectron spectra of the group VIA hexacarbonyls

Abstract

He(I), He(l1) and valence region X-ray photoelectron spectra are reported for the Group VIA hexacarbonyls and are assigned on the basis of published molecular orbital calculations. A pronounced increase in intensity of the first photoelectron band is observed on descending the group, and multiplet structure is clearly discernible on this band in the case of W(CO)6.As a starting point in the study of the U.V. photoelectron (pe.) spectra of transition metal complexes, the Group VIA hexacarbonyls are attractive in view of their high symmetry and volatility. The experience gained from an investigation of these molecules should provide a firm basis on which to develop an understanding of the u.v.-p.e. spectra of more complex systems containing the CO ligand. A preliminary report of the He(1) p.e. spectra has appeared but no detailed assignment was advanced.l Other ionisation studies of the Group VIA hexacarbonyls include electron impact work 2-5 and photoionisation,6* which yield only the first ionisation potential,? and also X-ray photoelectron studies primarily concerned with the core levels of the molecules. We report here the high resolution p.e. spectra of chromium, molybdenum and tungsten hexacarbonyls excited by He(1) radiation (58.4 nm). These show some significant differences from those previously reported. In addition, since assignment of these spectra depends to such a large extent on relative band intensities, we report the spectra excited by He(I1) radiation (30.4 nm) and also the valence region X-ray p.e. spectra. EXPERIMENTAL u.v.-p.e. spectra were recorded on a Perkin-Elmer PS 16 spectrometer modified by the inclusion of the Perkin-Elmer heated inlet system and a 2 mm bore boron nitride discharge capillary in the lamp unit. The use of this capillary, together with a low helium pressure and high current density (150 mA, 10 kV), gave approximately 10 % of the He(I1) line at 30.4 nm.This proved sufficient to give He(I1) spectra of satisfactory intensity (ca. 300 c/s). Efficient differential pumping on the lamp minimizes the intensity of the helium ionisation line at 24.58 eV, but the presence of this structure in low intensity is useful for calibration purposes.Early work is summarised in Part 1 of this ~e r i e s .~

Valence ionization spectra of group six metal hexacarbonyls studied by the symmetry-adapted cluster-configuration interaction method

“…Previous assignments and arguments relied on the similarity of these three carbonyls. 21,[23][24][25] Indeed, their ionization spectra are very similar, even though the differences in ionization energies of the atoms are more than 1 eV between the group six metals. 29,30 The lowest ionized state of these three carbonyls was studied by electron momentum spectroscopy and Hartree-Fock and density functional theory ͑DFT͒ calculations.…”

“…Based on Koopmans' theorem, Higginson et al 21 identified seven peaks in this region; however, their assignment involved uncertainties. Other studies identified only three or four peaks in this region.…”

“…They have been widely studied as a prototype for understanding the chemical bonds of organometallic compounds 18,19 or a model for adsorption of carbon monoxide on metal surfaces. 20 On the ionizations in the valence region, He I and He II PES, 21 x-ray PES ͑XPES͒, 22 synchrotron radiation PES ͑SRPES͒, 23,24 and PIES ͑Ref. 25͒ have been reported for Cr͑CO͒ 6 , Mo͑CO͒ 6 , and W͑CO͒ 6 .…”

“…The present calculations well reproduced the ionization energies observed experimentally using PES. [21][22][23][24] A consistent assignment was provided for the experimentally identified peaks of the group six metal hexacarbonyls. This study is a typical example of the application of the SAC-CI method to transition metal compounds.…”

Electron momentum spectroscopy of metal carbonyls: a reinvestigation of the role of nuclear dynamics

“…Because of their high volatility and octahedral symmetry, they are also most useful molecular models for studies of bond formation between carbonyl groups and metal surfaces. Investigations of the outermost valence orbitals of these compounds comprise the ultra-violet [He I, He II] and X-ray photoelectron experiments by Higginson et al [42] and various studies [43][44][45] employing Electron Momentum Spectroscopy (EMS). In the EMS studies of Cr(CO) 6 , Mo(CO) 6 , and W(CO) 6 by Chornay et al [43], and by Rolke et al [44], the measured (e, 2e) ionization cross-sections for the HOMO were found to be much larger than expected in the low momentum region, according to standard theoretical models of (e, 2e) ionization processes in the high energy limit, and to be largely inconsistent with the nodal characteristics of an uncorrelated (Hartree-Fock) 2t 2g orbital at p = 0.…”