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 .~

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The He I and He I1 spectra of the compounds M(CO)5X and M2COlo (M = Mn, Re ; X = C1, Br, I), valence region X-ray photoelectron spectra of M2(CO)lo, and the He I spectra of some related compounds are reported and discussed. From observations of intensity changes it is deduced that in M(CO)51 and Re(CO),Br the orbital ionisation sequence for the low ionisation potential region is e(X) < b,(M) < e(M) < al(M-X Q bonding), but that in M(CO),C1 and Mn(CO),Br the sequence of e orbital ionization potentials is reversed. These assignments remove several inconsistencies which existed between earlier assignments and observations upon other compounds. Probable orbital sequences for other M(CO),L compounds are discussed.

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