The photoelectron spectra excited by helium 21.2 eV radiation in the complexes Mn(CO)5X, where X = H, C1, Br, I, CH3, CF3, COCF3 and Mn(C0)5, have been determined. The ionization potential data are discussed in terms of a simple molecular orbital description of the electronic structures.PHOTOELECTRON SPECTRA OF Mn(CO),X The hydride, and also the methyl and perfluoro-methyl compounds, show two PE bands in region A. The relative intensities of these bands are close to 2 : 1 in Mn(CO),H, and about 3 : 1 in Mn(CO),CF3 : in the methyl compound, the intensity ratio is inverted but approximately 1 : 2. Invoking our intensity criterion, (1) above, we therefore assign these two LP to the mainly metal e(dx,,dy,) and b2(dxy) m.o., as shown in table 1. The hydride case is illustrated in fig. 2(b). The acyl compound, Mn(CO),COCF,, also apparently has two IP in region A , but these are incompletely resolved. We presume they have the same origin as the IP of Mn(CO),H, etc., assigned previously.The PE spectra of the chloride and bromide exhibit three low-energy bands, while Mn(CO),I has four bands. We assume that the first two bands of the latter compound are the components, split under spin-orbit interactions, of a band which corresponds to the first band in the PE spectrum of either Mn(CO),Br or Mn(CO),CI. Now the only terms of the Mn(CO),X+ ion which are subject to multiplet splitting