Photoelectron Spectra and Bonding in Phosphorus Compounds'

The results of (low energy) photoelectron spectroscopy render possible a better appreciation of bonding in molecules. The application of the new experimental method is demonstrated utilizing representative phosphorus compounds, and the close symbiosis delineated with molecular orbital models. Among the topics discussed are: the element P4, phosphine PH3 and derivatives, phosphorus substituent effects in organic compounds and phosphorus multiple bonding. 'Actually, everything is much more complicated'2 MOLECULAR STATES AND MOLECULAR ORB1TALS On absorption or emission of energy a molecule is transferred from one molecular state to another with different electron distribution. Measurements therefore only yield information concerning the energy difference between two molecular states and the electron distributions within them. It is the intention of the chemist to interpret and to rationalize these data, which stimulates the development and the application of simplifying models. Although, for instance, connection lines between atoms in structural formulas are rather topological symbols, their identification with localized electron pairs has led to the conceptualization of bond models—including the definition of model quantities like covalent radii, bond enthalpies or electronegativities by artificially subdividing individual molecules into more or less independent sub-units. On the other hand, molecular orbital models —derived from linear combinations of e.g. one-electron basis functions— useful as they are for correlations with experimental data tend to overemphasize electron delocalization. Summarizing, choosing the appropriate model3 to describe molecular states is not so much a question of verifiability but rather one of usefulness and applicability. Photoelectron spectroscopy, the new measurement technique7 to determine all (mono) ionization potentials of chemical compounds, is based on the well-known photoionization process of a neutral species M to its radical cation M, M+hv+Me+ee (1) in which the photon energy hv in excess of the ionization energy IE(M) * Correspondence address: 6 Frankfurt/Main, Theodor-Stern-Kai 7.