COVALENCY EFFECTS IN THE STANDARD ENTHALPIES OF FORMATION OF TRIVALENT LANTHANIDE AND ACTINIDE HALIDES

The effect of covalency on the standard formation enthalpies of crysta l trihalides of lanthanides and actinides M, Delta H-f(o)(MX3,c), (X = F, Cl, Br and I) is discussed. The enthalpies, including covalency ar e derived, i) using a deviation of effective charges of M and X from t he ionic distribution, resulting in the change of ionization energies of M and electron affinity of X when trihalides are formed, and, ii) t aking into account the covalent contribution from overlap of valent at omic orbitals of M and X with a change in charge. For a given X, coval ency results from three contributions, reduction of the negative charg e on X decreasing monotonically with Z of M, reduction of the positive charge on M increasing irregularly with Z and overlap of X and M atom ic orbitals which increases with Z. The covalent contribution changes irregularly across the two series. The difference Delta H-f(o)(AnX(3), c)-Delta H-f(o)(LnX(3),c) for An-Ln pairs shows two trends: it decreas es from the beginning (Ac-La) to the middle of the series (Am-Eu) and then increases towards the end (Yb-No).