Mass Spectrometric Determination of the Dissociation Energies of LaAu, CeAu, PrAu, and NdAu and Predicted Stability of Gaseous Monoaurides of Electropositive Metals

Abstract

The Knudsen‐cell mass spectrometric method has been used to study the exchange reactions LnAu(g)+Au(g)=Ln(g)+Au₂(g), where Ln stands for a lanthanide metal. For Ln=C, the third‐law and second‐law enthalphies ${\mathrm{\Delta H}}_0°$ in kilocalories per mole were obtained as 23.4 ± 0.4 and 22.5 ± 1.2, respectively. The third‐law enthalphies, in kilocalories per mole, for Ln=La, Pr, and Nd resulted in 29.6 ± 1.0, 20.3 ± 0.6, and 16.4 ± 1.5, respectively. The third‐law investigation of the exchange reactions CeAu(g)+Ln(g)=LnAu(g)+Ce(g) yielded the reaction enthalpies ${\mathrm{\Delta H}}_0°$ in kilocalories per mole of: − 3.8 ± 2.2 for Ln=La, + 3.1 ± 0.8 for Ln=Pr, and + 6.4 ± 1.5 for Ln=Nd. Combining the enthalphies of the first reaction with the known dissociation energy of Au₂(g) and using the value obtained for $D_0°$ (CeAu) as a standard for the second exchange reaction, the following dissociation energies, $D_0°$ , in kilocalories per mole have been derived for the gaseous lanthanide auride molecules: LaAu, 79 ± 5; CeAu, 75.0 ± 3.5; PrAu, 72.5 ± 5; and NdAu, 69 ± 6. The corresponding standard heats of formation ${\mathrm{\Delta H}}_{\text{f,298}}°$ in kilocalories per mole of these gaseous intermetallic compounds are 111 ± 6, 113 ± 5, 99.7 ± 5.5, and 96.5 ± 6.5, respectively. The high values for the dissociation energies of LaAu, CeAu, PrAu, and NdAu are explained in terms of the Pauling model of a single covalent bond to which a significant extra ionic contribution is added. Using this model, the dissociation energies of the diatomic alkali, alkaline‐earth, and rare‐earth aurides have been calculated. These gaseous intermetallic compounds are expected to be intermediate in their ionic character to the corresponding iodides and hydrides.