The dissociation energies of gaseous BaPd and BaRh and calculated dissociation energies of selected diatomic group IIA–platinum metal intermetallic molecules

The high-temperature Knudsen effusion mass spectrometric technique has been used to measure the second law and third law enthalpies of the reactions BaPd(g) =Ba(g)+Pd(g) and BaAu(g)+Pd(g) =BaPd(g)+Au(g). The third law enthalpies ΔH°298 (in kJ) have been obtained as 224.3±2.4 and 35.8±1.8. The corresponding second law values are 216.8±8.9 and 30.9±9.8 kJ, respectively. From these reaction enthalpies the following thermodynamic properties in kJ mol−1 (kcal mol−1) were derived: D°298[BaPd(g)]=221.8±5.0 (53.0±1.2), D°0[BaPd(g)]=220.0±5.0(52.6±1.2), ΔHf,298[BaPd(g)]=336.8±10 (80.5±2.5), and °Hf,0[BaPd(g)]=338.5±10 (80.9±2.5). The dissociation energy of BaRh in kJ mol−1 (kcal mol−1) has been determined as D°0[BaRh(g) =257.4±25 (61.5±6.0) or D°298[BaRh(g)]=259.4±25 (62.0±6.0). The dissociation energies of BaPd and BaRh have been interpreted in terms of an empirical valence bond approach assuming double bond formation and utilizing a 5d orbital of barium in BaPd. Values for the bond energies of the diatomic molecules of barium with the platinum metals and of SrPt calculated by the model are also given.