Hydration change reaction of light REE^<3+>(aq) series-II : Contrasting nephelauxetic effects between hydrated [REE(H_2O)_9]^<3+> and [REE(H_2O)_8]^<3+> series and the tetrad effects in their hydration enthalpies and REE-OH_2 distances
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Light REE^<3+>(aq) is a mixture of two distinguishable species of nonahydrate and octahydrate in equilibrium with respect the hydration change reaction that [REE(H_2O)_8]^<3+> (aq)+H_2O(1)=[REE(H_2O)_9]^<3+>(aq), and the apparent hydration number of light REE^<3+>(aq) is changing across the series. Here is discussed nephelauxetic effects of the two REE^<3+>(aq) series from ΔH^0_r data for the hydration change reaction and the tetrad effect variations of their hydration enthalpies (ΔH_<hyd>) and REE-OH_2 distances. The ΔH^0_r data combined with those data for solution of REE(ES)_3・9(H_2O) with ES=C_2H_5SO_4^- reported previously, suggest that Racah (E^1 and E^3) parameters for light REE^<3+> like Nd^<3+> decrease in the following order: [REE(H_2O)_9]^<3+>(aq)>REE(ES)_3・9(H_2O)>[REE(H_2O)_8]^<3+>(aq). The average REE^<3+>-OH_2 distance also decreases in this order. Hence this is a nephelauxetic series that Racah parameters are reduced with decreasing REE^<3+>-OH_2 distance. The Racah parameters for [REE(H_2O)_9]^<3+>(aq) series are fairly large and comparable with those of gaseous free REE^<3+>. No significant tetrad effect is expected in its series variation of (-ΔH_<hyd>). By contrast, [REE(H_2O)_8]^<3+>(aq) series has significantly smaller Racah parameters than gaseous free REE^<3+>, and a convex tetrad effect is expected in the series variation of (-ΔH_<hyd>). Since (-ΔH_<hyd>) of [REE(H_2O)_8]^<3+>(aq) in heavy REE series is fitted to a linear function of the reciprocal of observed REE-OH 2 distance in REECl_3 solution, this relation has been extended to light REE series, and a similar relation for [REE(H_2O)_9]^<3+>(aq) in light REE series has also been determined from our thermodynamic mixture model and observed REE-OH 2 distances in REECl_3 (REE=La, Pr and Nd) solutions. The reciprocal of REE-OH_2 distance in [REE(H_2O)_8]^<3+>(aq), when plotted against the atomic number of REE, indicates a small convex tetrad effect whereas such an effect is absent in [REE(H_2O)_9]^<3+>(aq), in accordance with their nephelauxetic effects. Interestingly, (-ΔH_<hyd>) of [REE(H_2O)_8]^<3+>(aq) satisfies a linear function of 1/(REE-OH^2), and at the same time, the series variations of both (-ΔH_<hyd>) and 1/(REE-OH_2) exhibit small convex tetrad effects when plotted against the atomic number of REE. This is analogous to the relationship between the lattice enthalpy of REEO_<1.5>(cubic) and REE-O distance. The property of (-ΔH_<hyd>) and the lattice enthalpy of REEO_<1.5> can be understood from the viewpoints: i) the existence of REE(III) cluster with the equilibrium configuration having the minimum energy by itself, and ii) the classical limit in quantum-chemical energetics where Planck's constant (h) equals zero. It is emphatically noted that the observed tetrad effects and RSPET can complement the classical ion model with a tactic by which internal structures of the real REE^<3+> ions are legitimately introduced into our thermodynamic discussion.