Actinide metals thermochemical data

Electronegativities, Xp. shown in bold text and rounded to one decimal place, are taken from Allred or from Pauling. The values are based on thermochemical data analysed using Pauling s approach for the elements in their normal oxidation state, viz. for Sc, Y and La, M for Cu. Ag and Au, for the other d-block metals, Ln for the lanthanides and An for the actinides. 

What type of compounds will be covered in this overview. Strictly, only compounds which have at least one metal-carbon bond (with the exception of metal carbides) are called organometaiiic. However, this definition was not adopted in this chapter because many families of compounds that are relevant in organometaiiic chemistry would not be included (e.g., alkoxides). By metal we mean any element from groups 1 (except hydrogen), 2, 3 (including lanthanides and actinides), 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 (except boron), 14 (except carbon and silicon), and 15 (antimony and bismuth only). Most of the available thermochemical data for all these species are freely available in a single on-line database the NIST Chemistry WebBook Unless stated otherwise, all the data included in this chapter were quoted from that reference. 

Johansson (1978) and Brooks etal. (1984) utilized the energy difference (f d transitions) between trivalent (f"ds ) and tetravalent (f" M s ) cerium and other f-element metals to estimate thermochemical data for compounds. A similar approach has been used by Mikheev et al. (1986) and Spitsyn et al. (1985) to include the divalent as well as the tetravalent state. Again, a critical issue has been the behavior of the heavy actinides with respect to f- d transitions, and the need to utilize and to interpret the few experimental measurements on these actinides, which are discussed below (e.g., section 2.4.1.3). Johansson and Munck (1984) defined a function P (M) that removes the intershell multiplet coupling energy from the atomic reference state (A ,.,up,i j is the energy difference between the baricenter and the lowest level of a multiplet). For the lanthanides P (M) is significantly smoother than P(M),-except for an anomaly at Yb that may be due to an error in experimental data. Unfortunately, Johansson and Munck (1984) point out that spectroscopic data on the heavy actinides are inadequate to correct P(M) to P (M) for the actinides. 

Each of the four kinetic regions observed in fig. 14 corresponds to control of the bulk hydriding rate by a separate process. The decrease in r as P approaches P is clearly thermodynamic in nature because r approaches zero as the thermochemical activity gradient driving the reaction approaches zero. As described by Haschke (1991), the approach to equilibrium gives rise to the rate decreases observed at high temperature in Arrhenius data for the U-l-H system under isobaric conditions. Unlike the U-UH3 system, equilibrium pressures for the metal-hydride systems of the lanthanides and transuranium actinides are not in the 0.1 to 1 bar range frequently employed in hydride... 

Alternatively, Bratsch and Lagowski (1985a, b, 1986) proposed an ionic model to calculate the thermodynamics of hydration AGj, A/fJ and ASj using standard thermochemical cycles. This model is based on the knowledge of the values of quantities such as the enthalpy of formation of the monoatomic gas [A/f (M )], the ionization potential sum for the oxidation state under consideration and the crystal ionic radius of the metal ion. This approach, however, is difficult to apply for the actinides since the ionization potentials are, for the most part, unavailable. To overcome this problem, the authors back-calculated an internally consistent set of thermochemical ionization potentials from selected thermodynamic data (Bratsch and Lagowski 1986). The general set of equations developed are ... 

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