Inorganic compounds and coordination complexes of the lanthanoids

All the lanthanoids are soft white metals. The later metals are passivated by an oxide coating and are kinetically more inert than the earlier metals. Values of E° for half-reaction 24.6 lie in the range —2.0 to —2.4 V, and the small variation indicates that variations in A H°, IE values and AhydFf° (which are considerable) effectively cancel out. 

The carbides Ln2C3 and LnC2 are formed when the metals are heated with carbon. The LnC2 carbides adopt the same structure as CaC2 (see Section 13.7), but the C—C bonds (128 pm) are significantly lengthened (119 pm in CaC2). They are metallic conductors and are best formulated as Ln [C2] (e ). Lanthanoid borides were discussed in Section 12.10 halides are described below. 

The discussion in this section is necessarily selective. Most of the chemistry concerns the +3 oxidation state, with Ce(IV) being the only stable +4 state (equation 24.8). 

Compounds such as KCeF4, NaNdF4 and Na2EuCl5 are made by fusion of group 1 metal fluorides and LnF3. These are double salts and do not contain complex anions. Several discrete hexahalo anions of Ln(II) are known, e.g. )YbLl -. 

CeF4 crystallizes with an a-ZrF4 structure. What is the coordination numher of each Ce centre in the solid state  

Values of A/P (defined above) for the lanthanoid metals can be determined using the data in Table 25.4. The trend in Ahyd (Ln, g) is a consequence of the lanthanoid contraction, and offsets the general increase in the sum of the ionization energies from La to Lu. The variations in Aiiy ii °(Ln, g), TiIE and Aa/f°(Ln) effectively cancel out, and values of EH° are similar for all the metals with the exception of europium. The trend in values of follows from the trend in EH°. However, actual values must (i) be determined from AG° rather than A/f° and (ii) be related to (defined as 0 V) for the reduction of H (aq) to jH2(g) (see Tables 8.2 and 8.3 and related discussion). 

In addition to values of for the Ln /Ln couple. Table 25.4 lists values of for half-reaction 25.8, this being of greatest importance for Sm, Eu and Yb. 

Using data from Table 25.4, determine A. f°(298K) for the half-reaction  

Assuming that the sign and magnitude of AG for the reduction of Gd (aq) can be approximated to those of A. f , explain why Ln= /Ln negative (Table 25.4) even though Aff = 

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Chapter 24 Inorganic compounds and coordination complexes of the lanthanoids 749... 

See the general references in the Introduction, and some more-speciahzed books [4, 6-58], Some articles in journals discuss actinide complexation and thermodynamics at elevated temperatures [59] classifying lanthanoids by multivariate analysis, albeit with results that seem hard to defend [60] designing sequestering agents for Pu and other actinoids [61] lanthanoid compounds with complex inorganic anions, part of a thematic issue on lanthanoid chemistry [62] Pm, discovery and chemistiy [63] recent Sc chemistiy [64] actinoid complexes [65] the transuranium elements [66] actinoid complexes with OH and [67] coordination numbers [68] the aqueous chemistiy and thermodynamics of Eu [69] photooxidation-reduction of Np and Pu [70] review of Pm [71] Rth thermochemistiy [72] unusual oxidation states of Ln and An [73] and Rth chemistiy [74]. 

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