Actinide complexes coordination numbers

Although no single crystal X-ray work has been done on the cyclopentadienide complexes of the trivalent actinides, it is clear that they have structures similar to those of the known homologous lanthanides. Both the trivalent lanthanides and actinides behave as Lewis acids and form adducts to complete their coordination spheres. An optimum formal coordination number of ten is indicated and their structures seem to be dictated by a maximization of electrostatic interactions within the steric constraints of the ligands. 

Relatively few structural analysis of the decacoordinated lanthanide complexes have been reported. Although the lanthanides and actinides are long suspected as candidates for having decacoordination, it was not until 1965 that Lind, Lee and Hoard 211) were able to establish decacoordination for La(III) ion in monoclinic crystals of composition [La(EDTAH)] 7H2O. To date, only about ten or so decacoordinated structures are known, but many lanthanide complexes may have a coordination number of ten. 

The simplest of the cubic structures is the primitive cubic structure. This is built by placing square layers like the one shown in Figure 1.1 (a), directly on top of one another. Figure 1.9(a) illustrates this, and you can see in Figure 1.9(b) that each atom sits at the corner of a cube. The coordination number of an atom in this structure is six. The majority of metals have one of the three basic structures hep, cep, or bcc. Polonium alone adopts the primitive structure. The distribution of the packing types among the most stable forms of the metals at 298 K is shown in Figure 1.10. As we noted earlier, a very few metals have a mixed hcp/ccp structure of a more complex type. The structures of the actinides tend to be rather complex and are not included. 

Most commonly, metal ions M2+ and M3+ (M = a first transition series metal), Li+, Na+, Mg2+, Al3+, Ga3+, In3+, Tl3+, and Sn2+ form octahedral six-coordinate complexes. Linear two coordination is associated with univalent ions of the coinage metal (Cu, Ag, Au), as in Ag(NH3)2+ or AuCL Three and five coordination are not frequently encountered, since close-packing considerations tell us that tetrahedral or octahedral complex formation will normally be favored over five coordination, while three coordination requires an extraordinarily small radius ratio (Section 4.5). Coordination numbers higher than six are found among the larger transition metal ions [i.e., those at the left of the second and third transition series, as exemplified by TaFy2- and Mo(CN)g4 ] and in the lanthanides and actinides [e.g., Nd(H20)93+ as well as UC Fs3- which contains the linear uranyl unit 0=U=02+ and five fluoride ligands coordinated around the uranium(VI) in an equatorial plane]. For most of the metal complexes discussed in this book a coordination number of six may be assumed. 

Only a few structures of thiocyanate complexes are known for the lanthanides and actinides, i.e. for Er,341 Th342 and U.343 They all contain terminal N-bonded NCS and have as an interesting aspect a high coordination number, e.g. as in [NEt4]4[Th(NCS)8].342... 

The extensive chemistry of amido complexes, and, more particularly, of alkylamido complexes, reveals that the planar form is almost invariably found, along with bridging amides (221). Much attention has been paid to the synthesis of metal amido complexes of early transition metals, lanthanides and actinides. The amido group, particularly where it is bulky, confers unusual low coordination numbers on the metals and can also produce materials with considerable kinetic stability toward attack by nucleophiles (42, 67). However, the relevance of this extensive and fascinating chemistry to nitrogen fixation is somewhat problematic. 

Table 5.10 Coordination Numbers and Coordination Geometry of Some Actinide Complexes...

Forty years ago, very little was known about lanthanide complexes. By analogy with the d-block metals, it was often assumed that lanthanides were generally six coordinate in then-complexes. We now know that this is not the case, that lanthanides (and actinides) show a wider variety of coordination number than do the d-block metals, and also understand the reasons for their preferred choice of ligand. 

As for the lanthanides, actinide complexes display high coordination numbers. A study of the aqua ions of early actinides makes an interesting comparison (Table 11.1 lists numbers of water molecules and bond lengths). 

It will be noted that the transitions are often broader than those found in the spectra of lanthanide complexes - and indeed the later actinides, see Section 12.2.4. The 5f energy levels are more sensitive to coordination number than are the corresponding levels in the lanthanides since there are bigger crystal-field effects, one sees pronounced differences between the spectra of 6-coordinate [UCle] and of U" +(aq) (Figure 12.5), leading to the conclusion that the uranium(iv) aqua ion was not six coordinate (most recent EX-AFS results suggest a value of 9 or 10, see Table 11.1). Figure 12.6 displays another example of the difference in spectra between similar complexes of different coordination number. 

The coordination chemistry of the actinides in aqueous environments can be segregated along two lines, low valency (di-, tri- and tetravalent) and high valency (penta- and hexavalent). For actinide ions with a low valency, the coordination chemistry is dominated by ionic bonding. As a result, the coordination number and geometry of these aqueous complexes is dictated by the steric bulk and electronic... 

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