Lanthanide chemistry Complex stability

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Arnold and co-workers conducted a range of work with tethered (bidentate) NHCs, in which the NHC was anchored to the metal centre covalently as well as via the carbene. This imparted additional stability to the complex, allowing the isolation of species such as lanthanide-NHC complexes 108 and 109 (Figure 2.6). However, it has been shown that the NHC-metal bond is often still quite labile and therefore this might have implications for catalysis. This lability can be used for interesting chemistry such as the functionalisation of silanes and the capture of CO2 and CS2. ... 

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. 

This essay on the lanthanides has repeatedly drawn attention to a problem of immense importance in both chemistry and biochemistry. The role of water in controlling the stability, the structure, and the lability of coordination compounds. In fact the emphasis extends from coordination compounds to the surfaces of solids47. The role of water is then bound to be extremely important not only in complex chemistry and catalysis but in the growth and properties of crystals and amorphous materials. We can illustrate the problems outside Ln(III) chemistry48,49. ... 

Fig. 4.38. Stability constants in water at 298 K and = 0.1 M, for lanthanide complexes with the (2.2.1) cryptand and the bibracchial lariat ethers (2.1)DA and (2.2)DA. From data reported by J.-C.G. Biinzli, in Handbook on the Physics and Chemistry of Rare Earths, eds K.A. Gschneidner Jr., L. Eyring, Vol. 9, Ch. 60, North Holland,...

The chemistry of these heterometallic compounds based on the M—O—motif covers main-group elements, transition metals, and lanthanides. The generation of the M—O—motif (21) requires the successful s)mtheses and stabilization of well-defined hydroxides. A considerable effort has been ongoing to stabilize terminal hydroxides of main-group and transition metals (22). Recently, a number of well-defined hydroxides of main-group and transition metals 1-11 (Chart 1) have been made (23-35) by careful hydrolysis of suitable precursors. Some of these hydroxides were used as building blocks to synthesize heterometallic complexes with M—O—backbones by reaction with catalyti-cally active transition metal complex precursors. 

With almost all of the conceivable coordination chemistry of the expanded porphyrins still left to be explored, it cannot be over-stres that the potential for new chemistry is enormous. This is i rticularly true when account is made of the fact that the chemistry of the metalloporphyrins has played a dominant role in modern inorganic chemistry. What with the possibility to enhance the stability of imusual coordination geometries (and, perhaps oxidations states) and the ability to form stable coordination complexes with a variety of unusual cations including those of the lanthanide and actinide series, the potential for new inorganic and organometallic discoveries are almost unlimited. For instance, as with the porphyrins, one may envision linear arrays of stacked expanded porphyrin macrocycles which may have unique conducting properties and/or which could display beneficial super- or semiconducting capabilities. Here, of course, the ability to coordinate not only to cations but also to anions could prove to be of tremendous utility. 

Kremer, C., Torres, J., Dominguezb, S., and Mederos, A. (2005) Structure and thermodynamic stability of lanthanide complexes with amino acids and peptides. Coordination Chemistry Reviews, 249, 567-590. 

Improvement in fractional separations in terms of the var5dng stabilities of lanthanide complex species is a well-known and highly important area of applied chemistry (34) S6). The complex species are often useful in analytical determinations. Certain of them possibly can be applied in constructing amorphous transparent or liquid laser devices. 

The structure and behavior of lanthanides and actinides in ILs has recently been summarized in detail by Binnemans and the reader is referred to this excellent review for more details [71]. Among others, it has been pointed out that solvation of the metal ions by the IL components is a major issue in f-element coordination chemistry [71, 79, 218, 219], Understanding the solvation is also a key to understanding f-element extraction processes. Among others, theoretical studies suggest that Ln3+ (Ln = La, Eu, Yb) are surrounded by six PF6 anions in Bmim-PF6 and by 11-13 imidazolium ions in the second ionic sphere. The same authors also suggest that free lanthanide cations, that is, cations without a solvation shell, are poorly soluble in Bmim-PF6 [149, 152, 220-223], They also report that, while [LnCls]3 complexes are unstable in the gas phase, they can be stabilized by solvation in ILs. 

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