Complexes of the Heavier Alkaline-Earth Metals

Many compounds of the heavier alkaline-earth metals with organic ligands are described in the literature, for which from spectroscopic data -interactions can be anticipated. Recent reviews by Gowenlock (108) and Lindsell (86,108) provide comprehensive information in this field of chemistry. 

Organic fragments which can serve as n ligands include cyclopentadienyl, indenyl, and fluorenyl systems as well as polynuclear aromatic anionic and dianionic species. From IR and NMR data it has been concluded that the bonding in these complexes may be regarded as fully ionic. 

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Romero N, Rofca S-C [c], Sarazin Y, et al. Highly fluorinated tris(indazolyl)borate sdylamido complexes of the heavier alkaline earth metals synthesis, characterization, and efficient catalytic intramolecular hydroamination. Chem Eur J. 2015 21 4115-4412. 

It is also possible to isolate bis(carbene) complexes involving the heavier alkaline earth metals. Thus, the reaction of two equivalents of 4 (R = Me or (Bu, R = H) with calcium, strontium and barium bis(trimethylsilyl)amides [M N(SiMe3)2 2(thf)2] (M = Ca, Sr, Ba) resulted in the displacement of two thf molecules to afford the corresponding biscarbene species, 19 (19). The solubilities and stabilities of these complexes were found to decrease from calcium to barium. 

Some typical complexes of beryllium have already been considered, and it was seen that complex formation is favored if the complexing atoms become incorporated into a cyclic structure. Indeed, in the case of the heavier alkaline-earth metals (whose radii are far greater and... 

When one goes from Be to heavier alkaline earth elements like Mg and Ca, the situation becomes even more complex since the tendency to form hybrid sp orbitals decreases as a group is decended. Indeed, it has been shown [80, 82] that in order to correctly describe the bonding in clusters of the heavier alkaline earth metals, one has to employ large basis sets which indude not only p but also d polarization functions. Furthermore, correlation has to be taken into account simultaneously. 

From the trend in acidities of the hydrogen halides in water, it follows that fluoride is the most basic or nucleophilic of the halides and iodide the least basic if the hydrogen ion is considered the reference acid. It should be recalled (p. 169) that this order of halide basicities is the same as that toward small, multicharged ions with rare-gas structures (for example, Be2+, A 3, and Si4+). A different, and sometimes reversed, order of basicities or nucleophilicities is observed toward certain ions of the post-transition metals (for example, Cu+, Hg +). For a number of ions (for example, Be+2, B+3 and Ta+6), fluoride complexes may exist in aqueous solution, whereas the other halo-complexes do not. Only a few of the elements having positive valence states form no halo-complexes the most important of these are carbon, the rare earths, the alkali metals, and the heavier alkaline-earth metals. 

The alkaline and rare-earth metals, and positive actinide ions, generally have greater affinity for —0 groups as electron donors. Many transition metals complex preferentially with enoHc —0 and some nitrogen functions. PolarizabiUty of the donor atoms correlates with stabiUty of complexes of the heavier transition metals and the more noble metal ions. 

In 1998, the same group reported the synthesis of magnesium metallocene NHC complexes, in addition to metallocene NHC complexes of other group 2 elements.21 The adducts show an interesting trend in the nature of the metal-carbene bonds which increase as metal radii increase. The trends are reflected in both the solid state structures (metal-carbene bond length increases) and the NMR spectra (downfield shift of C2) of the adducts (Table 1). In addition to mono-NHC complexes, the heavier alkaline earth elements (Sr and Ba) are capable of forming stable bis-NHC adducts. 

Within the c/x plot for bidentate complexation there is a very constrained region containing all the classical (and some exotic or nltratrace) essential metal ions other than Cn, which exert biocatalytic functions in the center. The heavier alkaline earths Ca. Sr (for corals and foraminifers) and Ba (in desmides) are essential elements bnt hardly for pnrposes of biochemical catalysis, that is, substrate binding measured by c and x does not matter for heavy alkaline earth essentiality. 

The similarity of the log K values for the bound macrocycles to those involving the unbound macrocycles suggests that prediction of metal separations using silica gel-bound macrocyclic ligands should be possible. Thus, one has a powerful means to predict separations using available data compilations [5]. Such separations have been studied. For example, 18-crown-6 gel material 2 was used to separate 0.001 M concentrations of the alkaline earth cations [14]. The log A values for the association of unbound 18-crown-6 with Sr and Ba in water are 2.72 and 3.87, respectively [5]. The log K value for 18-crown-6-Ca interaction should be between 0.9 and 1.9 as described in Section 3. Magnesium ions do not complex with 18-crown-6. All the heavier alkaline earth cations were separated from Mg " by their being retained on the gel 2 column. Sr and Ba were selectively retained on the column over Ca by factors of 54 and 339, respectively, and Ba was retained... 

The H NMR spectra of only a few heavier alkaline earth metal alkoxide derivatives that are soluble in suitable organic solvents have been studied so far. The complex Ba2(OCPh3)4(THF)3 shows only a single line for aryl ring protons down to —43°C, and the NMR spectrum also shows only one aryl environment. ... 

The formation of alkaline earth metal bis[bis(tiialkylsilyl)amides] has been discussed in detail elsewhere." Like all heavier group 2 metal bis[bis(tiisalkylsilyl)amides], the complex [(Ca N(SiMe3)2 2)2] has a dimeric structure both in solution and the solid state (Figure 3.8), in which the calcium atoms are in a distorted trigonal planar environment. 

Ate complexes of the alkaline earth metals are most prominent for magnesium, and to a significantly lesser extent for the heavier metals. The compounds are often observed in the presence of encapsulating donors such as crown ethers or cryptates a representative example is shown in Figure 28. ... 

The separation of isotopes of alkaline earth metals by ion-exchange chromatography (Be and Ca), using the band elution technique, and by chemical exchange reactions (Ca), using macrocyclic polyether complexes, has been assessed. The separation factors for Be and Ca decrease with increase in mass of the isotopes, and were found to be of the same order as those determined previously. Enrichment of the heavier isotopes of Ca by reaction (1), where L represents a macrocyclic polyether (c.g. DCH18C6, DB18C6), has also been shown to be effective. ... 

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