Divalent uncommon

C2 . This coordination number is, of course, found in numerous molecular compounds of divalent elements, but is relatively uncommon otherwise. In many cases where stoichiometry might imply its occurrence, a higher coordination number actually occurs because some ligands form bridges between two central atoms. In terms of the more conventional types of coordination compound—those with a rather metallic element at the center—it is restricted mainly to some complexes of... 

Uncommon IPRs were tested recently. Polymerized acyl monoglydnate surfactant was found to be as effective as sodium dodecylsulfate for the resolution of organic amines [126]. For the analysis of pyridine-based vitamins in infant formnlas, dioc-tylsulfosuccinate produced a unique retention pattern [133], Among bizarre IPRs, tris(hydroxymethyl)aminomethane was used for the determination of cyclamate in foods. It was selected over different ion-pair reagents such as triethylamine and dibu-tylamine, based on sensitivity and time economies [134]. Hexamethonium bromide, a divalent IPR, was used successfully to separate sulfonates and carboxylates [135]. 

Concretely, this chapter will first deal with the molecular chemistry of uncommon divalent rare-earth compounds, excluding those of Eu, Yb and Sm that have now been defined as "common". We will describe the molecular chemistry of the salt-like rare-earth diiodides Ndl2, Dyl2 and... 

Tml2, and that of other isolated uncommon divalent complexes there are also two endohedral metallofullerenes Sc Cg4 (Takahashi et al., 1995) and Tm C82 (Kirbach and Dunsch, 1996 Pichler et al., 1997) for which spectroscopic evidence points to the divalent nature of Sc and Tm, respectively they are worth mentioning here but they will not be discussed further. We will also consider the important issue of chemical systems in which uncommon divalent species may be present or have been postulated as intermediates, or displaying divalent-like reactivity. We will also report on the few low-valent scandium species that have been reported and finally on the zero-valent compounds. 

SCHEME 2 Direct synthesis of uncommon divalent diiodides from the metal powders and diiodine. 

The other isolable molecular compoimds of uncommon divalent rare earths have been imtil now mostly organometallic complexes featuring cyclopentadienyl ligands or cyclopentadienyl "equivalents" such as pyrazolylborates. The vast majority of characterised compounds contain Tm", the less reactive of the uncommon divalent rare earths (which unfortunately is also the most expensive). Many other stable compounds seem within reach, given that a good steric protection is achieved, such as for instance amides or alkoxides. Also, it may be possible to synthesise more heteroleptic complexes such as LR°I with an open coordination sphere, which could lead to a very rich new chemistry. A pending question finally involves the possibility of finding yet other complexes of rare earths in new divalent states (for instance Y " "). 

As far as reactivity is concerned, much is still to be discovered. No reports have yet appeared on the reactivity of uncommon divalent... 

Francois Nief s chapter reports on the molecular chemistry of rare earths in uncommon low-valence states zerovalent, monovalent, and also divalent, but to the exclusion of Sm, Eu, and Yb which have been widely studied and are thus considered as common. The relevance of this subject stems from the discovery in the past 10-15 years of stable molecular... 

In brittle micas, the interlayer cations are divalent (e.g., Ca " ). The bond is stronger and although the layered structure still imparts basal cleavage they are more brittle. Brittle micas are uncommon minerals and not of any real interest. 

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