Nitrate complexes scandium

It might be expected that, since the Sc + ion has a considerably greater crystal radius than the corresponding tripositive ions of the 3d metals (compare Sc + at 0.885 A with Ti +, Cr +, and Fe + at 0.810, 0.755, and 0.785 A, respectively), coordination numbers greater than six might be conunon in scandium complexes. With the exception of the aqua ion (and nitrate complexes), this has not been realized, but studies of scandium chemistry are still relatively rare (partly on account of cost). Aqua complexes and simple hydrates have been discussed in Solvento Complexes of the Lanthanide Ions. 

Relatively few complexes of N-donor ligands are known. One of the best characterized is with the tridentate ligand terpyridyl, which reacts with scandium nitrate to form Sc(terpy )(N03 )s here scandium is connected to 9 atoms, but one Sc-O bond is considerably longer than the others, so a coordination number of 8.5 has been assigned. A range of porphyrin and phthalocyanine complexes exist syntheses often involve the high-temperature routes typical of the transition metals but recently a high-yield low-temperature route has been utilized to make octaethylporphyrin (OEP) complexes ... 

The coordination of the nitrate groups in the complexes of phosphine and arsine oxides already discussed is generally symmetrically bidentate. However, in Rb2[Sc(N03)5] there are three biden-tate and two monodentate nitrates, resulting in eight-coordination for scandium, in contrast to (N0" ")2[Sc(N03)5] , where one monodentate and four bidentate nitrates give nine-coordinate scandium. ... 

The classical tridentate ligand terpyridyl (terpy) forms a nine-coordinate complex with scandium nitrate [Sc(terpy)(N03)3].52 This compound with chelating nitrates exhibits a relatively rare high coordination number for scandium. 

The actual situation that exists in solutions of rare earth salts in either aqueous or non-aqueous solution is complicated by concentration effects that can lead to both inner and outer sphere anion coordination as well as to the possibility of hydroxo-containing species particularly in the case of scandium. Inner sphere complexes containing halide ions, nitrate ions, sulfate ions (Choppin, 1971) and even the perchlorate ion (Fratiello et al., 1973 Silber, 1974) have been identified. Consequently, the interpretation of results obtained from measurements of conductance, density, partial molal volume, reaction kinetics, formation constants, and solution spectra can be extraordinarily complicated. 

The fact that the scandium(III) ion has the 3d electronic configuration severely limits the kinds of physical methods that can be brought to bear on determining the nature of the chemical environment in its complexes. The Sc nuclide has 100% natural abundance and a high sensitivity compared to protons and NMR studies using Sc may well prove to be a powerful tool (Melson et al., 1974). NMR spectra of aqueous solutions of scandium(III) chloride, perchlorate, nitrate, and sulfate have been obtained and it has been demonstrated that the chemical shifts for Sc at very low concentrations for the chloride, bromide, and perchlorate are independent of the anion and may be characteristic of the [Sc(OH)] ion. At higher concentrations the chemical shifts are both anion- and concentration-dependent and reflect the formation of bromo and chloro complexes and in the case of the perchlorate, polymeric hydroxo species. The chemical shifts for the nitrate and sulfate are different from the others and indicate that even at low concentrations there must be significant quantities of the respective complexes. 

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