Metal nitrates stoichiometry ratios

Within each coordination number the oxyanion may function as a monodentate, a bidentate or, very occasionally, a tridentate ligand to an individual metal cation. With the higher anion coordination numbers the M O distances to different metal atoms may not be identical. In practice, coordination numbers of 1 and 2 predominate and are independent of the three main structural types X02, X03 and X04. The coordination number 3 is essentially confined to tetrahedral X04 anions, and the much less numerous coordination numbers of 4-12 also involve mainly tetrahedral X04 type anions, particularly in their anhydrous oxyacid salts M(XO )9 or double salts M M(XO )(f+,. The stoichiometry number, p, is very often a funciton of the oxyanion/metal ratio of the preparative conditions. The higher p, the lower the coordination number of the oxyanion and the more the bonding is likely to involve a monodentate rather than a bidentate function. Nevertheless, the latter is very little influenced by the stoichiometry p. This is illustrated for the bidentate nitrate ion in the six structures (l)-(6),29,31-34 in which p increases from one to six and the bonding role of the nitrato group is essentially unchanged. 

The ratio of the size of the metal ion and the radius of the internal cavity of the macrocyclic polyether determines the stoichiometry of these complexes. The stoichiometry of these complexes also depends on the coordinating ability of the anion associated with the lanthanide. For example, 12-crown-4 ether forms a bis complex with lanthanide perchlorate in acetonitrile while a 1 1 complex is formed when lanthanide nitrate is used in the synthesis [66]. Unusual stoichiometries of M L are observed when L = 12 crown-4 ether and M is lanthanide trifluoroacetate [67]. In the case of 18-crown-6 ligand and neodymium nitrate a 4 3 stoichiometry has been observed for M L. The composition of the complex [68] has been found to be two units of [Nd(18-crown-6)(N03)]2+ and [Nd(NCh)<--)]3. A similar situation is encountered [69] when L = 2.2.2 cryptand and one has [Eu(N03)5-H20]2- anions and [Eu(2.2.2)N03]+ cations. It is important to note that traces of moisture can lead to polynuclear macrocyclic complexes containing hydroxy lanthanide ions. Thus it is imperative that the synthesis of macrocyclic complexes be performed under anhydrous conditions. 

The reactants used were the more soluble cation sources, generally the nitrates. The aqueous solutions of these cation sources were combined in a post transition metal to noble metal ratio appropriate for the ultimately desired pyrochlore stoichiometry. For lead ruthenate syntheses the reactant Pb Ru ratio was required to be slightly higher than the intended final Pb Ru ratio because of the high solubility of lead relative to ruthenium. For bismuth ruthenate syntheses the Bi Ru ratio was required to be slightly lower than the intended final Bi Ru ratio because of the higher solubility of ruthenium. 

If, however, the l,3-diaminopropan-2-ol is added to a methanolic solution of lanthanide(III) nitrate or thiocyanate and dap (in the molar ratio 1 1 1), and the reaction mixture refluxed for 2-3 h, kinetically stable Ln2(L798)(N03)4(H20)s (Ln = La, Pr) or Ln2(L798)(NCS)4(H20)4 (Ln = La, Pr, Nd, Sm, Eu) respectively are claimed to have been isolated [185], Unfortunately, the products were characterised only by elemental analysis, IR and NMR spectra, which do not permit a reliable assignment of the macrocycle formed as the 2 + 2 condensation product. For labile metal ions such as the lanthanides the sequence in which reagents are added should not have a critical influence on the stoichiometry of condensation. Nevertheless the occurence of a 2 + 2 condensation mechanism initiated by coordination of dap to a lanthanide(III) ion (in accordance with synthetic procedure employed) could not be totally excluded. 

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