The thermal characteristics of potassium oxalato lanthanates: K3LnOx3 · n H2O (Ln=La−Tb) and K8Ln2Ox7 · 14 H2O (Ln=Tb−Yb,Y)

The title compounds were studied by TG, DTA, DSC, IR and absorption spectroscopy. The complexes go through dehydration (70–200°C), an irreversible exothermic process (in air or N₂ atmosphere, 250–300°C) and decomposition to a mixture of oxides and carbonates (385–700°C). The exothermic process occurs without weight loss and corresponding heats of reaction fall in the range 0–26 kJ/mol. The absorption spectrum of the Nd complex in the range 5000–6000 Å was employed to monitor perturbations in the coordination sphere of Nd³⁺ arising from the exothermic process. Involvement of the Nd³⁺ cation is implied and the heats of reaction show a close relationship to the radii of Ln³⁺. The interpretation of these data was made with the aid of valuable structural information obtained previously.     

Cyclometallation. Palladium 2-arylpyridine complexes

A number of new cyclometallated compounds of palladium with 2-arylpyridines have been synthesized and characterized. Palladium acetate has been proven to be a more useful starting material than Li₂PdCl₄, since the resulting acetato-bridged dimers, [Pd(OAc)(2-arylpyridine)]₂, unlike the chloro-bridged dimers, are conveniently soluble in common organic solvents. The effect of varying substituents on the aryl nucleus supports the concept that after initial N-complexation, the 2-position of the aryl nucleus undergoes electrophilic attack by the palladium atom. Detailed NMR studies of the soluble acetato compounds showed that the 6-heteroaryl and “ortho” to the Pd-C bond protons in the complexes are shifted (≈0.75 and 0.5 ppm, respectively) upfield from the ligand position. These shielding effects are believed to be due primarily to through-space interactions of overlying aromatic rings and secondarily to through-bond (Pd-to-ligand) effects. Both acetato- and chloro-bridged dimers will react with certain ligands to give mononuclear species containing the intact cyclometallated ligand.     

Some Recent Developments in the Chemistry of Transition Metal Oxocations

The range of stability of transition metal oxocations in aqueous solution is discussed. Species with a metal-oxygen bond of order higher than unity are particularly interesting. Bond orders can sometimes be determined from bond lengths and angles, but generally only by infrared spectroscopy. – The frequency range of the metal-oxygen stretching vibration, previously assumed as about 900–1100 cm^?1, must according to these investigations be extended at least to 780 cm^?1 if all multiple bonds, particularly those in trans-dioxo complexes, are to be included. Broad bands down to 650 cm^?1 are ascribed to metal-oxygen bridges. — The existence of cations such as ZrO²⁺ and VO, or of the “well-known” titanyl ion, TiO²⁺, has never been proved. In many cases polynuclear, oxygen-bridged oxometal species must be assumed.