The mixed metal oxalates, FeCu(ox)₂.3H₂0, CoCu(ox)₂.3H₂0, and NiCu(ox)₂.3.5H₂0, [ox = C₂0₄] have been prepared by coprecipitation from solution. The thermal behaviour of these compounds in nitrogen and in oxygen has been examined using thermogravimetry (TG), thermomagnetometry (TM), differential scanning calorimetry (DSC) and evolved gas analysis (EGA), and results are compared with results obtained for Cuox and Mox.yH₂0. The thermal behaviour of the mixed oxalates, MCU(OX)₂.xH₂0, differed from that of the individual metal oxalates, Cuox, Coox.2H₂0, Niox.2H₂0 and Feox.2H₂0. All three mixed oxalates on heating in N₂, first dehydrate and then decompose in at least two overlapping endothermic stages. Both CO and CO₂ were evolved in proportions which varied with the surrounding atmosphere, and from compound to compound, and with extent of reaction of a given compound. The mixed oxalates, MCU(OX)₂.xH₂0, do not show the exothermic behaviour characteristic of Cuox, and reasons for this are discussed. Thermochemical calculations were done and the enthalpies of formation of the hydrates and dehydrated oxalates were determined. It was found that the enthalpy of mixing was very small or within experimental error. X-ray powder diffraction patterns for the individual and mixed oxalates were compared. The pattern for Cuox differs from the patterns obtained for the other oxalates, confirming suggestions that Cuox has a structure different to most other transition metal oxalates. The kinetics of dehydration and decomposition of the mixed oxalates were investigated, using isothermal and programmed temperature TG and DSC experiments. The yield and composition of evolved gases during isothermal decomposition were measured and compared with the enthalpy changes. X-ray photoelectron spectroscopy studies provided some information on the electron environment of the metal atoms in the various oxalates.

The crystal structure of Ba [Cu(C₂0₄)₂(H₂O)].5H₂O has been determined using single crystal X-ray diffractometry. It crystallises in the triclinic system, space group Pī , with a = 6.539(2) Å, b = 9.211(3) Å, c = 10.928(3) Å, a = 85.42(3)°, β = 79.22(3)° , γ = 80.30(3)°, V = 636.08(8)ų and Z = 2. The structure consists of [Cu(C₂0₄)₂(H₂O)]²⁻ ions weakly bridged by barium ions and water molecules. The copper(II) ions are in a tetragonally elongated square-pyramidal environment with some trigonal distortion. The two oxalate groups occupy the equatorial positions and a water molecule occupies the axial position. The barium ion is surrounded by nine oxygens: five oxygens from water molecules and four oxygens from oxalate groups. The thermal behaviour of Ba [Cu(C₂0₄)₂(H₂O)].5H₂0 in N₂ has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (~80°C), but continues until the onset of the decomposition (~280°C). The decomposition takes place in two major stages. The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311 ±30 kJ mol⁻¹. The overall enthalpy change for the decomposition of Ba[Cu(C₂0₄)₂]in N₂ was estimated from the combined area of the peaks of the DSC curve as -347 kJ mol⁻¹. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the α-time curves could be described by the three-dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125 ±4 kJ mol⁻¹ and (1.38 ±0.08)x10¹⁵ min⁻¹, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model with n = 5. The other process was slow and could also be described by the contracting-geometry model , but with n = 2. The values of Eₐ and A were 206 ±23 kJ mol⁻¹ and (2.2 ±O.5)xl0¹⁹min⁻¹, respectively, for the fast process, and 259 ±37 kJ mol⁻¹ and (6.3 ±1.8)x10²³min⁻¹, respectively, for the slow process.The crystal structure of zinc oxalate dihydrate ([Zn(C₂0₄)(H₂O)₂]n) has also been determined by X-ray diffraction methods. It crystallises in the monoclinic system, space group C2/c with a = 11.786(2) Å, b = 5.397(1)Å, c = 9.712(1) Å, B = 126.19(5)°, V = 498.58(8)ų, Z = 4 and R = 0.037 for 435 absorption-corrected independent reflections and 50 parameters. The asymmetric unit consists of half the monomeric unit [Zn(C₂0₄)(H₂O)₂). The structure consists of infinite, linear chains of zinc ions bridged by oxalate groups. The geometry of the coordination polyhedron surrounding the zinc ion is octahedral, with the oxalate oxygens occupying the equatorial positions and water molecules occupying the axial positions.