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Solubility
As a general rule, oxalate complexes tend to be insoluble in water. There are exceptions; the group 1 elements from the periodic table (potassium, sodium, etc.) form soluble oxalate salts. The most infamous of the insoluble oxalates is calcium oxalate. Urine normally contains compounds intended to prevent calcium oxalate from forming, but in some patients it crystallizes in the kidneys or other parts of the urinary tract, forming hard lumps called kidney stones.
Binding Sites
Many ligands like water only bind to the metal ion in the complex at one site. Oxalate, however, binds to an ion at two sites, with two of the oxygen atoms on the oxalate ion donating electrons to the central metal ion. For this reason, chemists call it a bidentate ligand (bi meaning two and dentate meaning toothed). Sometimes oxalate can also form complexes with a metal ion where other ligands are also involved. The iron (II) oxalate complex, for example, features two water molecules that also act as ligands.
Spectrochemical Series
The Heisenberg uncertainty principle says you can't know both an electron's location and its momentum with absolute precision at the same time. Nonetheless, you can calculate where in an atom the electrons will probably be, and the regions of space where they are likely to be found are called orbitals. When ligands like oxalate bind to a metal ion, some of the ion's orbitals end up higher in energy than others. The difference in energy is called the field splitting, and it's important in determining the color of the complex and its magnetic properties. Some ligands induce a bigger field splitting than others. Ranking ligands in order from lowest to highest field splitting gives a list called the spectrochemical series. Oxalate is midway up the spectrochemical series, so it causes a larger ligand field splitting than chloride or fluoride ions but a smaller one than cyanide ions or carbon monoxide.
Oxalate Poisoning
Some species of plants can accumulate high levels of oxalate in their tissues, either in the form of insoluble iron or calcium oxalate complexes or in the form of soluble potassium and sodium oxalate salts. The oxalate can bind to calcium in an animal's digestive tract, forming insoluble crystals and decreasing calcium absorption. Alternatively, soluble oxalate salts may enter the bloodstream and combine with calcium there, potentially leading to kidney failure if ingested in large quantities. Some of these plants can represent a possible hazard to livestock.