When this polyatomic ligand is bonded to a trimethylsilyl group, but not a similar tert-butyl group, it rapidly interconverts which of its atoms is bonded to the group. Compounds of transition metals with this ligand form the widest variety of, and best studied, coordination polymers. This non-carbene species displays the canonical example of Umpolung, allowing it to catalyze the (*) benzoin condensation. Out of common bridging ligands, this ligand is the simplest in which two different atoms both readily bond to metal centers. The most common method of gold refining (10[1])forms a water-soluble (10[1])complex of gold with this ligand. Iron(II) complexed to six instances of this ligand is a component (10[1])of Prussian blue. For 10 points, name this toxic anion made of a triple-bonded carbon and nitrogen. ■END■ (10[1])