text: Several homoleptic (complexes with only one kind of ligand) complexes have been characterized by X-ray crystallography. The inventory includes octahedral complexes  (M = Co, Rh). Square-planar homoleptic complexes are also known for Pt(II) and Pd(II). The potassium salts of  (M = Zn, Cd) feature homoleptic complexes with four O,O-bidentate nitrite ligands.