Transition metal complexes of pyridine-N-oxides encompass coordination complexes that contain pyridine-N-oxides as ligands. Transition metal complexes of amine-N-oxides encompass coordination complexes that contain amine oxides as ligands. The two classes of complexes are similar.
Pyridine-N-oxides and amine oxides are classified as L ligands in the covalent bond classification method. In the usual electron counting method, they are two-electron ligands. With respect to HSAB theory, they are classified aa hard, as is typical for oxygen-based ligands.
Amine oxides are moderate bases. For , the pK<sub>a</sub> is around 9.7. Pyridine-N-oxides are significantly weaker bases.
Particularly common are the octahedral homoleptic complexes of the type where M = Mn(II), Fe(II), Co(II), Ni(II). Many variations of pyridine N-oxide are known, such as the dioxides of 2,2'- and 4,4'-2,2'-bipyridine. Complexes derived from the trioxide of terpyridine have been crystallized as well.
Pyridine-N-oxides bind to metals through the oxygen. According to X-ray crystallography, the M-O-N angle is approximately 130ð in many of these complexes. As reflected by the pKa of 0.79 for , pyridine N-oxides are weakly basic ligands. Their complexes are generally high spin, hence they are kinetically labile.
Oxides of tertiary amines, which are more basic than pyridine-N-oxides, form a variety of metal complexes. One such complex is derived from N-methylmorpholine-N-oxide and osmium tetroxide. This adduct is a probable intermediate in the Upjohn dihydroxylation.
Zinc pyrithione is a coordination complex of a sulfur-substituted pyridine-N-oxide. This zinc complex has useful fungistatic and bacteriostatic properties.