The conjugate base of hydroxamic acids forms is called a hydroxamate. Deprotonation occurs at the  group, with the hydrogen atom being removed, resulting in a hydroxamate anion . The resulting conjugate base presents the metal with an anionic, conjugated O,O chelating ligand. Many hydroxamic acids and many iron hydroxamates have been isolated from natural sources.
They function as ligands, usually for iron. Nature has evolved families of hydroxamic acids to function as iron-binding compounds (siderophores) in bacteria. They extract iron(III) from otherwise insoluble sources (rust, minerals, etc.). The resulting complexes are transported into the cell, where the iron is extracted and utilized metabolically.
Ligands derived from hydroxamic acid and thiohydroxamic acid (a hydroxamic acid where one or both oxygens in the  functional group are replaced by sulfur) also form strong complexes with lead(II).