η-bonding is the most common motif seen in coordination chemistry of dioxygen.  Such complexes can be generated by treating low-valent metal complexes with oxygen.  For example, Vaska's complex reversibly binds O (Ph = CH):
:IrCl(CO)(PPh) + O  IrCl(CO)(PPh)O
The conversion is described as a 2 e redox process: Ir(I) converts to Ir(III) as dioxygen converts to peroxide. Since O has a triplet ground state and Vaskas complex is a singlet, the reaction is slower than when singlet oxygen is used. The magnetic properties of some  η'-O complexes show that the ligand, in fact, is superoxide, not peroxide.
Most complexes of η-O are generated using hydrogen peroxide, not from O.  Chromate ([CrO)]) can for example be converted to the tetraperoxide [Cr(O)].  The reaction of hydrogen peroxide with aqueous titanium(IV) gives a brightly colored peroxy complex that is a useful test for titanium as well as hydrogen peroxide.