In monometallic complexes, aldehydes and ketones can bind to metals in either of two modes, η-O-bonded and η-C,O-bonded.  These bonding modes are sometimes referred to sigma- and pi-bonded. These forms may sometimes interconvert.
The sigma bonding mode is more common for higher valence, Lewis-acidic metal centers (e.g., Zn). The pi-bonded mode is observed for low valence, electron-rich metal centers (e.g., Fe(0) and Os(0)).
For the purpose of electron-counting, O-bonded ligands count as 2-electron "L ligands": they are Lewis bases.  η-C,O ligands are described as analogues of alkene ligands, i.e. the Dewar-Chatt-Duncanson model.
η-C,O ketones and aldehydes can function as bridging ligands, utilizing a lone pair of electrons on oxygen. One such complex is , which features a  ring.