Oxygen metal-ligand bonds

This expanded structure allowed two water molecules to bind by two hydrogen bonds to the two ionized carboxylate groups and by a oxygen/metal ligand bond to the sodium ion. The position of this water molecule is displayed as in Fig. 3. Water molecule 2... 

Metal-Oxygen and Metal-Ligand Bond Lengths for Metals Commonly Used in the Aldol Condensation ... 

An alternative mechanism is for the peroxide or hydroperoxide ligand to attack the substrate directly i.e., O—O bond cleavage could be concerted with attack on substrate. Another possibility is that the oxygen atom is inserted in a metal-ligand bond prior to transfer to the substrate. Neither of these alternative mechanisms has been demonstrated experimentally. These various possibilities remain to be considered as more information about the monooxygenase enzymes becomes available. 

---