Structural measure of metal-ligand covalency from the bonding in carboxylate ligands

The data set of over 40 000 crystal structures containing the carboxylate group that has been reported to the CSD has been used to extract structural changes to the carboxylate group upon binding to different elemental centers. We find quantifiable structural changes to the carboxylate group depending on the elemental center it is interacting with. The trends follow those traditionally associated with covalency; elements exhibiting electronegativity closest to that of oxygen exhibit the largest structural change. In addition, we find the measure is extendable to transition metal systems where we observe the trends of Pauling neutrality not only are maintained but also are quantifiable; i.e., the structural change increases with oxidation state, i.e., II < III < IV, and decreases with an increase in coordination number, 4c > 5c > 6c. Further, the measure gives us a quantifiable measure of the difference between the covalencies of the long and short bonds of Cu(II) complexes. From the bond lengths of the bound carboxylate arm, we are able to derive bond orders and hence calculate the covalent character in the adjoining metal-carboxylate bonds. As such, we have a structurally derived quantification of metal-ligand covalency.