Structural measures of element-oxygen bond covalency from the changes to the delocalisation of the carboxylate ligand

The data set of more than 40,000 crystal structures containing the carboxylate group that have been deposited in the CSD has been used to examine the structural changes that occur in the carboxylate C-O bond lengths upon binding to different elemental centres. We report here quantifiable structural changes that are dependent on the elemental centre with which the group is interacting. For the main-group elements the trends are entirely periodic and 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 both the transition metals and the lanthanoids and actinoids. Amongst the transition metals the trends of Pauling neutrality are not only maintained, but are quantifiable. The difference between the two C-O bond lengths increases with oxidation state and decreases with an increase in coordination number. All of the lanthanoids exhibit covalency within error of each other and the bonds to the actinoids are found to be more covalent than those to the lanthanoids. From the data analysis we are able to derive a correlation between the lengths of the two carboxylate arms that allows us to quantify percentage covalent character defined in terms of the resonance contributions to the carboxylate group.