Bond valence parameters for alkali- and alkaline-earth-oxygen pairs: derivation and application to metal-organic compounds

New bond valence R-0 parameters were derived for alkali- and alkaline-earth-oxygen pairs encountered in metal-organic compounds. A total of 7693 metal-oxygen bond lengths were extracted from structures deposited in the Cambridge Structural Database (CSD) and containing at least one carboxylate ligand bound to the metal. One hundred structures were individually analyzed for each metal. R-0 values were computed using two different approaches: geometric mean and graphical fitting. In both cases, the bond valence parameter b was fixed at 0.37 angstrom. R-0 values obtained using a geometric mean approach were 1.744 (Na-O), 2.094 (K-O), 2.228 (Rb-O), 2.412 (Cs-O), 1.661 (Mg-O), 1.933 (Ca-O), 2.090 (Sr-O), and 2.265 angstrom (Ba-O). These parameters yielded more accurate metal bond valence sums (BVS) and lower root-mean square deviations (RMSD) than those computed using R-0 values currently available in the literature,(1) which had been derived from the structures of inorganic oxides. The most significant improvement was observed for sodium (this work mean BVS = 1.01 v.u., RMSD = 0.116 v.u.; literature mean BVS = 1.18 v.u., RMSD = 0.226 v.u.; v.u.: valence units) and magnesium (this work mean BVS = 2.00 v.u., RMSD = 0.038 v.u.; literature mean BVS = 2.18 v.u., RMSD = 0.100 v.u.). More accurate bond valence sums and lower variances were also observed when the new set of R-0 values was applied to metal-organic compounds featuring oxygenated ligands such as ethers, ketones, aldehydes, alcohols, and esthers. These parameters should be applicable to the structural and crystal-chemical analysis of metal-organic compounds ranging from coordination complexes to organic-inorganic extended hybrids.