On the coordination in metal saccharinates. Implications for bond-order models of metal-ligand binding

The structures of saccharinates retrieved from the Cambridge Structural Database were used to discuss the coordination properties of deprotonated saccharin. The series of the first-row metal(II) saccharinato isomorphs and of triphenylstannyl saccharinates were analyzed within the bond valence model (BVM). The "relative radius" parameter of the saccharinato ligand for the M(Ow)(4)(Nsac)(2) type of coordination was estimated (1.424 Angstrom) from correlation of the metal-N(saccharinato) distances with the Shannon-Prewitt ionic radii. Making use of the exponential bond distance-bond order (BDBO) relation of Pauling within the BVM, ligand-specific mean bond order slims (MBOS) were recently derived for several ligands. Coupled with the coordination number (CN), they are predictive for the metal-ligand bond lengths. Using parameterized power function instead of the exponential form of the BDBO relation, a new set of MBOS's is derived here: isothiocyanate 2.56 +/- 0.06; pyridine 1.84 +/- 0.16; imidazole 2.02 +/- 0.12; chloride 2.05 +/- 0.10; water 1.54 +/- 0.03. The two sets of MBOS values can be used to predict the metal-ligand distances nearly equally well, showing that the distances are solely predetermined by the MBOS and CN values, independently of the particular form of BDBO relation used.