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 Å) 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 sums (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.