M^II chemistry

The structural chemistry of Sn^II and Pb^II compounds is very complex and diverse. The M²⁺ ions contain the (ns)² configuration and therefore a nonbonding electron pair that can have a stereochemical affect analogous to that in molecules. So the Sn coordination located in SnO (1) depicts tin with four oxygen neighbors on one side and a 'vacant' coordination site actually occupied through the lone-pair. Sn^II sulfide and halides comprise polymeric structures with identical stereochemical characteristics, but Pb^II compounds come out to be more ionic, and less influenced through the nonbonding electrons. One form of PbO has the similar structure like SnO, but the structures of several other compounds are identical to those found with the larger M²⁺ ions in group 2, instances being PbS (rocksalt) and PbF₂ (fluorite). Solubility patterns of a few Pb^II salts also parallel those found in group no.2 (example insoluble sulfate and carbonate) but variation appear with softer anions: so PbS is insoluble in the water, heavier halides insoluble in cold water but more soluble in hot.

The aqueous M²⁺ ions are quite acidic, Sn²⁺ particularly so and depicts typical amphoteric behavior, going through strong protolysis to create polymeric hydroxo species, that dissolve in alkali to create the pyramidal [Sn(OH)₃]^-. Pb²⁺ forms complexes with a class b pattern of stability similar to that of Cd²⁺ (see Topics E3 and G4) even though it does not complex with NH₃ in aqueous solution.