Cadmium: Coordination Chemistry

The coordinative behavior of cadmium(II) is typical of a soft acid. Evidence of this fact is its strong interactions with S2− and HS− groups leading to the formation of highly stable complexes. Interactions with ligands containing oxygen-donor groups are also well known. The chemistry of CdII shows versatile coordination abilities, giving structures ranging from simple complexes to 1D, 2D, and 3D polymeric architectures. These polymers exhibit interesting physical properties. CdII is able to substitute ZnII in the active site of several Zn-enzymes and to interfere with the metabolism of CaII. This is why a strong interest has been devoted in the past decade to the coordination chemistry of cadmium. The coordinative behavior of the cadmium(II) ion resembles that of mercury(II) and, in a lesser extent, of zinc(II). The main coordination numbers observed for CdII are 4, 5, and 6. Owing to the larger size, CdII assumes coordination number 6 more easily than ZnII. In CdMe2 and dihalides in gas phase, CdII only binds to two ligands, but in solid and solution phase, the coordination number is higher than 2. Coordination number 3 characterizes CdX3− (X = halide) in organic media; however, in aqueous solution, CdBr3− is pyramidal owing to the binding of one or more water molecules. Coordination numbers higher than 3 are observed in solid MCdX3. In aqueous solution, CdX42− anions are tetracoordinated. Most CdX2L2 complexes show a coordination number higher than 4, but a distorted tetrahedral geometry is found in CdX2L2 when L is a P- or S-donor. Cd2+ is found to be five coordinated in CdCl53−. A very large number of Cd compounds are six coordinated, for example, [Cd(H2O)6]2+, [Cd(acac)2], and [Cd(acac)3]− (acac = acetylacetonate) have a CdO6 core. Six-coordinated CdN6 ions are formed with mono- and bidentate ligands such as imidazole or ethane-1,2-diamine. In the solid state, CdX2 compounds are six coordinated. Halide adducts of the type CdX4L2 are common; halogen bridging leads to an approximately octahedral CdII for Cl− and Br−, but I− gives either four and five-coordinated CdII. A few seven-coordinated compounds are known: in the solid state, each CdII ion in [Cd(H2O)(CH3OC6H4COO)2]n is seven coordinated in a distorted pentagonal bipyramidal geometry. Eight coordination is rare. CdF2 contains a CdF8 core in a fluorite structure. In Cd(NO)3·4H2O, the four water molecules are square planar and bidentate NO3− anions lie above and below this plane.