Possible role of relativistic effects in the plasticity of the coordination geometry of Cadmium(II).: A voltammetric study of the stability of the complexes of cadmium(II) with 12-crown-4,15-crown-5 and 18-crown-6 in aqueous solution and the structures of [Cd(benzo-18-crown-6)(NCS)2] and [K(18-crown-6)][Cd(SCN)3]

A differential pulse voltammetric study of complexes of Cd(II) and Pb(II) with crown ethers is reported. Measured log K-1 values for Cd(II) with 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane), 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane), and 12-crown-4 (1,4,7,10-tetraoxacyclododecane) are respectively 2.53 (+/-0.06), 1.97 (+/-0.07), and 1.72 (+/-0.08) and for Pb(II) with 18-crown-6 is 4.17 (+/-0.03), all at 25 degreesC in 0.1 M LiNO3. Cd(II) is smaller than is usually associated with strong bonding with crown ethers. The high log K-1 values for Cd2+ with crown ethers found here are discussed in terms of distortion of Cd(II) by relativistic effects. The resulting plasticity of the coordination geometry of the Cd(II) ion allows it to meet the metal ion size requirements of all the crown ethers, allowing high log K-1 values to occur. Crystal structures for [Cd(bz-18-crown-6)(SCN)(2)] (1) (bz-18-crown-6 = benzo-1,4,7,10,13,16-hexaoxacyclooctadecane) and [K(18-crown-6)][Cd(SCN)(3)] (2) are reported. 1 was triclinic, space group P1, a = 8.5413(2), b = 10.0389(2), and c = 13.4644(2) Angstrom, alpha = 94.424(1), beta = 102.286(1), and gamma = 93.236(1)degrees, Z = 2, and final R = 0.023. 2 was orthorhombic, space group Cmc2(1), a = 14,7309(3), b = 15.1647(3), and c = 10.6154(2) Angstrom, Z = 4, and final R = 0.020. In 1, the Cd occupies the cavity of the bz-18-crown-6 with long average Cd-O bond lengths of 2.65 Angstrom and is N-bonded to the thiocyanates with short average Cd-N bonds of 2.12 Angstrom. In [Cd(bz-18-crown-6)(SCN)(2)], the linear coordination involving the Cd and the two N-bonded thiocyanate groups in 1 is discussed in terms of the role of relativistic effects in the tendency to linear coordination geometry in group 12 metal ions. In 2 Cd forms a polymeric structure involving thiocyanate bridges between Cd atoms and K+ occupies the cavity of the crown ether. 2 highlights the fact that cadmium is almost never S-bonded to thiocyanate except in bridging thiocyanates.