The role of relativistic effects (RE) in the structures of Cd(II) complexes with crown ethers, and the reason the ‘soft’ Cd(II) strongly prefers to bind to SCN− through N, are considered. The synthesis and structures of [Cd(18-crown-6)(thiourea)2] (ClO4)2.18-crown-6 (1) and [Cd(Cy2-18-crown-6)(NCS)2] (2) are reported. (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane; Cy2-18-crown-6 = cis–anti–cis-2,5,8,15,18,21-hexaoxatricylo[20.4.0.0(9,14)]hexacosane). In 1 Cd is coordinated in the plane of the crown which has close to D3d symmetry, with long Cd–O bonds averaging 2.688 Å. The two thiourea molecules form relatively short Cd–S bonds that average 2.468 Å, with an S–Cd–S angle of 164.30°. This structure conforms with the idea that Cd(II) can adopt a near-linear structure involving two covalently-bound donor atoms (the S-donors) with short Cd–S bonds, which resembles gas-phase structures for species such as CdCl2. The structure of 2 is similar, with the two SCN− ligands N-bonded to Cd, with short Cd–N bonds of 2.106 Å, and N–Cd–N angle of 180°. The crown in 2 forms long Cd–O bonds that average 2.698 Å. Molecular mechanics calculations suggest that a main reason Cd(II) prefers to bind to SCN− through N is that when bound through S, the small Cd–S–C angle, which is typically close to 100°, brings the ligand into close contact with other ligands present, and causes steric destabilization. In contrast, the Cd–N–C angles for SCN− coordinated through N are much larger, being 171.4° in 2, which keeps the SCN− groups well clear of the crown ether. DFT (density functional theory) calculations are used to generate the structures of [Cd(18-crown-6)(H2O)2]2+ (3) and [Cd(18-crown-6)Cl2] (4). In 3, the Cd(II) is bound to only three O-donors of the macrocycle, with Cd–O bonds averaging 2.465 Å. The coordinated waters form an O–Cd–O angle of 139.47°, with Cd–O bonds of 2.295 Å. In contrast, for 4, the Cd is placed centrally in the cavity of the D3d symmetry crown, with long Cd–O bonds averaging 2.906 Å. The Cl groups form a Cl–Cd–Cl angle of 180°, with short Cd–Cl bonds of 2.412 Å. With ionically bound groups on the axial sites of[Cd(18-crown-6)X2] complexes, such as with X = H2O in 3, the Cd(II) does not adopt linear geometry involving the two X groups, with long Cd–O bonds to the O-donors of the macrocycle. With covalently-bound X = Cl in 4, short Cd–Cl bonds and a linear [Cl–Cd–Cl] unit results, with long Cd–O bonds to the crown ether.
