The concept developed by Pearson [1] on the hard and soft acids and bases rationalizes that soft cations show a pronounced affinity for coordination with softer ligands, while hard cations prefer coordination with harder ligands. In these complexes, mercury is usually tetrahedrally S-coordinated, while the ion of the metal may be surrounded by four or six ligands. For instance, the SCN-ligands in MnHg(SCN)4 bridge the tetrahedrally coordinated N-bonded Mn atoms and the tetrahedrally coordinated S-bonded Hg atoms into a 3-D network with space group View the MathML source. MnHg(SCN)4 exhibits high hyperpolarizability due to the extended π-conjugation of the Mn–NCS–Hg bridges and their parallel alignments in the 3-D crystal structure [2]. This 3-D network provides a larger domain for polarizability, which in turn induces a large macroscopic nonlinearity. As second-order nonlinear optical (NLO) materials, bimetallic thiocyanates such as ZnCd(SCN)4, ZnHg(SCN)4, CdHg(SCN)4 and MnHg(SCN)4 (abbreviated as ZCTC, ZMTC, CMTC and MMTC, respectively) exhibit efficient second harmonic generation (SHG) at short wavelengths.