Two novel complexes trans-[Fe(MBPT)2(NCS)2] (1) and cis-[Fe(mMBPT)2(NCS)2] (2) have been synthesized and their structure characterized at 293 K by single-crystal X-ray crystallography [MBPT (or mMBPT)=4-p(or m)-methylphenyl-3,5-bis(pyridin-2-yl)-1,2,4-triazole]. Complexes 1 and 2 are isomers, and 1 crystallizes in the triclinic space group P1h with Z=1, a=9.120(3)Å, b=9.2563(18) Å, c=11.576(2) Å, α=76.256(15)°, β=76.405-(18)°, γ=84.983(19)°, and V=922.1(4) Å3 in contrast to 2 in the monoclinic space group C2/c with Z=4, a=21.851(3) Å, b=13.4113(15) Å, c=16.859(3) Å, β=128.153(9)°, and V =3885.0(9) Å3. Both complexes have a similar pseudo-octahedral [FeN6] core with the NCS-groups in the trans arrangement in 1 but cis in 2. Variable-temperature magnetic susceptibility and IR spectral measurements reveal that 1 shows an abrupt high-spin T low-spin (HS T LS) transition centered at T1/2 around 231 K, in contrast to 2 staying in a high-spin state in the observed temperature range of 75-300 K. The extended X-ray absorption fine structures (EXAFS) spectra confirm that the average Fe-N distance for the HS state in 1 is 0.2 Å longer than that of the LS state. Complex 1 represents the first spin-crossover iron(II) complex with triaryltriazole and trans-thiocyanate ligands.

The new Schiff base ligand 2-chlorobenzaldehyde thiosemicarbazone (L, 1) has been synthesized and characterized by spectral techniques and single-crystal X-ray analysis. Crystals of 1 are monoclinic, space group P21/n with a=12.964(4) Å, b=5.131(5) Å, c=4.970(1) Å, β=94.32(2)°, and Z=4. The thiosemicarbazone moiety adopts a configuration with N(1) cis to N(3) and places the E configuration about both the bonds C(1)-N(2) and C(2)-N(3). The monodentate behavior of the neutral ligand (L) has been investigated in two cadmium halides CdL2 Br2 (2) and CdL2I2 (3). Compound 2 crystallizes in space group Cc with a =8.175(1) Å, b=14.176(1) Å, c=21.073(1) Å, β=94.02(1)°, and Z=4. Compound 3 crystallizes in space group P1h with a=10.9577(1) Å, b=16.174(1) Å, c=7.878(1) α=100.50(1)°, β=109.39(1)°, γ =83.67(1)°, and Z= 2. The coordination geometry about the cadmium(II) atom in compound 2 conforms to a tetrahedral configuration with two sulfur atoms from two unequivalent neutral ligands and two bromide atoms. Whereas the coordination geometry about the cadmium(II) atom in compound 3 is (4+1) distorted trigonal bipyramidal with two iodide atoms and one sulfur atom in the equatorial plane, the other thiosemicarbazone sulfur and the iodide atom I(1a) of an adjacent moiety occupy the axial positions. Compound 2 exhibits powder SHG efficiencies ca. 20 times that of urea, whereas compound 3 does not exhibit any SHG efficiency. Theory and experiment suggested that intermolecular contact is the main factor controlling the SHG efficiencies of compounds.