Variable Temperature FTIR Spectroscopy Of Transition Metal Complexes Using The SCN Reporter Ligand.

Abstract

Due to its large oscillator strength, as well as its position in the IR spectrum (ca. 2100 cm-1) which is relatively free of interferences, the CN stretch absorption in transition metal thiocyanate and iso-thiocyanate complexes has long been used as a diagnostic measure of both the ligand binding mode, as well as an indication of molecular point-group symmetry. In bis-SCN complexes of the first-row transition metals having (distorted) D, symmetry, cis complexes are expected to show two absorbances, corresponding to the sym and asym stretching modes, while for trans complexes (D,) having inversion symmetry, the sym stretch should be IR forbidden and Raman allowed, while the asym stretch is IR allowed and Raman forbidden. Similar considerations apply to square planar complexes (Cav and D). In this study, a number of octahedral and square planar transition metal bis-thiocyanate (isothiocyanate) complexes of Mn(II), Fe(II), Co(II), Ni(II),Cu(II), Ru(II) and Pt(II) have been synthesized, and characterized. by variable temperature IR spectroscopy both in KBr and Kel-F grease mull matrices. Depending on the steric requirements of the other ligands, the characteristic signature of the pseudohalide stretching mode is found to depend critically on the (near) degeneracy of the sym and asym CN stretching modes. Low temperature IR data, as well as supporting nmr and Raman spectroscopic results are required to completely characterize the geometry of these inorganic and metal organic complexes.