Infrared spectroscopy and coordination chemistry

Abstract

An infrared spectral study in the region 240-2000cm^-1 was undertaken to investigate the effect of coordination number, oxidation state, anion and stereochemistry on the Metal-Nitrogen (M-N) and Metal-Halogen (M-X) bond vibrations in about 70 complexes of Cr(III) , Mn(II), Mn(III), Fe(II) , Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Cd(II), Sn(II), Sn(IV), and Hg(II) using 1,10-phenanthroline (phen) and 2,2'-bipyridine (bipy) as ligands. It has been found that the M-N frequencies do not follow the stability order of the divalent metal complexes. The Jahn-Teller distortion in Cu(II) complexes has been noted in ligand band splittings in the 600cm^-1 region. The steric hindrance of phen over bipy has also been observed. The weakening of M-X bond in going from MX2 to MX2•L to MX2•L2 to MX2•L3 (where L = phen or bipy; X = halogen, M = metal) has also been observed and it has been inferred that this might be diagnostic in the identification of inorganic complexes. Complexes of several tetrahalometallates of the general formula (phenH) (MCl4)•HCl or (bipyH)(MCl4) where M= Fe(III), Cr(III) and also (bipyH2)(MC14) or (phenH)Z(MCl4) HC1, where M= Co(II), Zn(II), Cd(II), etc., have been prepared for the first time and ℧3(M-X) mode has been assigned and discussed with respect to the crystal field theory and solid state interaction. The thermal decomposition of some of the acid salts resulted in some interesting products. For example, (bipyH)(FeCl4) and (phenH)(FeCl4)•HCl on heating at 110°C gave rise to products of simplest composition (FeCl2•bipy)•Cl and (FeCl2•phen)•Cl respectively. The ℧3(M-X) broad intense band which is at ~380cm^-l in (FeCl4)^- has been split into two broad and intense bands at 380 and 333cm^-1 with several weak bands around 300cm^-1. Alternative formulations such as (FeCl2•2L)(FeCl4) (where L = phen or bipy) have been proposed and the metal-metal interactions discussed. The yellow species obtained by mixing FeC13 directly with phen and bipy appeared to have distorted octahedral structures with three M-X bond vibrations around 300cm^-1. Magnetic susceptibility, electrolytic conductance and some x-ray diffraction data are consistent with the above formulations. The ℧(M-X) for (CrC14)^- in (phenH)(CrC14) (phen•HC1)•6H2O has been assigned at 380cm^-1. The thermal decomposition of this salt gives rise to CrCl3 •phen•phen•HC1•H2O. A square planar structure has been assigned for (CrC14)^- in view of crystal field stabilization energy. The ℧3(M-X) mode in divalent tetraha1ometa11ates has been assigned, discussed and the spectra of the metal salts have been compared with protonated ligands. The thermal decomposition of these salts gave rise to 4-coordinated compounds with the anion going into the first coordination sphere.