Complexes of tetracyanobiimidazole--V. Self-association of Ir(I) complex anions

Abstract

The self-association of the planar anion [Ir(CO)2Tcbiim]- where H2Tcbiim is 4,4',5,5'-tetracyano-2,2'-biimidazole has been studied in acetontrile solution by analysis of the charge-transfer spectra. At low concentrations (~ 10-5 M) monomer units prevail. At intermediate concentrations (~ 10-3 M) dimerization is evident and a very strong ([var epsilon] = 18 500 M-1 cm-1) new absorption appears at 475 nm. A novel procedure was used to obtain this value and to estimate the equilibrium constant for dimerization, 13.2 M-1 at 24.4[deg]C. From the temperature dependence of the spectra, [Delta]H is calculated to be -28 to -30 kJ M-1. At higher concentrations (~ 10-2 M) spectral changes consistent with further oligomerization are observed.