Electronic Structures of Bis- and Monothiophene Complexes with Fe, Co, Ni: A Density Functional Theory Study

A density functional theory study for the bis- and monothiohene complexes of Fe, Co, and Ni (MT2 and MT, T = thiophene, M = Fe, Co, Ni) was performed to understand their coordination geometries, bonding properties, vibration spectra and singlet excited state spectra. The typical metal coordination exists in the complexes. The Fe-thiophene coordination has the highest stability, with Ni-thiophene being the second highest, and Co-thiophene the lowest, Bisthiophene complexes of Co and Ni prefer to homolytically dissociate to their monothiophene ones and free thiophene. Frequency calculation shows that the ligand-M-ligand asymmetric stretching vibration in bisthiophene complexes shows a strong absorption, at 435.2, 495.7, and 383 cm(-1) for Fe(eta(4)-T)(2), Co(eta(2) -T)(2) and Ni(eta(2)-T)(2), respectively. The M-S stretching vibration in monothiophene complexes shows a strong absorption in the far-infrared region, at 209, 156, and 150 cm(-1) for Fe(eta(4)-T), Co(eta(4)-T) and Ni(eta(5)-T), respectively. The excited state spectra indicate that the characteristic absorption wavelengths of the complexes have a red shift of more than 12.40 eV compared to free thiophene, at 3.54, 1.64, 3.83, 2.75, 1.43, and 2.58 eV for Fe(eta(4)-T)(2), Co(eta(2)-T)(2), Ni(eta(2)-T)(2), Fe(eta(4)-T), Co(eta(4)-T), and Ni(eta(5)-T), respectively.