Theoretical study of 3d-metal porphyrin π-complexes acetylene

The structural parameters, energies, and spectroscopic characteristics of acetylene π-complexes of metalloporphyrins M(P)(π-C2H2), where M is a 3d-metal atom in different multiplicity states have been calculated by the density functional theory B3LYP method. It has been shown that the activation of the coordinated acetylene molecule is manifested in (i) a sharp weakening of its C-C bond, (ii) a 20°–40° decrease of the bond angles φ(HCC) and an elongation by 0.05–0.10 Å of the R(CC) bond, (iii) a long-wavelength shift of the νstr(CC) stretching mode by 300–500 cm−1, (iv) considerable electron density transfer from the porphyrin ring (P ring) to the π-ligand, and (iv) a strong displacement (0.5–0.6 Å) of the M atom from the P ring plane toward the π-ligand and the dome distortion of the P ring. There is a trend in the behavior of the activation effects along the 3d series and with a change in the electronic state multiplicity of the complexes.