Comparison of rotational barriers of dithiocarbamate ligand in the four- and five-coordinate complexes Ir(PH₃)(CO)(H₂dtc) and Ir(PH₃)₂(CO)(H₂dtc): A theoretical viewpoint

The addition of PH₃ ligand to a four-coordinate model complex such as Ir(PH₃)(CO)(H₂dtc) leads to three different five-coordinate structures which have a general formula as Ir(PH ₃)₂(CO)(H₂dtc). Pure density functional theory (BP86) was used to study the metal-H₂dtc ligand interactions in these complexes. Rotational barriers of dithiocarbamate ligand in four- and five-coordinate complexes have been investigated by density functional theory calculation at the same level. The results show that the rotational barriers decrease in all five-coordinate complexes compared to four-coordinate ones. It seems that a π-interaction exists between carbon and nitrogen of dithiocarbamate ligand. The results also show that in four-coordinate complex π-interaction of C-N bond is more significant than that of five-coordinate complexes. Molecular orbital calculation in these complexes shows that in addition to σ-interactions between some of HOMOs of H₂dtc ligand and orbitals of central metal, π-interactions also exist. A similar calculation revealed that the π-interactions in four-coordinate complex are stronger than its counterpart in five-coordinate complexes. Comparing four- and five-coordinate complexes, we found that because of the weakness of π-interactions in the latter the π-interactions becomes stronger in C-S bond while it weakens that of the C-N bond. If the π-interaction is decreased in C-N bond, it leads to a decrease in the rotational barrier in the H₂dtc ligand. The ONIOM method is used to model effect of replacing hydrogens on phosphorus and nitrogen atoms by phenyl and ethyl groups, respectively, on four-coordinate complex.