Theoretical studies of π-loading and structural diversity in Cp3MX (M = Zr, Hf; X = H, CH3, OR, NR2) compounds

Density functional theory has been used to explore the bonding and geometric preferences of tris-Cp zirconium and hafnium compounds of the type Cp3MX. In most cases, two primary isomers were found: a “3-5” isomer, which contains three η5-Cp ligands, and a “2-5,1-1” isomer, which contains two η5-Cp and one η1-Cp ligands. The relative energies of these isomers are dependent upon the π-donor ability of the X ligand. For the tris-Cp methyl compounds, the “3-5” isomers are more stable by 1–4 kcal/mol. The “2-5,1-1” isomers of the tris-Cp alkoxide and amido compounds are more stable by 3–6 and 6–11 kcal/mol, respectively. Bonding analyses show these binding preferences can be understood in terms of π-competition between the tris-Cp ligand set and the lone pair of the X ligand. π-loading on the X ligand decreases the π-donor ability of the “3-5” tris-Cp ligand set, which increases the preference for the “2-5,1-1” ground state geometry in the complexes with strong π-donors. In addition, molecular orbital theory provides evidence for destabilization effects arising from filled–filled interactions between the “3-5” Cp3M fragment and the X lone pair.

DFT calculations have been used to explore the bonding and geometric preferences of Cp3MX (M = Zr, Hf; X=H, CH3, OR, NR2) compounds containing σ-donor and π-donor X ligands. The compounds with a σ-only X ligand were found to be more stable as the “3-5” isomer, while the compounds with a π-donor X ligand were more stable as the “2-5,1-1” isomer. This stability difference is explained using molecular orbital theory.Figure optionsDownload as PowerPoint slide