Geometry Optimization of a Ru(IV) Allyl Dicationic Complex:  A DFT Failure?

Area of Science:

• Computational Chemistry
• Organometallic Chemistry
• Quantum Chemistry

Background:

• Accurate geometry optimization is crucial for understanding organometallic complex behavior.
• Density Functional Theory (DFT) is widely used, but its performance for specific interactions needs evaluation.
• Ruthenium-allyl interactions are key in many catalytic processes.

Purpose of the Study:

• To assess the performance of various DFT functionals and basis sets for geometry optimization of a specific ruthenium complex.
• To evaluate the accuracy of DFT in describing weak ruthenium-carbon bonds, particularly in η(3)-allyl coordination.

Main Methods:

• Geometry optimization of [Ru(η(5)-C5H5)(η(3)-CH2CHCHC6H5)(CH3CN)2](2+) using five pure and four hybrid DFT functionals with VDZP, VTZP, and VQZP basis sets via Gaussian 03.
• Comparison of calculated geometries with X-ray crystallographic data of an analogous complex.
• Validation using the ADF program with larger basis sets and employing Møller-Plesset perturbation theory (MP2) calculations.

Main Results:

• All tested DFT functionals and basis sets poorly described the coordination mode of the η(3)-allyl ligand.
• Significant overestimation of the Ru-C bond distance for the substituted allyl carbon (0.23–0.50 Å) was observed across DFT methods.
• MP2 calculations yielded a more accurate Ru-C bond distance, with only a slight underestimation (0.07 Å).

Conclusions:

• Standard DFT functionals and basis sets are inadequate for accurately describing weak Ru-C interactions in this type of organometallic complex.
• The study highlights limitations of DFT for precise geometry optimization involving specific weak metal-ligand bonds.
• MP2 method provides a more reliable description for these weak interactions compared to the tested DFT approaches.