Electronic structure and bonding studies on triple-decker sandwich complexes with a P6 middle ring

Area of Science:

• Organometallic Chemistry
• Computational Chemistry
• Materials Science

Background:

• Triple-decker sandwich complexes featuring a central P6 ring and metal atoms (M) are of interest due to their unique electronic and structural properties.
• The mno Rule provides a guideline for skeletal electron counts in cluster frameworks, but deviations can lead to interesting structural variations.

Purpose of the Study:

• To investigate the structure and bonding of CpMP6MCp triple-decker sandwich complexes using Density Functional Theory (DFT) and hybrid Hartree-Fock DFT (HF-DFT) methods.
• To correlate the number of valence electrons (VE) with the observed geometries (planar symmetric, asymmetric, puckered) of the central P6 ring.
• To explore the electronic factors, including orbital occupancies and metal oxidation states, that dictate these structural preferences.

Main Methods:

• Computational studies employing DFT and hybrid HF-DFT calculations.
• Analysis of electronic structure, including valence electron counts (VE) and orbital occupancies.
• Comparison of calculated structures with predictions from the mno Rule.

Main Results:

• Complexes with 28 VE exhibit symmetric planar P6 rings, while 26 VE complexes show asymmetric planar geometries, and 24 VE complexes adopt puckered structures.
• Deviations from the mno Rule's 50 skeletal electrons, particularly in higher VE complexes, are shown to influence P6 ring distortions.
• Orbital occupancies (2a* and 2b*) and metal oxidation states (e.g., Ti +4) are key factors determining the specific geometries observed.

Conclusions:

• The valence electron count is a critical determinant of the geometric preferences (planar vs. puckered) in P6 triple-decker sandwich complexes.
• DFT and HF-DFT methods effectively elucidate the interplay between electronic structure and molecular geometry in these organometallic systems.
• The study supports the carbon-phosphorus analogy, suggesting similar bonding patterns can be found in related carbon- and phosphorus-based clusters.