Planar Hexacoordinate Carbons: Half Covalent, Half Ionic.

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Quantum Chemistry

Background:

• Carbon typically exhibits coordination numbers of 3 or 4.
• Previous studies have explored hypercoordinate carbon but not in planar, stable global minima.
• The D3h CO3Li3+ structure served as a foundational model.

Purpose of the Study:

• To report the first global minima containing a planar hexacoordinate carbon (phC) atom.
• To design and characterize novel CE3M3+ (E=S-Te, M=Li-Cs) compounds.
• To investigate the geometric and electronic criteria for stable phC.

Main Methods:

• Theoretical structure prediction and optimization.
• Density Functional Theory (DFT) calculations.
• Chemical bonding analysis (e.g., QTAIM, ELF).

Main Results:

• Fifteen novel CE3M3+ structures were identified as global minima.
• All structures satisfy geometric and electronic criteria for planar hexacoordinate carbon.
• Carbon atoms exhibit a unique bonding pattern: covalent to three chalcogens and ionic to three alkali metals.

Conclusions:

• The successful design of phC structures is achieved by replacing oxygen with heavier, less electronegative chalcogens.
• This strategy induces negative charge on carbon, facilitating electrostatic attraction with alkali-metal cations.
• The findings challenge traditional views of carbon coordination and open new avenues in chemical bonding research.