Aromaticity and antiaromaticity in transition-metal systems

Area of Science:

• Inorganic Chemistry
• Theoretical Chemistry
• Materials Science

Background:

• Aromaticity, traditionally applied to organic compounds, is now recognized in transition-metal systems.
• Recent discoveries in all-metal clusters highlight aromaticity and antiaromaticity in transition metals.
• The d-orbitals in transition metals introduce complex bonding, structures, and stability.

Purpose of the Study:

• To review aromaticity and antiaromaticity in transition-metal clusters and compounds.
• To discuss the unique chemical bonding and structural diversity introduced by d-orbitals.
• To explore the concept of delta-aromaticity and multi-fold aromaticity in these systems.

Main Methods:

• Review of existing literature on transition-metal clusters and compounds.
• Analysis of theoretical frameworks describing sigma, pi, and delta aromaticity.
• Examination of proposed counting rules for cyclic transition-metal systems.

Main Results:

• Transition-metal systems exhibit diverse aromaticity, including sigma, pi, and a novel delta-aromaticity.
• Multi-fold aromaticity, antiaromaticity, or conflicting aromaticity are characteristic features.
• Separate counting rules are proposed for cyclic transition-metal systems accounting for all three aromaticity types.

Conclusions:

• Multiple aromaticity and antiaromaticity phenomena are likely prevalent in transition-metal chemistry.
• This review aims to stimulate further research into the structure and bonding of transition-metal systems.
• Understanding aromaticity in transition metals could impact studies of metalloenzymes and biomolecules.