σ‐, π‐, δ‐, and ϕ‐Aromaticity

In recent years, there have been significant advances in extending the concepts of aromaticity and antiaromaticity into the realm of inorganic chemistry. These new developments have shown that aromaticity and antiaromaticity in metal systems are frequently multiple in nature. Two new types of aromaticity, namely, δ- and ϕ-aromaticity, were found in transition metal and f -element cyclic systems in addition to the σ- and π-types already known in organic chemistry. There are also now many examples of inorganic systems with conflicting aromaticity, which refers to the systems with simultaneous presence of aromaticity and antiaromaticity in different electronic (σ-, π-, δ-, or ϕ-) subsystems. It was shown that the antiaromaticity in any electronic subsystem could be interpreted in terms of formation of small areas of aromaticity leading to the island aromaticity.