Recent advances in aromaticity and antiaromaticity in transition-metal systems

The concept of aromaticity/antiaromaticity was recently extended to polynuclear transition metal compounds. In the current report we review recent advances made in the field of aromaticity and antiaromaticity in bare transition-metal clusters, clusters of transition metal suboxides and transition-metal clusters embedded in metalloorganic and inorganic compounds. We analyzed aromaticity in reported aromatic/antiaromatic species through the recently developed Adaptive Natural Density Partitioning method. We confirmed aromaticity in most cases though there are a few cases where no aromaticity or a different type of aromaticty was revealed by the AdNDP method. The peculiarity of aromaticity in polynuclear transition metal compounds is frequently its multifold (σ-, π-, and δ-) nature. Moreover, the presence of simultaneous multifold aromaticity/antiaromaticity leads to conflicting aromaticity which complicates assignment of aromaticity/antiaromaticity in such compounds.