All-Transition Metal Aromaticity and Antiaromaticity

Though aromaticity in compounds containing a transition-metal atom has already been discussed for quite a long time, aromaticity in all-transition metal systems have been recognized only recently. There are examples of sigma-, pi-, and delta-aromaticity based on s-, p-, and d-AO0s. We derived the counting rules for sigma-, pi-, delta-, and phi-aromaticity/antiaromaticity for both singlet/triplet coupled model triatomic and tetratomic systems so that one could use those to rationalize aromaticity and antiaromaticity in all-transition metal systems. These rules can be easily extended for any cyclic systems composed out of odd or even number of atoms. We elucidated the application of these rules to the all-transition metal cyclic systems: Au-3(+)/Au-3(-), Na2Zn3, Hg-4(6-), Mo3O92- Sc-3(-), Hf-3, and Ta-3(-) clusters. We believe that the use of concepts of aromaticity, antiaromaticity and conflicting aromaticity can be an important theoretical tool for deciphering chemical bonding in various known and novel chemical compounds containing transition metal atoms.