Theoretical study on [3]- and [4]radialene complexes CpM(C2nH2n) (n = 3, 4; M = Sc∼Ni): Special metal-aromatic interaction along with metal-alkene bonds

Abstract The coordination and electron configuration of radialene complexes CpM(C 2n H 2n ) (n = 3, 4) are investigated. For M = Sc, Ti, Fe, Co, Ni, the ground states of CpM(C 2n H 2n ) are low-spin states (singlet or doublet), while for M = V, Cr, Mn, the ground states are high-spin states (triplet or quartet). For M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, [3]radialene in the ground states of CpM(C 6 H 6 ) are ŋ 6 -, ŋ 6 -, ŋ 6 -, ŋ 4 -, ŋ 4 -, ŋ 4 -, ŋ 4 -, and ŋ 2 -coordinated; and [4]radialene in the ground states of CpM(C 8 H 8 ) are ŋ 8 -, ŋ 8 -, ŋ 6 -, ŋ 4 -, ŋ 4 -, ŋ 4 -, ŋ 4 -, and ŋ 2 -coordinated, respectively. Between metal atom and radialene in CpM(C 2n H 2n ), the metal-alkene interactions, which are caused by the exocyclic C C π-bonds that donate the lone pair electrons to the empty d-orbitals of metal atom, are dominant and fundamental. Besides, the special π-backbonding metal-aromatic interactions, which is overlapped by the lone pair on metal atom and the empty 2π-aromatic orbital on the cyclic C 3 or C 4 ring of [n]radialenes, may also exist in some ground states of CpM(C 2n H 2n ) complexes.