Structural motifs, thermodynamic properties, bonding and aromaticity of sandwich complexes formed by alkaline earth metals with pentafulvene. A theoretical approach

Structure of M(η5-C6H6)2 complexes, where M=(Be, Ma, Ca, Sr, Ba), and C6H6 is the pentafulvene, was optimized at the B3LYP/6–311++G(d,p)/ECP level of theory. Two types of complexes were found: (cis-) singlet ansa-metallocenes with the cis-arrangement of two pentafulvene rings and (trans-) triplet sandwich type pentafulvene complexes of trans-where two pentafulvene molecules are rotated in respect to each other by various degrees. All metals except beryllium are bonded in η5-mode of coordination. The charge transfer of 1.6–1.8 e from the metal atom to the pentafulvene ring estimated by Natural Population Analysis results in aromatization of the fulvene rings, which is documented by three aromaticity indices: electronic pEDA, geometric HOMA and magnetic NICS(1)ZZ. For sandwich complexes pEDA and NICS(1)ZZ are linearly correlated. Bonding situation in systems under study was analyzed by Energy Decomposition Analysis at the B3LYP/TZP/ZORA level. The electrostatic contribution is dominant, especially in the case of cis-complexes, however the covalent contribution is substantial, especially for beryllium complex. Exceptionally low dissociation energy of magnesium complex can be explained by the fact that the electrostatic and orbital contributions are not fully compensating the high Pauli repulsion term.