Malar Density functional theory analysis of some triple-decker sandwich complexes of iron containing cyclo-P 5 and cyclo-As 5 ligands

Structure and bonding in triple-decker cationic complexes [(η5-Cp)Fe(μ, η:η-E5) Fe(η5-Cp)]+ (1: E = CH, 2: E = P, 3: E = As) and [(η5-Cp)Fe(μ, η:η5-Cp)Fe(η5-E5)]+ (E = P, As) are examined by density functional theory (DFT) calculations at the B3LYP/6-31+G* level. These species exhibit the lowest energy when all the three ligands are eclipsed. In the complexes with bifacially coordinated cyclo-E5, the perfectly eclipsed D5h sandwich structure a is found to be a potential minimum. The energy difference between the fully eclipsed and the staggered conformations b and c are within 1.0, 2.1, and 6.3 kcal/mol, respectively, for E = CH, P, andAs. The isomeric species with monofacially coordinated cycloE5 (E =P, As), [(η5 -Cp)Fe(μ, η :η5-Cp)Fe(η5-E5)]+ are predicted to be about 30 and 60 kcal/mol higher in energy , respectively, for E = P and As. The calculations predict that the bifacially coordinated cyclo-E5 (E =P,As) undergoes significant ring expansion leading to “loosening of bonds” as observed experimentally. The consequent loss of aromaticity in the central cyclo-E5 indicates that significant π -electron density from the ring can be directed towards bonding with the iron centers on both sides. The diffuse nature of the π orbitals of cyclo-P5 and cyclo-As5 can lead to better overlap with the iron d-orbitals and result in stronger bonding. This is reflected in the bond order values of 0.377 and 0.372 for the Fe-P and Fe-As bonds in 2a and 3a, respectively. The natural population analysis reveals that the Fe atom that is coordinated to a cyclo-E5 (E = P, As) possesses a negative charge of −0.23 to −0.38 units due to transfer of electron density from the inorganic ring to the metal center.