Terminal and bridging ligand effects on M(I)-M(I) multiple bonding: A DFT investigation of the coordination in (X)[M2Cl]L2 complexes (M = Cr, Fe, L = CO, PEt3, X = Cl, allyl, Cp, and indenyl)

A theoretical DFT investigation has been achieved on (X)[M2Cl]L-2 (M = Cr, Fe, L = CO, PEt3, and X = allyl, cyclopentadienyl, and indenyl) complexes by means of BP86 and B3LYP* functionals. The geometry optimizations showed an eta(3)-coordination for the allyl, while Cp and indenyl ligands adopt the same eta(5)-coordination mode and the chloride acts as bridging ligand. The metal-metal bond distances are very sensitive to the replacement of the CO with PEt3 (triethylphosphine) with regard to the electron-donation and electron pi-backdonation properties for the former and only to the electron-donation for the later. This replacement impacts drastically the metal-metal bonding. Changing the metal from Cr to Fe increases the metal-metal bond distance, decreases the bond order and the oxidation sate of Fe, in agreement with the decrease of Wiberg bond indices (WBI) and Mayer bond order values (MBO). The findings showed that the Cl- and allyl(-) behave as 4-electron donors, while Cp- and Ind(-) behave as 6-electron donors leading to outstanding electron properties stemming from different coordination, which are clarified by the sigma-donation and pi-backdonation properties, which are highlighted by the orbitals' populations and the energy decomposition analysis (EDA). Based on the aforementioned information, the chromium and iron complexes of Cl- and allyl(-) are of 22- and 24-metal valence electrons (MVE), where those of Cp- and Ind(-) are of 26- and 28-MVE, respectively. It is well noted that the metal-metal multiple bonding overcomes the electrons' deficiency.