Photoelectron spectroscopy and computational studies of electron delocalization in organometallic complexes

Abstract

Metal-metal and metal-ligand electron delocalization in organometallic complexes, such as biferrocene, bis(mu-fulvalenediyl)diiron, CpM(CO)₄, (Cn*)RhMe₃, and (P₃)RhMe₃, (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane and P₃ = CH₃C(CH₂PMe₂)₃ are examined by gas-phase photoelectron spectroscopy. Theoretical studies at various computational levels are also carried out to understand the delocalization. The metal-metal electron delocalization is studied using biferrocene and bis(mu-fulvalenediyl)diiron molecules. Compared with monometallic molecules, these molecules show a different degree of electron delocalization through a bridging ligand depending on the energy match between each ferrocene fragment. The extended π system of the ligand has greater impact on the ligand-based ionizations than on the metal-based ionizations. Further splitting and broadening of the metal-based ionization band is observed for the ring-tilted ferrocene. The metal-ligand electron delocalization is studied with two type of molecules. The first is (η⁵-C₅H₄R)M(CO)₄ (R = H, M = V, Nb, Ta; R = SiMe₃, M = Nb, Ta; R = COCH₃, M = Nb) which is used to probe the metal-carbonyl interaction. In general, these molecules exhibit both extensive delocalization of metal electron density and geometric sensitivity to the electron configuration. Such strong delocalization dampens the substitutent effect introduced onto the Cp ring or at the metal center. A consistent splitting of 0.4 eV is observed for the Cp-based ionization band for all of the molecules due to a dynamic Jahn-Teller distortion of the positive ions. The other type of molecule is (L)RhMe₃ [L = Cn* and P₃]. The amine molecule exhibits a negligible backbonding from the metal to the ligand while the phosphine molecule shows a strong metal-phosphine interaction provided by the electron donation from the metal to the phosphines. As a comparison, spectra of ligands Cn*, Cn, and P₃ show a similar band profile for the first ionization band. Theoretical studies demonstrate that the metal-phosphine backbonding accounts for the different band profiles in photoelectron spectra. A theoretical study of the relaxation energy of metal-based ionizations of CpM(CO)n (M = V, Nb, Ta, n = 4; M = Mn, Tc, Re, n = 3; M = Co, Rh, Ir, n = 2) in the Hartree-Fock formalism has been carried out. The results show that the electron relaxation energy of these molecules can be well understood in terms of Slater's concepts of electron shielding and effective nuclear charge.