title: A counterintuitive structural effect of metal-metal bond protonation and its electronic underpinnings.
abstract: Protonation across the metal-metal bond in the complexes [(CO)(2)M(mu-dppm)(mu-PtBu(2))(mu-H)M(CO)(2)] (M=Fe or Ru, dppm=Ph(2)PCH(2)PPh(2)) induces M-M bond shortening of up to about 0.05 A. DFT calculations on simplified iron models reproduce this trend well. Conversely, the computations show that the M-M distance in the dimer [{Cp*Ir(CO)}(2)] lengthens with two consecutive protonations, but there are no crystal structure determinations to highlight the effects on the Ir-Ir bond. DFT calculations and the analogous cobalt system confirm that the transformation of a two-electron, two-center (2e-2c) bond into a 2e-3c bond is accompanied by the predicted elongation. An MO analysis indicated similar nature and evolution of the M-M bonding these cases. In particular, the HOMOs of the mono-hydrido cations [Cp(CO)M(mu-H)M(CO)Cp](+) (M=Ir, Co) have evident M-M bent-bond character, and hence subsequent protonation invariably causes a decrease in the bond index. The Fe(2) and Co(2) systems have also been analyzed with the quantum theory of atoms in molecules (QTAIM) method, but in no case was an M-M bond critical point located unless an artificially shorter M-M distance was imposed. However, the trends for the atoms-in-molecules (AIM) bond delocalization indexes delta(M-M) confirm the overall M-M bond weakening on protonation. In conclusion, all the computational results for the iron system indicate that the paradigm of a direct correlation between bond strength and distance is not always applicable. This is attributable to a very flat potential energy surface and various competing effects imposed by the bridging ligands.
authors.affiliation: Istituto di Chimica dei Composti Organo Metallici (ICCOM-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Florence, Italy. carlo.mealli@iccom.cnr.it
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Phillips
authors.forename: Andrea
authors.initials: A
authors.lastname: Ienco
authors.forename: Joachim
authors.initials: J
authors.lastname: Reinhold
authors.forename: Hans-Christian
authors.initials: HC
authors.lastname: Böttcher
authors.forename: Carlo
authors.initials: C
authors.lastname: Mealli