Binuclear cyclopentadienyliridium hydride chemistry: Terminal versus bridging hydride and cyclopentadienyl ligands

Abstract The cyclopentadienyliridium hydrides Cp₂Ir₂H_n (Cp = η⁵-C₅H₅; n = 6, 4, 2, 0), and CpIrH_n (n = 4, 2) related to the experimentally known pentamethyl cyclopentadienyliridium hydrides (Formula presented.) (Cp∗ = η⁵-Me₅C₅; n = 6, 2) and Cp∗IrH₄ have been investigated by density functional theory. The lowest energy Cp₂Ir₂H_n (n = 6, 4, 2) structures are predicted to have terminal Cp rings with the central Ir₂ unit bridged by two hydrogen atoms. For the hexahydride Cp₂Ir₂H₆, such doubly bridged Cp₂Ir₂(μ-H)₂H₄ structures are bent, leading to trans and cis structures of similar energies with an unbridged Cp₂Ir₂H₆ isomer lying only ∼4 kcal/mol above the bridged structures. This suggests fluxional behavior consistent with experimental data on the temperature dependence of the proton NMR spectrum of the closely related (Formula presented.). The tetrahydride Cp₂Ir₂H₄ is predicted to undergo slightly exothermic disproportionation into Cp₂Ir₂H₆ + Cp₂Ir₂H₂ and thus not be a viable species. This is consistent with the failure to find any (Formula presented.) in the (Formula presented.) systems. The doubly bridged dihydride Cp₂Ir₂(μ-H)₂ is a particularly favorable species since it lies more than 18 kcal/mol in energy below any other isomer. Higher energy Cp₂Ir₂H_n (n = 4, 2) structures have one or two bridging Cp rings and exclusively terminal hydrogen atoms. Related structures are the lowest energy structures for the hydride-free Cp₂Ir₂. A higher energy Cp₂Ir₂ structure consists of two CpIr units linked solely by an Ir-Ir bond. Analysis of the frontier molecular orbitals indicates this Ir-Ir bond to be the quadruple bond required to give each iridium atom the favored 18-electron configuration. However, this quadruple bond is a 2σ + 2π bond with no δ components and thus differs from the σ + 2π + δ quadruple bond found in the long-known Re₂Cl₈^2-.