All-Organometallic Analogues of Zeise's Salt for the Three Group 10 Metals

Ethene has been found to be able to split the electron-deficient pentafluorophenyl bridging system in [NBu₄]₂[{M(C₆F₅)₂}(μ-C₆F₅)₂] to give the corresponding mononuclear compounds [NBu₄][M(C₆F₅)₃(η²-C₂H₄)] (M = Pt (1), Pd (2)) in reasonable yield. Compounds 1 and 2 are well-behaved species and have been isolated and characterized by analytical and spectroscopic methods. The crystal structure of 2, as established by X-ray diffraction methods, reveals that the Pd atom is in an approximately SP-4 environment defined by the ipso-C atoms of the three σ-bound C₆F₅ groups (C₆F₅-κ¹) and the midpoint between the doubly bonded C atoms of the metal π-bound ethene molecule (η²-C₂H₄). The ethene molecule is coordinated upright, and the CC bond length (133.6(6) pm) is the same as in the free ligand (133.7(2) pm). The nickel homologue [NBu₄][Ni(C₆F₅)₃(η²-C₂H₄)] (3), formed by the low-temperature reaction of [NBu₄]₂[Ni(C₆F₅)₄] with B(C₆F₅)₃ in the presence of C₂H₄, could not be isolated but only spectroscopically detected in solution. The experimentally established stability of the [M(C₆F₅)₃(η²-C₂H₄)]^- species has been found to follow the trend calculated by DFT methods for the M-(η²-C₂H₄) bond strength:  Pt > Pd > Ni. Furthermore, quantitative estimates of back-bonding in the [M(C₆F₅)₃(η²-C₂H₄)]^- and [MCl₃(η²-C₂H₄)]^- anions were obtained using NBO analyses of electron populations of the relevant donor−acceptor orbitals and the second-order stabilization energy associated with the charge transfer (CT) interactions describing the back-bonding phenomenon.