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.