Energetics of Halogen Bonding of Group 10 Metal Fluoride Complexes

A study is presented of the thermodynamics of the halogen-bonding interaction of C6F5I with a series of structurally similar group 10 metal fluoride complexes trans-[Ni(F)(2-C5NF4)(PCy3)(2)] (2), trans-[Pd(F)(4-C5NF4)(PCy3)(2)] (3), trans-[Pt(F){2-C5NF2H(CF3)}(PR3)(2)] (4a, R = Cy; 4b R = iPr) and trans-[Ni(F){2-C5NF2H(CF3)} (PCy3)(2)] (5a) in toluene solution. F-19 NMR titration experiments are used to determine binding constants, enthalpies and entropies of these interactions (2.4 <= K-300 <= 5.2; -25 <= Delta H degrees <= -16 kJ mol(-1); -73 <= Delta S degrees <= -49 J K-1 mol(-1)). The data for -Delta H degrees for the halogen bonding follow a trend Ni < Pd < Pt. The fluoropyridyl ligand is shown to have a negligible influence on the thermodynamic data, but the influence of the phosphine ligand is significant. We also show that the value of the spin-spin coupling constant J(PtF) increases substantially with adduct formation. X-ray crystallographic data for Ni complexes 5a and 5c are compared to previously published data for a platinum analogue. We show by experiment and computation that the difference between Pt-X and Ni-X (X = F, C, P) bond lengths is greatest for X = F, consistent with F(2p pi)-Pt(5d pi) repulsive interactions. DFT calculations on the metal fluoride complexes show the very negative electrostatic potential around the fluoride. Calculations of the enthalpy of adduct formation show energies of -18.8 and -22.8 kJ mol(-1) for Ni and Pt complexes of types 5 and 4, respectively, in excellent agreement with experiment.