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. 19F NMR titration experiments are used to determine binding constants, enthalpies and entropies of these interactions (2.4 ≤ K300 ≤ 5.2; −25 ≤ ΔHo ≤ −16 kJ mol–1; −73 ≤ ΔSo ≤ −49 J K–1 mol–1). The data for −ΔHo for the halogen bonding follow a trend Ni JPtF 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(2pπ)–Pt(5dπ) 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.