Utilizing all-electron Dirac-exact relativistic calculations with the Normalized Elimination of the Small Component (NESC) method and the local vibrational mode approach, the transition from metal-halide to metal halogen bonding is determined for Au-complexes interacting with halogen-donors. The local stretching force constants of the metal-halogen interactions reveal a smooth transition from weak non-covalent halogen bonding to non-classical 3-center-4-electron bonding and finally covalent metal halide bonding. The strongest halogen bonds are found for dialkylaurates interacting with Cl-2 or FCI. Differing trends in the intrinsic halogen-metal bond strength, the binding energy, and the electrostatic potential are explained. (C) 2017 Elsevier B.V. All rights reserved.