We report a combined experimental and theoretical investigation of MI2-(M = Cs, Cu, Ag, Au) to explore the chemical bonding in the group IA and IB diiodide complexes. Both photoelectron imaging and low-temperature photoelectron spectroscopy are applied to MI2- (M = Cs, Cu, Au), yielding vibrationally resolved spectra for CuI2- and AuI2-and accurate electron affinities, 4.52 +/- 0.02, 4.256 +/- 0.010, and 4.226 +/- 0.010 eV for CsI2, CuI2, and AuI2, respectively. Spin-orbit coupling is found to be important in all the diiodide complexes and ab initio calculations including spin-orbit effects allow quantitative assignments of the observed photoelectron spectra. A variety of chemical bonding analyses (charge population, bond order, and electron localization functions) have been carried out, revealing a gradual transition from the expected ionic behavior in CsI2- to relatively strong covalent bonding in AuI2-. Both relativistic effects and electron correlation are shown to enhance the covalency in the gold diiodide complex.