Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MX, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV-VI, II-VI, and III-V semiconductor nanocrystals. In particular, we show that the Lewis acid base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20-30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50-65% for red-emitting CH3NH3CdBr3- and (NH4)(2)ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.