Is Pseudohalide CN^- a Real Halide? A General Symmetry Consideration

Recently, in light of the significant attention devoted to pseudohalide CN- and cyano radical CN physico-chemical property studies and superhalide behavior exploration in CN--ligated metal compounds, the photoelectron angular distribution nature of pseudohalide CN- has been directly demonstrated via the photoelectron velocity map imaging technique to be comparable to Cl-. For the halide Cl-, photoelectrons were observed at 266 nm (4.66 eV) to peak, perpendicular to the laser polarization associated with the detachment of p-orbital symmetry. For the analogous pseudohalide CN-, photoelectrons were present at a peak in laser polarization at 266 nm, which can be explained as detachment from mainly atomic s-like orbital symmetry. Although both are often regarded as having the same high electron affinity and similarly strong chemical bonding capabilities to stabilize complexes, their photoelectron angular distributions are distinctly different, which indicates their intrinsically different electronic-structure symmetry (HOMO nature). The approach based on symmetry consideration in this work could be utilized to explain the photoelectron angular distributions of pseudohalide and classic halide ligands via the advanced photoelectron velocity map imaging tool.