Synthesis, Characterization, and Theoretical Investigation of Two-Coordinate Palladium(0) and Platinum(0) Complexes Utilizing π-Accepting Carbenes

An elegant general synthesis route for the preparation of two coordinate palladium(0) and platinum(0) complexes was developed by reacting commercially available tetrakis(triphenylphosphine)palladium/platinum with π‐accepting cyclic alkyl(amino) carbenes (cAACs). The complexes are characterized by NMR spectroscopy, mass spectrometry, and single‐crystal X‐ray diffraction. The palladium complexes exhibit sharp color changes (crystallochromism) from dark maroon to bright green if the C‐Pd‐C bond angle is sharpened by approximately 6°, which is chemically feasible by elimination of one lattice THF solvent molecule. The analogous dark orange‐colored platinum complexes are more rigid and thus do not show this phenomenon. Additionally, [(cAAC)2Pd/Pt] complexes can be quasi‐reversibly oxidized to their corresponding [(cAAC)2Pd/Pt]+ cations, as evidenced by cyclic voltammetry measurements. The bonding and stability are studied by theoretical calculations. Bent out of shape: π‐Accepting cyclic alkyl(amino) carbenes were employed for the preparation of two‐coordinate, bent palladium(0) and platinum(0) complexes. The palladium complex exhibits crystallochromism (from maroon to green) due to controlled bending of the C‐Pd‐C bond angle by approximately 6° during elimination of a lattice solvent molecule. The bonding and stability of these complexes are studied by theoretical calculations.