Electronic Spectra of the Tetraphenylcyclobutadienecyclopentadienylnickel(II) Cation and Radical

Properties of the tetraphenylcyclobutadienecyclopentadienylnickel­(II)
cation 1 and its tetra-o-fluoro derivative 1a have been measured and calculated. The B3LYP/TZP optimized
geometry of the free cation 1 agrees with a single-crystal
X-ray diffraction structure except that in the crystal one of the
phenyl substituents is strongly twisted to permit a close-packing
interaction of two of its hydrogens with a nearby BF4– anion. The low-energy parts of the solution electronic
absorption and magnetic circular dichroism (MCD) spectra of 1 and 1a have been interpreted by comparison
with TD-DFT (B3LYP/TZP) results. Reduction or pulse radiolysis lead
to a neutral 19-electron radical, whose visible absorption and MCD
spectra have been recorded and interpreted as well. The reduction
is facilitated by ∼0.1 V upon going from 1 to 1a. Unsuccessful attempts to prepare several other aryl substituted
derivatives of 1 by the classical synthetic route are
described in the Supporting Information.