Mapping the unpaired electron density in [Pt(bipy)L₂]^- (bipy = 2,2′-bipyridine; L = Cl^-, CN^-, ¹³CN^-): A combined EPR-ENDOR-theoretical study

The X-band EPR and electron-nuclear double resonance (ENDOR) spectra of the formally d⁹, 17-electron radical anions [Pt(bipy)L₂]^- (bipy = 2,2′-bipyridine; L = Cl^-, CN^-, ¹³CN^-) are reported. The ¹⁹⁵Pt (I = 1/2, 34%) anisotropic hyperfine matrices are analysed in terms of both 5d and 6p contributions to the singly occupied molecular orbitals (SOMOs). For L = Cl^- there is 5.0% 5d_yz and 7.6% 6p_z character, for L = CN^- there is <1% 5d_yz and 13.1% 6p_z. The bulk of the unpaired electron density is delocalised about the bipy π* system and ENDOR spectra reveal the superhyperfine couplings to the ¹H, ¹⁴N and ¹³C (L = ¹³CN^-) nuclei. The unpaired electron densities in the ¹⁴N 2p_z orbitals contributing to the π* systems are calculated to be 12% and 4% for L = Cl^- and CN^-, respectively. The cyanide π-orbitals make little contribution to the SOMO for [Pt(bipy)(CN)₂]^-. There is a good agreement with density functional theory (DFT) MO calculations on the radical anions. DFT calculations also show the atomic orbital compositions of the lowest unoccupied (LU) MO of [Pt(bipy)L₂]^- and the SOMO of [Pt(bipy)L₂]^- to be virtually identical.