MAPPING THE UNPAIRED ELECTRON-DENSITY IN [PT(BIPY)L-2](-) (BIPY = 2,2'-BIPYRIDINE L = CL-, CN-, (CN-)-C-13) - A COMBINED EPR-ENDOR-THEORETICAL STUDY

The X-band EPR and electron-nuclear double resonance (ENDOR) spectra o f the formally d(9), 17-electron radical anions [Pt(bipy)L-2](-) (bipy = 2,2'-bipyridine; L = Cl-, CN-, (CN-)-C-13) are reported. The Pt-195 (I = 1/2, 34%) anisotropic hyperfine matrices are analysed in terms o f both 5d and 6p contributions to the singly occupied molecular orbita ls (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 unp aired electron density is delocalised about the bipy pi system and EN DOR spectra reveal the superhyperfine couplings to the H-1, N-14 and C -13 (L = (CN-)-C-13) nuclei. The unpaired electron densities in the N- 14 2p(z) orbitals contributing to the pi systems are calculated to be 12% and 4% for L = Cl- and CN-, respectively. The cyanide pi-orbitals make little contribution to the SOMO for [Pt(bipy)(CN)(2)](-). There is a good agreement with density functional theory (DFT) MO calculatio ns oil the radical anions. DFT calculations also show the atomic orbit al compositions of the lowest unoccupied (LU) MO of [Pt(bipy)L-2] and the SOMO of [Pt(bipy)L-2](-) to be virtually identical.