The molecular and electronic structure of the planar nickel metal center. The natural population analysis has confirmed that metal M ++ does accept electrons from the ligands but to dithiolene (1c, R = H) and of related complexes derived from nickel dithiolene by replacement of Ni by Pd (palladium a lesser extent than expected. The empty d-orbitals of M ++ are only partly occupied in the molecular ground state. The dithiolene, 2c, R = H) or by Pt (platinum dithiolene, 3c, R = H), or by replacement of S by NH (nickel diiminolene, 1a, positive charge of the metal decreases in the order Ni Ͼ Pd Ͼ Pt. The 1 H chemical shifts and the nucleus-independent R = H), O (nickel dioxylene, 1b, R = H) or Se (nickel diselenolene, 1d, R = H), were studied by density functional chemical shifts (NICSs) of the ring moieties calculated by GIAO-DFT display a pronounced electron delocalization. In theory using the B3LYP functional and the valence triple-zeta basis set 6-311+G* for all atoms except Pd and Pt. For the agreement with the calculated C-C bond lengths the 1 H chemical shifts and the NICS values show a marked bond latter atoms the quasirelativistic effective core potentials of the Stuttgart group were employed. The molecular structure delocalization. The NICS values show a change of the aromatic delocalization in the order Ni Ͼ Pd Ͻ Pt and NH Ͼ of nickel dithiolene (1c, R = H) is satisfactorily reproduced by DFT calculations. The geometry of the corresponding O Ͻ S Ͻ Se. The wave numbers of the IR spectra of the complexes calculated by DFT are grouped in separate platinum complexes 3a-3d is more sensitive to relativistic effects, resulting in the contraction of the X-Pt bonds. As frequency regions. The very intense absorption of 1c (R = H) in the visible region of the spectrum is surprisingly well shown with the metal dithiolenes, the two ligands are structurally related to mononegative ions of open shell reproduced by ab initio single-only configuration interaction calculations. While the color band of the palladium complex structure. The C-C bond lengths of the complexes are close to those of aromatic and chain-type polymethine structures
is predicted to be red-shifted relative to the nickel complex, a blue shift is calculated on passing from the palladium to (about 1.4 A ˚). The nickel dithiolene (1c, R = H) and related complexes have D 2h symmetry and are 14 π-electron systems the platinum complex. The blue shift is, in part, due to the relativistic contraction of bond lengths in the Pt complexes. with 10 π-electrons at the ligands and 4 π-electrons at the [a]