Density functional derived structures and molecular properties of nickel dithiolenes and related complexes

The molecular and electronic structure of the planar nickel dithiolene (1c, R = H) and of related complexes derived from nickel dithiolene by replacem ent of Ni by Pd (palladium dithiolene, 2c, R = H) or by Pt (platinum dithio lene, 3c, R = H), or by replacement of S by NH (nickel diiminolene, 1a, R = H), O (nickel dioxylene, Ib, R = H) or Se (nickel diselenolene, 1d, R = H) , were studied by density functional theory using the B3LYP functional and the valence triple-zeta basis set 6-311+G* for all atoms except Pd and Pt. For the latter atoms the quasirelativistic effective core potentials of the Stuttgart group were employed. The molecular structure of nickel dithiolen e (1c, R = H) is satisfactorily reproduced by DFT calculations. The geometr y of the corresponding platinum complexes 3a-3d is more sensitive to relati vistic effects, resulting in the contraction of the X-Pt bonds. As shown wi th the metal dithiolenes, the two ligands are structurally related to monon egative ions of open shell structure. The C-C bond lengths of the complexes are close to those of aromatic and chain-type polymethine structures (abou t 1.4 Angstrom). The nickel dithiolene (1c, R = H) and related complexes ha ve D-2h symmetry and are 14 pi-electron systems with 10 pi-electrons at the ligands and 4 pi-electrons at the metal center. The natural population ana lysis has confirmed that metal M++ does accept electrons from the Ligands b ut to a lesser extent than expected. The empty d-orbitals of M++ are only p artly occupied in the molecular ground state. The positive charge of the me tal decreases in the order Ni > Pd > Pt. The H-1 chemical shifts and the nu cleus-independent chemical shifts (NICSs) of the ring moieties calculated b y GIAO-DFT display a pronounced electron delocalization. In agreement with the calculated C-C bond lengths the 1H chemical shifts and the NICS values show a marked bond delocalization. The NICS values show a change of the aro matic delocalization in the order Ni > Pd < Pt and NH > O < S < Se. The wav e numbers of the LR spectra of the complexes calculated by DFT are grouped in separate frequency regions. The very intense absorption of 1c (R = H) in the visible region of the spectrum is surprisingly well reproduced by ab i nitio single-only configuration interaction calculations. While the color b and of the palladium complex is predicted to be red-shifted relative to the nickel complex, a blue shift is calculated on passing from the palladium t o the platinum complex, The blue shift is, in part, due to the relativistic contraction of bond lengths in the Pt complexes.