.DELTA.*MM-.PI.L Odd Electron Delocalization onto Aromatic Bridging Ligands in a Paramagnetic Dirhodium Complex and Intermolecular .PI.-Stack Interaction in Crystal.

The odd electron in the delta(RhRh)* orbital of (3,1)-[Rh-2(mhq)(4)py](+) (1(+); Hmhq = 4-methyl-2-quinolinol) was estimated to be delocalized onto each pi system of the four mhq bridges to the extent of ca. 9% based on its H-1 NMR paramagnetic shifts. The experimental spin distribution in the mhq ligands is consistent with a model of mixing of the pi HOMOs of the mhq ligands into the delta(RhRh)* singly occupied molecular orbital (SOMO). In crystals of (3,1)-[Rh-2(hq)(4)py] (2; Hhq = 2-quinolinol) and its cationic radical salts, 2 or 2(+) molecules form intermolecular pi-stack arrangements of hq ligands. Intermolecular interaction between 2(+) molecules in crystal was studied by examining electronic spectra, magnetic susceptibility, ESR, and electrical conductance of its salts.