Preparation and spectral and electrochemical characterization of dirhodium(II) complexes with bridging 1,8-naphthyridine ligands: 2,7-bis(2-pyridyl)-1,8-naphthyridine, 5,6-dihydrodipyrido[2,3-b:3'2'-j][1,10]phenanthroline, 2-(2-pyridyl)-1,8-naphthyridine and 1,8-naphthyridine. X-ray crystal structure of tris(.mu.-acetato)(2,7-bis(2-pyridyl)-1,8-naphthyridine)dirhodium(II) Hexafluorophosphate