Spectral data from metalloporphyrin-ligand complexes show shifts in maxima which are significantly larger for the B (Soret) transitions than for the Q (visible) transitions, in contrast to previous findings not involving ligands. The shifts in the B transitions are ascribed primarily to a stereoelectronic effect. The data suggest weaker configuration interaction than previously assigned. The possibility that visible absorption bands in porphyrins may be due to n -* * and 1 -* transitions is considered. Evidence in favor of this interpretation is presented, based on the examination of the spectra of monoprotonated porphyrins and on spectra in fluorosulfonic acid-antimony pentafluoride solution. Other possible interpretations of the data are also discussed.It is generally agreed that the absorption bands of porphyrins and metalloporphyrins observed in the region 350-700 µ are due to it -* * transitions. Simple Hiickel calculations by Longuet-Higgins, Rector, and Platt4 assign the degenerate pair of eg orbitals as lowest empty orbitals and a2u and alu as highest and next highest filled orbitals, respectively. The near-ultraviolet absorption (Soret band) is then ascribed to a transition of the type alu -eg, while the visible bands are ascribed to a2u -* eg transitions. This theory regards both transitions as allowed and does not account for the large difference in intensity observed between the visible and ultraviolet transitions. Platt5 has modified this one-electron approach by assuming strong configuration interaction between alu and a2u components. These are supposed to be nearly degenerate and give a combined "f" orbital, in Platt's notation, which has angular momentum of 4 au. The(1) (a) Part XXXVII of Porphyrin Studies; for part XXXVI, see

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