In contrast to porphyrins and chlorins, the direct metalation of bacteriochlorins is difficult. Nevertheless, Cu2+ and Zn2+ can be introduced into bacteriopheophytin in acetic acid, whereas Cd2+ can be inserted in dimethylformamide. The former reactions depend on the substituents of the isocyclic ring : they are facilitated if enolization of the beta-ketoester system is inhibited. Starting with [Cd]-bacteriochlorophyll-a or its 13(2)-hydroxy derivative, a series of metallo-bacteriochlorins with central divalent ions Pd2+, Co2+ Ni2+, Cu2+, Zn2+, and Mn2+ have been obtained by transmetalation. Like in the parent Mg complex, the four principal optical transitions are well-separated in these complexes, and their responses to changes in the central metal and its coordination state can be followed in detail. The energies of the Q(y) and B-x transitions are almost independent of the central metal, whereas the Q(x) and B-y transition energies change significantly, depending on the central metal as well as the presence of additional axial ligands. If the complexes are grouped by their coordination number, empirical linear correlations exist between these shifts and the ratio chi(M)(P)/r(M)(i), where chi(M)(P) is Pauling＇s electronegativity value and r(M)(i) is the ionic radius of the metal. A similar correlation was found for those H-1 NMR signals influenced mainly by the ring current and for the redox potentials. This observation was in contrast with the linear relationships with chi(M)(P) alone, found for metal-substituted porphyrins. The spectral variations influenced by the central metal and its state of ligation are attributed, within the framework of the four-orbital model, to the electrostatic interaction of the electron densities in the four orbitals with the effective charge of the central metal ions, which is most pronounced for the a(2u) orbital (HOMO-1). Ligation studies have revealed that addition of the first axial ligand decreases the effective charge of the central metal by approximately 50% and addition of the second axial ligand by another 20% with respect to the absence of axial ligands. The singlet-triplet splitting deduced from fluorescence and phosphorescence measurements is similar for [Pd]-, [Cu]-, [Zn]-, and [Mg]-BChl (4550 +/- 100 cm(-1)).