Resonance Raman (rRaman) spectroscopy has been used extensively in the studies of the heme chemistry of carbon monoxy adducts. In porphyrin systems, the axial ligand vibrational modes of the CO, v(Fe-CO) and v(CO), are enhanced with Soret excitation via an A-term (Franck-Condon) mechanism, but are not expected with Q excitation (B-term or vibronic mechanism). For the first time, these modes have been obtained with Q, as well as Soret excitation in rRaman spectra of CO complexes of ferrous chlorins. The enhancement with Q,excitation arises from an A-term mechanism of Raman scattering for these chlorins owing to their reduced molecular symmetry. Thus, in iron chlorins or other heme systems with reduced molecular symmetry, axial ligand vibrational modes may be enhanced with a, excitation if they are observed with Soret excitation in the corresponding iron porphyrins. These findings show rRaman spectroscopy to be exceptionally valuable in the study of chlorin chromophores with a, enhancement using red or orange-red excitation. Furthermore, the method appears to be selective for chlorin cofactors in proteins containing multiple heme centers such as cytochrome bd oxidase (see, for example, Sun; et al. Biochemistry 1995, 35, 2403-2412). It has been known that v(Fe-CO) and v(CO) frequencies of CO complexes of iron porphyrins and heme proteins exhibit linear correlations, falling into distinct sets for complexes possessing the same fifth ligand (for example, Ray; et al. J. Am. Chem. Sec. 1994, 126, 162-176). In this work, we have found that v(Fe-CO) and v(CO) of iron-chlorin-CO complexes also respond to the nature of the opposite axial ligand and fellow the same correlations derived from porphyrin systems. Thus, the reduction of one of the pyrrole rings of porphyrins has little effect on v(Fe-CO) and v(CO) frequencies, and their correlation behavior may perhaps be used to ascertain the identity of the proximal ligand of the chlorin in a protein system of unknown coordination, as in cytochrome bd oxidase.