The far infrared spectra of metal chloride complexes of pyridine in relation to their structures

The infrared spectra of the complexes M(py)₂Cl₂] (py = pyridine; M = Mn, Fe, Co, Ni, Cu, Zn) have been determined within the range 650–150 cm^?1. Assignments of the metal—nitrogen and metal—halogen stretching frequencies (vM—N and vM—Cl) are made on the basis of the band shifts induced by deuteration of the pyridine ring and by varying the coordinated metal ion. If the band of lowest frequency is assigned to a bending mode, the number of vM—N and vM—Cl bands observed agrees with the number required by symmetry considerations based on the known structures of the complexes. On the basis of this work, some earlier assignments have been revised.