The infra-red spectra of pyridine complexes and pyridinium salts

The main qualitative differences between the spectra of pyridine, co-ordinated pyridine and the pyridine ion are discussed and an assignment is made for all the bands observed in the spectra of co-ordinated pyridine. In view of the small shifts observed when comparing the spectra of pyridine complexes, it is concluded that the electron density over the pyridine ring remains almost constant whatever the acceptor atom and it is suggested that back-bonding plays a part in the bonding of these complexes. some of the N-H bands in the spectra of the pyridinium ion have been identified by deuteration and the spectra of this ions are discussed.