A normal coordinate analysis of 4,4'-bipyridyl (4,4'-bipy) is carried out to establish consistent vibrational assignment and to determine a Urey-Bradley force field. The potential energy distribution of 4,4'-bipy is calculated which has provided certainty for the assignments. Additional information is obtained from the vibrational spectra of M (4,4'-bipy)Cl-2 complexes (where M = Zn, Cu or Cd) and calculated wave numbers of complexed 4,4'-bipy. Several vibrational modes of 4,4'-bipy in the IR and Raman spectra of the complexes are found to have upward shifts in frequency compared to those in the free molecule and the shifts are metal dependent. An explanation supported by the normal coordinate analysis is provided in terms of coupling with low frequency vibrations, particularly the M-N stretching frequencies.