Effect of changing oxidation state on the metal–ligand vibrations of [Cr(2,2′-bipyridyl)3]n+ type complexes

In the series of tris-(2,2′-bipyridyl) complexes of chromium, the Cr–N stretching frequencies change very little by changing the oxidation state of chromium from III to 0, this unexpected result indicating that the strength of the Cr–N bond does not change appreciably over a wide change in the oxidation state of chromium.