On the Relative Intensity-Concentration Relationship of Carbonyl Bands in the Solution Infrared Spectra of Metal Carbonyl Compounds of C4v Symmetry

The theory on the relative intensities of CO stretching bands of metal carbonyl compounds of C^4v symmetry has been advanced by orgel, inter alios. ² For complexes of formula M(CO)₅Xⁿ a = -1, 0), three infrared active bands are expected, 2A₁ + E. The two A₁ bands consist of modes which are predominantly vibration of the CO trans to X (A₁ ⁽¹⁾) and the symmetrical stretch of the four equatorial CO's (A^{1 (2)}). The A₁ ⁽²⁾ band is nearly forbidden, and owes its intensity to the contribution of the axial CO stretch to the normal mode. The end result of this coupling between axial and radial CO stretching symmetry modes would appear to be an aid in assigning the bands in the carbonyl stretching region to the appropriate symmetry modes; the relative intensities are weak (A₁ ⁽²⁾), strong (E), and medium (A₁ ⁽¹⁾) Although this weak-strong-medium pattern is invariably observed in the spectra of