Assignment of the CO stretching absorptions in C 4 v metal pentacarbonyl derivatives. The method of local oscillating dipoles

Abstract

Metal pentacarbonyl derivatives of composition M(CO) 5 X should have structures with C 4 v symmetry and thus are expected to show three fundamental absorption bands in the CO stretching region of the infrared due to 2 a 1 + e modes. When M = Re and X = Cl, only two fundamental bands appear. As the size of M is decreased or that of X increased, a weak band on the high-energy side of the main peaks is observed to increase in intensity. By use of a simple oscillating local dipole picture in describing the different carbonyl stretching modes of these molecules, it is shown that the weak high-energy band can be assigned to one of the a 1 modes which, due to cancellation of moments, cannot be observed if four of the CO groups are coplanar. However, as these four groups are forced out of the plane this mode becomes active. Assignment of the other two fundamentals is made on the basis of position and intensity of the bands.