Carbonyl stretching vibrations planar complexes

FIGURE 4-22 Carbonyl Stretching Vibrations of cis- and tranx-Dicarbonyl Square Planar Complexes. 

FIGURE 4.26 Carbonyl Stretching Vibrations of c/s- and frans-Dicarbonyi Square Planar Complexes. 

It is often useful to consider a particular vibrational mode for a compound. For example, useful information often can be obtained from the C—O stretching bands in infrared spectra of metal complexes containing CO (carbonyl) ligands. The following example of cis- and trawi-dicarbonyl square planar complexes shows the procedure. 

The carbonyl stretches of a square planar complex, M(CO)4, can be derived from a basis made up of four vectors, one along each of the C—O bonds (pointing from C to O). From this basis, derive the reducible representation that makes up the C—O stretches and use the reduction formula to identity the irreducible representations it contains. Identify which vibrations will be IR active and which Raman active and use the projection operator method to sketch out each of the four vibrations. 

The square-planar Ni(I) complexes form five-coordinate Ni(I) carbonyl complexes when CO gas is introduced into solutions of the complexes, because they contain electron-rich Ni(I) ions capable of it back-donation (66,135). The CO binding constants and carbonyl vibrational frequencies are summarized in Table XI. Because of the back-bonding interaction between the Ni(I) atom and the CO ligand, CO stretching frequencies for the Ni(I) carbonyl complexes decrease as NiL+ becomes a more powerful reductant, which is represented by the Ev2 values (66). 

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