Question

the C-O strectching frequency has the foloowing value sthe four tertacarbonyl complexes shown below. What does this tell you about the M-CO bond in the theses complexes? Expalin the direction of the trend.

complex   CO Frequency

Ni (Co)₄ 2046 cm^-1

Co(CO)₄^- 1883 cm^-1

Fe(CO)_4^2- 1788 cm^-1

Answer 1

Stabilized by M —> L backbonding (d —> p*). Remember the HOMO and LUMO of CO; there is a combination of L—>M sigma bonding and M—>L pi backbonding. For the M—>L pi backbonding you need an electron-rich metal (low oxidation state). This effect is mirrored in the trends of CO stretching frequencies--high backbonding causes more electron density in CO p* orbital, weakening C—O bond, lowering stretching frequency.

With increasing charge on the metal, greater pi donation, lowering CO bond order.

  Bridging CO's have lower C—O stretching frequencies (v ~ 1800 cm-1) than terminal CO's (v ~ 2000 cm-1). Note, however, that the electron count alone does not predict presence of bridging CO's since an M2(CO)2 unit has the same number of electrons either way.