Try and rationalise what you are seeing

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Carry out an electron count on carbon monoxide, CO. It has 10 electrons, 6 of which are involved in bonding. The remaining 4 are divided between the C and the O atom, meaning the C atom has 2 electrons for metal bonding (note that the following is only one of the resonance forms of carbon monoxide).

 

•	Thus the C is bonded to the Mo via a sigma bond; the lone pair on the C is donated to an empty metal sigma orbital:

•	There is a secondary bonding effect: the Mo atom is 6-coordinate and is d6, thus the orbital splitting diagram will resemble the following: i.e. the t2g orbitals are filled however the eg orbitals are empty.

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The donation of electrons from the CO to the metal via the sigma bond effectively increases the electron density on the metal. The vacant carbon monoxide pi antibonding orbitals are of similar size to the filled metal t2g orbitals, thus the additional electron density on the metal (arising from the MoC bond) is donated from the metal t2g orbitals back to carbon antibonding orbitals as shown below.

 

 

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As the bonding is from the metal to the ligand rather than the usual ligand to metal, and because it involves pi orbitals, the phenomenon is termed pi back-bonding. This effect occurs in all metals however the effect is observer more commonly in metals with at least d4 electron configurations (fewer d electrons mean the metal has less electron density to donate back to the CO). We have used Mo in this example only to restrict the hitlist size.

•	But how does this affect the MoC and CO distances?

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Donation of electrons from the CO to the Mo strengthens the MoC bond. But this effect increases the amount of electron density on the metal, which in turn means the metal has more electron density which it donates back to the CO. As the electron density involved in back-bonding is donated to the CO antibonding orbitals, this effectively lengthens the CO bond.

•	Return to ConQuest and look at refcodes DUSHAA and FOFTOK.

 

	DUSHAA	FOFTOK
MoC distance	1.887Å	2.012Å
CO distance	1.228Å	1.140Å

 

•	These structures are at opposing ends of the scattergram and illustrate the effect described above.