Question

How does CO act as a ligand?

Answer 1

Look at the Lewis structure of carbon monoxide....

Explanation:

...the Lewis structure I would write is....

`""^(-):C-=O:^(+)`

...i.e. the HOT end of the ligand is the carbon, i.e. the donor with negative charge....and this typically binds to a metal centre in a Lewis base, Lewis acid interaction.....`Mlarr""^(-):C-=O:^(+)`.

However, carbon monoxide is often found coordinated to LOW valent transition metal centres, i.e. metals in low oxidation states....i.e. `Cr^0`, `Ni^0`...we invoke some mechanism by which the metal can back-donate electron density to the ligand, and this is what we mean we speak of the `pi-"acidity"` of carbon monoxide, it accepts electrons as well as donating them.... It engages in `sigma-"bond"` donation, but it also engages in `pi` back-donation, because the `pi-"antibonding orbitals"` of carbon monoxide are (i) empty, and (ii), the right geometry, and (iii) the right energy to receive electron density from the FILLED `"d-orbitals"` of the metal centre....

The bonding interaction is positive overall...i.e. the metal carbon bond is strengthened....but because electron density is DONATED to an empty antibonding orbital of the `C-=O`, with respect to carbon monoxide, the `C-O` bonding interaction is diminished, and the physical manifestation of this is the decrease in the `nu_(CO)` stretching frequency in the IR spectrum....