Why is carbon monoxide a “weak” lewis base?

A lewis base is substance that donates an unshared pair of electron.

In CO, a lone pair is present on both carbon and oxygen each. Moreover, lone pair of electron on carbon is localised so CO will act as lewis base. What causes the CO to become weak lewis base?

Also tell, what "Lewis acidity of CO" would mean? Actually , I read that lewis acidity of CO is very small? But I am not able to understand it.

• It isn't weak, just soft base and one of strongest pi acceptors. – Mithoron Dec 10 '17 at 0:48

$\ce{CO}$ is usually considered a Lewis base, as the lone pair on carbon readily donates it's electrons, for example forming transition metal coordination complexes. Although oxygen is more electronegative than carbon, the carbon is more rich in electron density in the $\ce{CO}$ molecule as it basically has a full formal negative charge. Additionally, products formed by the carbon bonding to Lewis acids are much more favorable that products that would be formed by bonding to the oxygen atom.

It is also possible for CO to act as a Lewis acid however. When acting as a ligand with certain low-valent transition metals, the metal can redistribute electron density back to the $\ce{CO}$ ligand in a process called back-donating. In this case the $\ce{CO}$ molecule accepts electrons via it's antibonding $\mathrm{\pi}$ orbitals, making it act as a Lewis acid.

• By question was why it is "weak "base? – Ka Sikh May 7 '17 at 5:15
• I edited my question. – Ka Sikh May 7 '17 at 15:37
• @KaSikh , sorry I misunderstood that the reason for CO being a weak rather than strong Lewis base was the crux of your question. I'm glad you edited that into your question, though maybe it was just confusion on my part ;) I'll try to come up with a good explanation, but the cutoff between strong and weak for Lewis acid/bases is not nearly as well defined as that for Brønsted–Lowry. You need to have some point of reference; the degree of reaction with BF3 is one example I've seen. But there are general trends, mainly corresponding to polarizability, electronegativity, etc. I'll edit my ans – airhuff May 7 '17 at 21:14
• I appreciate it. It would be nice if you also explain lewis acidity of CO as mentioned at last in question. – Ka Sikh May 8 '17 at 18:54
• I hope to get to it soon. Notice that the second paragraph of my answer discusses the Lewis acidity of CO. – airhuff May 8 '17 at 18:57