Ligand exchange reactions... are they one way or reversible? Are ligand exchange reactions one-way  reactions or reversible? I know this is a very silly question but its not said outright in any place...

For example, in my high school chemistry book, these two ligand exchange reactions are shown:

$$\ce{[Cu(H2O)6]^{2+} (aq) + 4Cl- (aq) &lt;=&gt; [CuCl_4]^{2-} (aq) + 6H2O (l)}$$

$$\ce{[Cu(H2O)6]^{2+} (aq) + 4NH3 (aq)-&gt; [Cu(NH3)4(H2O)2]^{2+} (aq) + 4H2O (l)}$$

The first is reversible but the second is one-way

Also, my teacher stated that ligand exchange reactions are irreversible... this confused me further

Can someone please tell me what is right...
Let's take the binding of $\ce{O2}$ and $\ce{CO}$ to hemoglobin as example. $\ce{CO}$ does bind much better than $\ce{O2}$!

We can consider the reaction of oxygen-loaded hemoglobin with $\ce{CO}$ a ligand exchange reaction. If that would be completely irreversible, the logical consequences in the case of an intoxication would be: [...] six feet under ;)

Instead, intoxicated patients are treated with oxygen at higher pressure.

To me, that sounds like Le Chatelier in the ICU and it only makes sense in the case of equilibria.