When is a ligand strong field or weak field?

I have seen many examples where a strong field ligand is written for a particular ion .

For example, we have $\ce{Co(H2O)6^3+}$. What is the hybridization of $\ce{Co^{3+}}$? Now, we know $\ce{H2O}$ comes later in the spectrochemical series, thus it should be considered a weak field ligand. But for $\ce{Co^3+}$, $\ce{H2O}$ acts as a strong field ligand and pairs up the electron causing the hybridization to be d2sp3. This doesn't make sense. If we can see that a ligand is weak with respect to the spectrochemical series and it turns out that its not just for a particular ion, why is it so?

Also, another example is where $\ce{NH3}$ is supposed to be a strong field ligand but comes out to be a weak field ligand just for a particular ion.

Why does this happen? How do I know when to consider a ligand strong or weak?

• chemistry.stackexchange.com/a/73027/43942 – Berry Holmes May 4 '17 at 17:36
• Instead of asking when is the ligand strong- or weak-field you should be asking whether the complex is high- or low-spin, as there are obviously more factors than the ligand. Ligand X can produce a high-spin complex with metal M whereas it might produce a low-spin complex with N. Do you then call X a strong- or a low-field ligand? It's ambiguous - as for the reasons, a typical undergrad inorganic chemistry textbook will describe them. Also you can't get a good answer by using hybridisation or CFT - a proper rationalisation will need MOT – orthocresol May 4 '17 at 19:31
• Nope its d2sp3. – Lakshya Gupta May 5 '17 at 15:06