Can the trend of spectrochemical series vary/fluctuate according to the central metal atom (CMA) that is being used?
For example, for the CMA $\ce{Ni^2+}$ the ligand $\ce{H2O}$ will have more CFSE than $\ce{NH3}$ because $\ce{NH3}$ acts as a weak field ligand (WFL) for $\ce{M^2+},$ even worse then $\ce{H2O},$ though $\ce{NH3}$ is placed higher then $\ce{H2O}$ in the spectrochemical series.
The spectrochemical series of ligands, our instructor clarified, gives only a rough idea of the relative strengths of ligands, and may vary for different Central Metal Atoms (CMAs).
An important example that I have encountered is of $\ce{[Co(H2O)6]^+^3}$, wherein $\ce{H2O}$ acts as a Strong Field Ligand (SFL), despite its lower position in the series.
Also, JD Lee mentions in his Concise Inorganic Chemistry (5th ed., Wiley publications) that there is an increase in CFSE as we go down the group in d-block, and several instances may be found where the ligands designated as WFL begin to act as SFL for 4-d or 5-d members.
Overall, I believe there are certain deviations from the series, but it generally works well.
