Ammonia is a strong or weak field ligand?

[Wrichik Basu]

Ammonia is present in the spectrochemical series near the middle, just after water. Is it a strong field ligand, or a weak field ligand?

Also, take into consideration the complex $[Ni(NH_3)_4 Cl_2]$. Chloride ion is towards the left end of the spectrochemical series which means it is a weak field ligand. Will the complex be a high spin or a low spin one?

 

[TeethWhitener]

Also, take into consideration the complex [Ni(NH3)4Cl2][Ni(NH3)4Cl2][Ni(NH_3)_4 Cl_2]. Chloride ion is towards the left end of the spectrochemical series which means it is a weak field ligand. Will the complex be a high spin or a low spin one?

Draw out the d-orbital configurations for Ni(II) high spin and low spin. What do you notice?

 

[Wrichik Basu]

Draw out the d-orbital configurations for Ni(II) high spin and low spin. What do you notice?

Ni(II) has 8 electrons in its 3d-orbital.

For low-spin complex:
$t_{2g}: d_{xy}^2 \, d_{yz}^2 \, d_{xz}^2$
$e_g: d_{x^2-y^2}^2 \, d_{z^2}^0$

For high-spin complex:
$t_{2g}: d_{xy}^2 \, d_{yz}^2 \, d_{xz}^2$
$e_g: d_{x^2-y^1}^2 \, d_{z^2}^1$

What conclusion can I draw from these?

 

[mjc123]

Not that high-spin Ni(II) has nine d electrons, certainly.

 

[TeethWhitener]

Also remember Hund’s Rule.

 

[Wrichik Basu]

My mistake. Corrected ones:

For low-spin complex:
$t_{2g}: d_{xy}^2 \, d_{yz}^2 \, d_{xz}^2$
$e_g: d_{x^2-y^2}^1 \, d_{z^2}^1$

For high-spin complex:
$t_{2g}: d_{xy}^2 \, d_{yz}^2 \, d_{xz}^2$
$e_g: d_{x^2-y^1}^1 \, d_{z^2}^1$

Both are same. Understood. Thanks @TeethWhitener

What about the first question:

Ammonia is present in the spectrochemical series near the middle, just after water. Is it a strong field ligand, or a weak field ligand?

 

[TeethWhitener]

Yes. For d⁰ through d³ and d⁸ through d¹⁰, there's really only one way to fill the d orbital states (EDIT: this is only true for octahedral complexes--once the geometry changes, the rules change), so high-spin vs. low-spin isn't a factor.

What about the first question:

One way to do this is to look at a case where the spin states do matter (d⁴ through d⁷) and see whether the ammine complexes are low-spin or high-spin. Looking at hexamminecobalt(III), you have a d⁶ cobalt center that's octahedral and low-spin, so this suggests that NH₃ is a reasonably strong field ligand.