Ammonia is a strong or weak field ligand?

  • #1
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Main Question or Discussion Point

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?
 

Answers and Replies

  • #2
TeethWhitener
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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?
 
  • #3
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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?
 
  • #4
mjc123
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Not that high-spin Ni(II) has nine d electrons, certainly.
 
  • #5
TeethWhitener
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Also remember Hund’s Rule.
 
  • #6
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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?
 
  • #7
TeethWhitener
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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.
 

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