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How does one come to know (in this table) which metal cations form ammonia complexes and which ones form hydroxy complexes on reaction with $\ce{NH4OH}$


  1. J.D Lee - Concise Inorganic Chemistry by Sudarshan Guha
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    $\begingroup$ You can't really know specific properties without experimentation, also you can make rules and generalisations for similar behaivour. I think a good starting point would be the Wikipedia article for the 'HSAB theory'. It mainly fulfills your need for the complex formation prediction. The theory even assigns values for acids and bases based on the valence state, ionisation energy and electron affinity called hardness. $\endgroup$ – Andrew Kovács Jun 19 '20 at 13:19
  • $\begingroup$ Can you please illustrate an example of this using HSAB principle as this has been extremely confusing $\endgroup$ – Schwarz Kugelblitz Jun 19 '20 at 13:29
  • $\begingroup$ You may need more time to understand the topic, assuming it is new to you. There is a picture of the periodic table (f block elements excluded) on the mentioned article found at the Theory paragraph. The picture associates strenght with color, and the bonds tend do be more stable when hard and hard or soft and soft are pairing. As your source states Zn and Cd do form amine complexes, while Al doesnt. $\endgroup$ – Andrew Kovács Jun 19 '20 at 13:50
  • $\begingroup$ Al is considered a hard acid while Zn is intermediate, Cd is soft. O donors are somewhat harder bases than N so based on the aforementioned assumption the softer Zn and Cd pair better with the softer N donor while Al pairs better with the harder O donor. $\endgroup$ – Andrew Kovács Jun 19 '20 at 13:50
  • $\begingroup$ I have come across HSAB principle before particularly in the context of conjugate addition.However it is a bit new for me to apply this in inorganic chemistry but I think I get the idea. How would you apply this for determining solubility ? $\endgroup$ – Schwarz Kugelblitz Jun 19 '20 at 23:04

Please be mindful, in my opinion, that your source material may not be of the highest quality.

For example, NH4OH is more accurately represented as aqueous ammonia (that is, NH3(aq), see, for example, this source's comment).

Second, Al(OH)3 does not form a complex with ammonia or, as your reference claims, is 'partially soluble' with aqueous ammonia to any significant extent, and if at all, likely due to the presence of OH-. See, for example, this ebook, Basic Analytical Chemistry discussing various metal compounds interactions with aqueous ammonia.

Third, a more accurate depiction of the action of ammonia on Cu(OH)2, for example, is the formation of a complex of the form, $\ce{[Cu(NH3)_x(H2O)y](+)}$, where x + y = 6, with also a possible charge of 2+ for cupric, where the concentration of ammonia influences the value of 'x' (supplying guidance on your question as to which possible complex may form).

Lastly, high strength ammonia is needed to react with some of the listed metal hydroxides and is characteristic of the particular metal. See my suggested source, that may address your question on which salts are (or, are not) soluble.

  • $\begingroup$ Do you have another link for "Ammonium hydroxide doesn't exist"? The one above requires to be purchased. $\endgroup$ – ba-13 Jun 20 '20 at 9:48
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    $\begingroup$ @B.Anshuman These previous chem.SE questions(here and here) will be of help. $\endgroup$ – Nilay Ghosh Jul 14 '20 at 6:02

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