My textbook represents the copper ion as "tetraaminecopper(II) sulfate-1-water", so I assumed that this complex ion has only 1 water of crystallization.

Then, in the Wikipedia article, they represent the ion as $\ce{[Cu(NH3)4(H2O)_n]SO4}$. The same formula is used but the number of water of crystallization is variable, denoted by $n$. So $n$ must have a range of values.

My question is what determines the value of $n$? What condition would cause $n$ to be larger or lower?

  • 1
    $\begingroup$ From the top of my head I only recollect tetraamminecopper(II) sulfate monohydrate. Probably that's the way Wikipeda article suggesting that this compound is hygroscopic and water content may deviate (e.g. $n≥1$). $\endgroup$
    – andselisk
    May 3, 2019 at 11:27

1 Answer 1


Tetraamminecopper(II) Sulfate or cuprammonium sulfate can be synthesized as its monohydrate (Ref.1):

A solution of $\pu{50 g}$ of finely divided $\ce{CuSO4 . 5 H2O}$ in $\pu{75 mL}$ of conc. ammonia and $\pu{50 mL}$ of water is filtered and precipitated by slow addition of $\pu{75 mL}$ of alcohol. After standing for several hours in the cold, the crystals are filtered on a Büchner funnel, washed with a mixture of alcohol and conc. ammonia (1:1) and then with alcohol and ether, and dried by suction.

$$\ce{CuSO4.5H2O + 4NH3 -> [Cu(NH3)4]SO4.H2O + 4H2O}$$

The properties of tetraamminecopper(II) Sulfate monohydrate is listed as follows (Ref.1):

Deep blue crystals, $\mathrm{d}^{20}_4$: 1.81. Solubility ($\pu{21.5 ^{\circ}C}$): $\pu{18.5 g} / \pu{100 g}\: \ce{H2O}$. Decomposes in air. Loses $\ce{H2O}$ and $\ce{2NH3}$ on heating to $\pu{120 ^{\circ}C}$; the remaining ammonia is evolved at $\pu{160 ^{\circ}C}$.

The crystallographic study of tetraamminecopper(II) Sulfate monohydrate was done several times, and most recently by Mazzi (Ref.2). In the abstract of Ref.1, the author states that:

$\ce{Cu(NH3)4SO4.H2O}$ is orthorhombie, $Pmcn-D^{16}_{2h}$, with four molecules in the unit cell. The determination of the structure was carried out by means of Patterson and Fourier projections on $(001)$ and $(100)$. The structure consists in layers parallel to $(010)$ formed by $\ce{Cu(NH3)4}$ squares and $\ce{SO4}$ tetrahedra held together by $\ce{H2O}$ molecules. Copper atoms, which lie on symmetry planes, have four nearest $\ce{NH3}$ neighbours ($\ce{Cu-N} = \pu{2.04-2.06 Å}$) and two more $\ce{H2O}$ neighbours ($\ce{Cu-O} = \pu{2.59-3.37 Å}$), which complete the distorted octahedral co-ordination group. Each $\ce{SO4}$ tetrahedron ($\ce{S-O} = \pu{1.40-1.43-1.46 Å}$) has two oxygen atoms linked with hydrogen bonds to adjacent $\ce{H2O}$ molecules ($\ce{OH...O} = \pu{2.67 Å}$).

All of these data directed to the comment of @andselisk who states that tetraamminecopper(II) sulfate is a monohydrate but, Probably why Wikipeda article suggesting it as $\ce{[Cu(NH3)4(H2O)n]SO4}$ that this compound may be hygroscopic, and thus, its water content may deviate (or vary). The crystal structure has confirmed it is a monohydrate.


  1. Georg Brauer, Ed., Handbook of Preparative Inorganic Chemistry – Volume 2; 2nd Edn, P. G. Stecher, Translation Ed., Translated by Scripta Technica, Inc.; Academic Press: New York, NY, 1965, “Section 19: Copper, Silver, Gold, by O. Glemser & H. Sauer,” p. 1021.
  2. F. Mazzi, “The crystal structure of cupric tetrammine sulfate monohydrate, $\ce{Cu(NH3)4SO4.H2O}$,” Acta Cryst. 1955, 8, 137–141 (https://doi.org/10.1107/S0365110X55000571).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.