What is the range of values of n in [Cu(NH3)4(H2O)n]SO4?

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](https://en.wikipedia.org/wiki/Tetraamminecopper(II)_sulfate), 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?




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



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


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



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> 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 $\pu{160 ^{\circ}C}$.
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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:



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> $\ce{Cu(NH3)4SO4.H2O}$ 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 ($\ce{OH...O} = \pu{2.67 Å}$). 
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All of these data directed to the comment of @andselisk who states that tetraamminecopper(II) sulfate is a *monohydrate* but, Probably why [Wikipeda](https://en.wikipedia.org/wiki/Tetraamminecopper(II)_sulfate) 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*. 


**References:**


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>).


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