# 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, 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?

• 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. $n≥1$). May 3, 2019 at 11:27

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.

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).