Water of crystallization

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Template:Short description In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions.[1] In some contexts, water of crystallization is the total mass of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation.

Upon crystallization from water, or water-containing solvents, many compounds incorporate water molecules in their crystalline frameworks. Water of crystallization can generally be removed by heating a sample but the crystalline properties are often lost.

Compared to inorganic salts, proteins crystallize with large amounts of water in the crystal lattice. A water content of 50% is not uncommon for proteins.

Applications

Knowledge of hydration is essential for calculating the masses for many compounds. The reactivity of many salt-like solids is sensitive to the presence of water. The hydration and dehydration of salts is central to the use of phase-change materials for energy storage.[2]

Position in the crystal structure

File:H-bondingFeSO47aq.tif
Some hydrogen-bonding contacts in Template:Chem2. This metal aquo complex crystallizes with water of hydration, which interacts with the sulfate and with the Template:Chem2 centers.

A salt with associated water of crystallization is known as a hydrate. The structure of hydrates can be quite elaborate, because of the existence of hydrogen bonds that define polymeric structures.[3] [4] Historically, the structures of many hydrates were unknown, and the dot in the formula of a hydrate was employed to specify the composition without indicating how the water is bound. Per IUPAC's recommendations, the middle dot is not surrounded by spaces when indicating a chemical adduct.[5] Examples:

For many salts, the exact bonding of the water is unimportant because the water molecules are made labile upon dissolution. For example, an aqueous solution prepared from Template:Chem2 and anhydrous Template:Chem2 behave identically. Therefore, knowledge of the degree of hydration is important only for determining the equivalent weight: one mole of Template:Chem2 weighs more than one mole of Template:Chem2. In some cases, the degree of hydration can be critical to the resulting chemical properties. For example, anhydrous Template:Chem2 is not soluble in water and is relatively useless in organometallic chemistry whereas Template:Chem2 is versatile. Similarly, hydrated Template:Chem2 is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of Template:Chem2 must therefore be protected from atmospheric moisture to preclude the formation of hydrates.

File:Ca(aq)6 improved image.tif
Structure of the polymeric Template:Chem2 center in crystalline calcium chloride hexahydrate. Three water ligands are terminal, three bridge. Two aspects of metal aquo complexes are illustrated: the high coordination number typical for Template:Chem2 and the role of water as a bridging ligand.

Crystals of hydrated copper(II) sulfate consist of Template:Chem2 centers linked to Template:Chem2 ions. Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water. A fifth water resides elsewhere in the framework but does not bind directly to copper.[6] The cobalt chloride mentioned above occurs as Template:Chem2 and Template:Chem2. In tin chloride, each Sn(II) center is pyramidal (mean Template:Chem2 angle is 83°) being bound to two chloride ions and one water. The second water in the formula unit is hydrogen-bonded to the chloride and to the coordinated water molecule. Water of crystallization is stabilized by electrostatic attractions, consequently hydrates are common for salts that contain +2 and +3 cations as well as −2 anions. In some cases, the majority of the weight of a compound arises from water. Glauber's salt, Template:Chem2, is a white crystalline solid with greater than 50% water by weight.

Consider the case of nickel(II) chloride hexahydrate. This species has the formula Template:Chem2. Crystallographic analysis reveals that the solid consists of [trans-Template:Chem2 subunits that are hydrogen bonded to each other as well as two additional molecules of Template:Chem2. Thus one third of the water molecules in the crystal are not directly bonded to Template:Chem2, and these might be termed "water of crystallization".

Analysis

The water content of most compounds can be determined with a knowledge of its formula. An unknown sample can be determined through thermogravimetric analysis (TGA) where the sample is heated strongly, and the accurate weight of a sample is plotted against the temperature. The amount of water driven off is then divided by the molar mass of water to obtain the number of molecules of water bound to the salt.

Other solvents of crystallization

Water is particularly common solvent to be found in crystals because it is small and polar. But all solvents can be found in some host crystals. Water is noteworthy because it is reactive, whereas other solvents such as benzene are considered to be chemically innocuous. Occasionally more than one solvent is found in a crystal, and often the stoichiometry is variable, reflected in the crystallographic concept of "partial occupancy". It is common and conventional for a chemist to "dry" a sample with a combination of vacuum and heat "to constant weight".

For other solvents of crystallization, analysis is conveniently accomplished by dissolving the sample in a deuterated solvent and analyzing the sample for solvent signals by NMR spectroscopy. Single crystal X-ray crystallography is often able to detect the presence of these solvents of crystallization as well. Other methods may be currently available.

Table of crystallization water in some inorganic halides

In the table below are indicated the number of molecules of water per metal in various salts.[7][8]

Hydrated metal halides
and their formulas		 Coordination sphere
of the metal		 Equivalents of water of crystallization
that are not bound to M		 Remarks
Calcium chloride
Template:Chem2		 Template:Chem2 Template:CNone example of water as a bridging ligand[9]
Titanium(III) chloride
Template:Chem2		 trans-Template:Chem2[10] two isomorphous with Template:Chem2
Titanium(III) chloride
Template:Chem2		 Template:Chem23+[10] Template:CNone isomeric with Template:Chem2.2H2O[11]
Zirconium(IV) fluoride
Template:Chem2		 Template:Chem2 Template:CNone rare case where Hf and Zr differpage=965-12|[12]
Hafnium tetrafluoride
Template:Chem2		 Template:Chem2Template:Mvar(Template:H2O-nl)Template:Mvar one rare case where Hf and Zr differpage=965-12|[12]
Vanadium(III) chloride
Template:Chem2		 trans-Template:Chem2[10] two
Vanadium(III) bromide
Template:Chem2		 trans-Template:Chem2[10] two
Vanadium(III) iodide
Template:Chem2		 Template:Chem2 Template:CNone relative to Template:Chem2 and Template:Chem2, Template:Chem2 competes poorly
with water as a ligand for V(III)
Template:Chem2 Template:Chem2 four
Chromium(III) chloride
Template:Chem2		 trans-Template:Chem2 two dark green isomer, aka "Bjerrums's salt"
Chromium(III) chloride
Template:Chem2		 Template:Chem2 one blue-green isomer
Chromium(II) chloride
Template:Chem2		 trans-Template:Chem2 Template:CNone square planar/tetragonal distortion
Chromium(III) chloride
Template:Chem2		 Template:Chem2 Template:CNone violet isomer. isostructural with aluminium compound[13]
Manganese(II) chloride
Template:Chem2		 trans-Template:Chem2 two
Manganese(II) chloride
Template:Chem2		 cis-Template:Chem2 Template:CNone cis molecular, the unstable trans isomer has also been detected[14]
Manganese(II) bromide
Template:Chem2		 cis-Template:Chem2 Template:CNone cis, molecular
Manganese(II) iodide
Template:Chem2		 trans-Template:Chem2 Template:CNone molecular, isostructural with FeCl2(H2O)4.[15]
Manganese(II) chloride
Template:Chem2		 trans-Template:Chem2 Template:CNone polymeric with bridging chloride
Manganese(II) bromide
Template:Chem2		 trans-Template:Chem2 Template:CNone polymeric with bridging bromide
Rhenium(III) chloride
Template:Chem2		 triangulo-Template:Chem2 Template:CNone heavy early metals form M-M bonds[16]
Iron(II) chloride
Template:Chem2		 trans-Template:Chem2 two
Iron(II) chloride
Template:Chem2		 trans-Template:Chem2 Template:CNone molecular
Iron(II) bromide
Template:Chem2		 trans-Template:Chem2 Template:CNone molecular,[17] hydrates of FeI2 are not known
Iron(II) chloride
Template:Chem2		 trans-Template:Chem2 Template:CNone polymeric with bridging chloride
Iron(III) chloride
Template:Chem2		 trans-Template:Chem2 two one of four hydrates of ferric chloride,[18] isostructural with Cr analogue
Iron(III) chloride
Template:Chem2		 cis-Template:Chem2 two the dihydrate has a similar structure, both contain Template:Chem2 anions.[18]
Cobalt(II) chloride
Template:Chem2		 trans-Template:Chem2 two
Cobalt(II) bromide
Template:Chem2		 trans-Template:Chem2 two
Cobalt(II) iodide
Template:Chem2		 Template:Chem2 Template:CNone[19] iodide competes poorly with water
Cobalt(II) bromide
Template:Chem2		 trans-Template:Chem2 Template:CNone molecular[17]
Cobalt(II) chloride
Template:Chem2		 cis-Template:Chem2 Template:CNone note: cis molecular
Cobalt(II) chloride
Template:Chem2		 trans-Template:Chem2 Template:CNone polymeric with bridging chloride
Cobalt(II) bromide
Template:Chem2		 trans-Template:Chem2 Template:CNone polymeric with bridging bromide
Nickel(II) chloride
Template:Chem2		 trans-Template:Chem2 two
Nickel(II) chloride
Template:Chem2		 cis-Template:Chem2 Template:CNone note: cis molecular[17]
Nickel(II) bromide
Template:Chem2		 trans-Template:Chem2 two
Nickel(II) iodide
Template:Chem2		 Template:Chem2 Template:CNone[19] iodide competes poorly with water
Nickel(II) chloride
Template:Chem2		 trans-Template:Chem2 Template:CNone polymeric with bridging chloride
Platinum(IV) chloride
Template:Chem2[20]		 trans-Template:Chem2 3 octahedral Pt centers; rare example of non-first row chloride-aquo complex
Platinum(IV) chloride
Template:Chem2[21]		 fac-Template:Chem2 0.5 octahedral Pt centers; rare example of non-first row chloride-aquo complex
Copper(II) chloride
Template:Chem2		 Template:Chem2 Template:CNone tetragonally distorted
two long Cu-Cl distances
Copper(II) bromide
Template:Chem2		 Template:Chem2 two tetragonally distorted
two long Cu-Br distances[17]
Zinc(II) chloride
ZnCl2(H2O)1.33[22]		 Template:Chem2 Template:CNone coordination polymer with both tetrahedral and octahedral Zn centers
Zinc(II) chloride
ZnCl2(H2O)2.5[23]		 Template:Chem2 Template:CNone tetrahedral and octahedral Zn centers
Zinc(II) chloride
Template:Chem2[22]		 Template:Chem2 Template:CNone tetrahedral and octahedral Zn centers
Zinc(II) chloride
ZnCl2(H2O)4.5[22]		 Template:Chem2 three tetrahedral and octahedral Zn centers
Cadmium chloride
CdCl2·H2O[24]		 Template:CNone water of crystallization is rare for heavy metal halides
Cadmium chloride
CdCl2·2.5H2O[25]		 CdCl5(H2O) & CdCl4(H2O)2 Template:CNone
Cadmium chloride
CdCl2·4H2O[26] 		Template:CNone octahedral
Cadmium bromide
CdBr2(H2O)4[27]		 Template:Chem2 two octahedral Cd centers
Aluminum trichloride
Template:Chem2		 Template:Chem2 Template:CNone isostructural with the Cr(III) compound

Examples are rare for second and third row metals. No entries exist for Mo, W, Tc, Ru, Os, Rh, Ir, Pd, Hg, Au. AuCl3(H2O) has been invoked but its crystal structure has not been reported.

Hydrates of metal sulfates

Transition metal sulfates form a variety of hydrates, each of which crystallizes in only one form. The sulfate group often binds to the metal, especially for those salts with fewer than six aquo ligands. The heptahydrates, which are often the most common salts, crystallize as monoclinic and the less common orthorhombic forms. In the heptahydrates, one water is in the lattice and the other six are coordinated to the ferrous center.[28] Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides.[29][30] Many monohydrates are known.[31]

Formula of
hydrated metal ion sulfate		 Coordination
sphere of the metal ion		 Equivalents of water of crystallization
that are not bound to M		 mineral name Remarks
MgSO4(H2O) [Mn(μ-H2O)(μ4,-κ1-SO4)4][31] Template:CNone kieserite see Mn, Fe, Co, Ni, Zn analogues
File:ICSD CollCode71346.png
Substructure of MSO4(H2O), illustrating presence of bridging water and bridging sulfate (M = Mg, Mn, Fe, Co, Ni, Zn).
MgSO4(H2O)4 [Mg(H2O)4(κ′,κ1-SO4)]2 Template:CNone sulfate is bridging ligand, 8-membered Mg2O4S2 rings[32]
MgSO4(H2O)6 [Mg(H2O)6] Template:CNone hexahydrate common motif[29]
MgSO4(H2O)7 [Mg(H2O)6] one epsomite common motif[29]
TiOSO4(H2O) [Ti(μ-O)2(H2O)(κ1-SO4)3] Template:CNone further hydration gives gels
VSO4(H2O)6 [V(H2O)6] Template:CNone Adopts the hexahydrite motif[33]
VSO4(H2O)7 [V(H2O)6] one hexaaquo[34]
VOSO4(H2O)5 [VO(H2O)4(κ1-SO4)4] one
Cr(SO4)(H2O)3 [Cr(H2O)3(κ1-SO4)] Template:CNone resembles Cu(SO4)(H2O)3[35]
Cr(SO4)(H2O)5 [Cr(H2O)4(κ1-SO4)2] one resembles Cu(SO4)(H2O)5[36]
Cr2(SO4)3(H2O)18 [Cr(H2O)6] six One of several chromium(III) sulfates
MnSO4(H2O) [Mn(μ-H2O)(μ4,-κ1-SO4)4][31] Template:CNone szmikite see Fe, Co, Ni, Zn analogues
MnSO4(H2O)4 [Mn(μ-SO4)2(H2O)4][37] Template:CNone Ilesitepentahydrate is called jôkokuite; the hexahydrate, the most rare, is called chvaleticeite with 8-membered ring Mn2(SO4)2 core
MnSO4(H2O)5 ? jôkokuite
MnSO4(H2O)6 ? Chvaleticeite
MnSO4(H2O)7 [Mn(H2O)6] one mallardite[30] see Mg analogue
FeSO4(H2O) [Fe(μ-H2O)(μ4-κ1-SO4)4][31] Template:CNone see Mn, Co, Ni, Zn analogues
FeSO4(H2O)7 [Fe(H2O)6] one melanterite[30] see Mg analogue
FeSO4(H2O)4 [Fe(H2O)4(κ′,κ1-SO4)]2 Template:CNone sulfate is bridging ligand, 8-membered Fe2O4S2 rings[32]
FeII(FeIII)2(SO4)4(H2O)14 [FeII(H2O)6]2+[FeIII(H2O)4(κ1-SO4)2]Template:Su Template:CNone sulfates are terminal ligands on Fe(III)[38]
CoSO4(H2O) [Co(μ-H2O)(μ4-κ1-SO4)4][31] Template:CNone see Mn, Fe, Ni, Zn analogues
CoSO4(H2O)6 [Co(H2O)6] Template:CNone moorhouseite see Mg analogue
CoSO4(H2O)7 [Co(H2O)6] one bieberite[30] see Fe, Mg analogues
NiSO4(H2O) [Ni(μ-H2O)(μ4-κ1-SO4)4][31] Template:CNone see Mn, Fe, Co, Zn analogues
NiSO4(H2O)6 [Ni(H2O)6] Template:CNone retgersite One of several nickel sulfate hydrates[39]
NiSO4(H2O)7 [Ni(H2O)6] morenosite[30]
(NH4)2[Pt2(SO4)4(H2O)2] [Pt2(SO4)4(H2O)2]2− Template:CNone Pt-Pt bonded Chinese lantern structure[40]
CuSO4(H2O)5 [Cu(H2O)4(κ1-SO4)2] one chalcantite sulfate is bridging ligand[41]
CuSO4(H2O)7 [Cu(H2O)6] one boothite[30]
ZnSO4(H2O) [Zn(μ-H2O)(μ4-κ1-SO4)4][31] Template:CNone see Mn, Fe, Co, Ni analogues
ZnSO4(H2O)4 [Zn(H2O)4(κ′,κ1-SO4)]2 Template:CNone sulfate is bridging ligand, 8-membered Zn2O4S2 rings[32][42]
ZnSO4(H2O)6 [Zn(H2O)6] Template:CNone see Mg analogue[43]
ZnSO4(H2O)7 [Zn(H2O)6] one goslarite[30] see Mg analogue
CdSO4(H2O) [Cd(μ-H2O)2(κ1-SO4)4] Template:CNone bridging water ligand[44]

Hydrates of metal nitrates

Transition metal nitrates form a variety of hydrates. The nitrate anion often binds to the metal, especially for those salts with fewer than six aquo ligands. Nitrates are uncommon in nature, so few minerals are represented here. Hydrated ferrous nitrate has not been characterized crystallographically.

Formula of
hydrated metal ion nitrate		 Coordination
sphere of the metal ion		 Equivalents of water of crystallization
that are not bound to M		 Remarks
Cr(NO3)3(H2O)9 [Cr(H2O)6]3+ three octahedral configuration[45] isostructural with Fe(NO3)3(H2O)9
Mn(NO3)2(H2O)4 cis-[Mn(H2O)4(κ1-ONO2)2] Template:CNone octahedral configuration
Mn(NO3)2(H2O) [Mn(H2O)(μ-ONO2)5] Template:CNone octahedral configuration
Mn(NO3)2(H2O)6 [Mn(H2O)6] Template:CNone octahedral configuration[46]
Fe(NO3)3(H2O)9 [Fe(H2O)6]3+ three octahedral configuration[47] isostructural with Cr(NO3)3(H2O)9
Fe(NO3)3)(H2O)4 [Fe(H2O)3(κ2-O2NO)2]+ one pentagonal bipyramid[48]
Fe(NO3)3(H2O)5 [Fe(H2O)5(κ1-ONO2)]2+ Template:CNone octahedral configuration[48]
Fe(NO3)3(H2O)6 [Fe(H2O)6]3+ Template:CNone octahedral configuration[48]
Co(NO3)2(H2O)2 [Co(H2O)2(κ1-ONO2)2] Template:CNone octahedral configuration
Co(NO3)2(H2O)4 [Co(H2O)4(κ1-ONO2)2 Template:CNone octahedral configuration
Co(NO3)2(H2O)6 [Co(H2O)6]2+ Template:CNone octahedral configuration.[49]
α-Ni(NO3)2(H2O)4 cis-[Ni(H2O)4(κ1-ONO2)2] Template:CNone octahedral configuration.[50]
β-Ni(NO3)2(H2O)4 trans-[Ni(H2O)4(κ1-ONO2)2] Template:CNone octahedral configuration.[51]
Pd(NO3)2(H2O)2 trans-[Pd(H2O)2(κ1-ONO2)2] Template:CNone square planar coordination geometry[52]
Cu(NO3)2(H2O) [Cu(H2O)(κ2-ONO2)2] Template:CNone octahedral configuration.
Cu(NO3)2(H2O)1.5 uncertain uncertain uncertain[53]
Cu(NO3)2(H2O)2.5 [Cu(H2O)2(κ1-ONO2)2] one square planar[54]
Cu(NO3)2(H2O)3 uncertain uncertain uncertain[55]
Cu(NO3)2(H2O)6 [Cu(H2O)6]2+ Template:CNone octahedral configuration[56]
Zn(NO3)2(H2O)4 cis-[Zn(H2O)4(κ1-ONO2)2] Template:CNone octahedral configuration.
Template:Chem2 [H2O–Hg–Hg–OH2]2+ linear[57]

Photos

See also

References

Template:Reflist

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