Lead(II) chloride

Lead(II) chloride has the formula PbCl₂. It is also known as lead chloride, lead dichloride, and plumbous chloride. PbCl₂ is one of the most important lead-based reagents. PbCl₂ also occurs naturally in the form of the mineral cotunnite.

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Solubility

The solubility of PbCl₂ in water is low (9.9 g/L at 20 °C) and for practical purposes it is considered insoluble. Its solubility-product constant (K_sp) at 25 °C (298 K) is 1.6 x 10^-5. It is one of only three commonly insoluble chlorides, the other two being silver chloride (AgCl) with K_sp = 1.8 x 10^-10 and mercury (I) chloride (Hg₂Cl₂) with K_sp = 1.3 x 10^-18.^[1]

Reactions

Precipitation from aqueous solution

Lead(II) chloride precipitates from solution upon addition of chloride sources (HCl, NaCl, KCl) to aqueous solutions of lead(II) compounds such as Pb(NO₃)₂.

Pb(NO₃)_2(aq) + 2 NaCl_(aq) → PbCl_2(s) + 2 NaNO_3(aq)

Pb(CH₃COO)_2(aq) + HCl_(aq) → PbCl_2(s) + 2 CH₃COOH_(aq)

basic PbCO₃ + 2 HCl_(aq) → PbCl_2(s) + CO_2(g) + H₂O^[2]

Soluble chloro complexes

Addition of chloride ions to a suspension of PbCl₂ gives rise to soluble complex ions. In these reactions the additional chloride (or other ligands) break up the chloride bridges that comprise the polymeric framework of solid PbCl_2(s).

PbCl_2(s) + Cl^- → [PbCl₃]^-_(aq)

[PbCl₃]^-_(aq) is isoelectronic with PCl₃.

PbCl_2(s) + 2 Cl^- → [PbCl₄]^2-_(aq)

Organometallic derivatives

Lead(II) chloride is the main precursor for organometallic derivatives of lead. The usual alkylating agents are employed, including grignard reagents and organolithium compounds:

2 PbCl₂ + 4 RLi → R₄Pb + 4 LiCl + Pb

2 PbCl₂ + 4 RMgBr → R₄Pb + Pb + 4 MgBrCl

3 PbCl₂ + 6 RMgBr → R₃Pb-PbR₃ + Pb + 6 MgBrCl^[3]

Notice how these reactions produce derivatives that are more similar to organosilicon compounds, i.e. that Pb(II) tends to disproportionate upon alkylation.

Conversion to oxides

PbCl₂ reacts with molten NaNO₂ to give PbO:

PbCl_2(l) + 3 NaNO₂ → PbO + NaNO₃ + 2NO + 2 NaCl

PbCl₂ is also used to prepare ceramics by cation replacement reactions:

xPbCl_2(l) + BaTiO_3(s) → Ba_1-xPb_xTiO₃ + xBaCl₂^[4]

Synthesis

PbCl₂ can be synthesized by several methods as discussed above, that is, by precipitation as a solid by reaction of an aqueous Pb(II) salts with a soluble chloride salts. An interesting reaction involves treatment of Pb(IV) oxide with HCl:

PbO_2(s) + 4 HCl → PbCl_2(s) + Cl₂ + 2 H₂O

Notice that lead(IV) chloride is not formed as it is unstable with respect to loss of Cl₂.

PbCl_2(s) also forms by the action of chlorine gas on Pb metal:

Pb + Cl₂ → PbCl₂

Uses of lead(II) chloride

• Molten PbCl₂ is used in the synthesis of lead titanate (PbTiO₃) and barium lead titanate (see reaction above).^[5]

• PbCl₂ is used in organometallic synthesis to make metallocenes, known as plumbocenes.^[6] It is also used to make compounds of the formula R₄Pb from Grignard reagents or organo-lithium compounds (see organometallic reactions above).^[7]
• PbCl₂ is used in production of infrared transmitting glass.^[8]
• PbCl₂ is used in production of ornamental glass called aurene glass. This stained glass has an iridescent surface formed by spraying with PbCl₂ and reheating under controlled conditions. Stannous chloride (SnCl₂) is used for the same purpose.^[9]
• PbCl₂ is used in synthesis of lead(IV) chloride (PbCl₄): Cl₂ is bubbled through a saturated solution of PbCl₂ in aqueous NH₄ forming [NH₄]₂[PbCl₆]. The latter is reacted with cold concentrated sulfuric acid (H₂SO₄) forming PbCl₄ as an oil.^[10]
• Pb is used in HCl service even though the PbCl₂ formed is slightly soluble in HCl. Addition of 6-25% of antimony (Sb) increases corrosion resistance.^[11]
• A basic chloride of lead, PbCl₂·Pb(OH)₂, is known as Patteson's white lead and is used as pigment in white paint.^[12]
• PbCl₂ is an intermediate in refining bismuth (Bi) ore. The ore containing Bi, Pb, and Zn is first treated with molten caustic soda to remove traces of acidic elements such as arsenic and tellurium. This is followed by the Parkes desilverization process to remove any silver and gold present. The ore now contains Bi, Pb, and Zn. It is treated with Cl₂ gas at 500 °C. ZnCl₂ forms first and is removed. Then PbCl₂ forms and is removed leaving pure Bi. BiCl₂ would form last.^[13]

Structure

  In solid PbCl₂, each lead is coordinated by 9 chloride ions - 6 lie at the apices of a trigonal prism and 3 lie beyond the centers of each prism face. PbCl₂ forms white orthorhombic needles.

Vaporized PbCl₂ molecules have a bent structure with the Cl-Pb-Cl angle being 98° and each Pb-Cl bond distance being 2.44Å.^[14] PbCl₂ is emitted from internal combustion engines that use ethylene chloride-tetraethyllead additives for antiknock purposes.

Cotunnite

PbCl₂ occurs naturally in the form of the mineral cotunnite. It is colorless, white, yellow, or green with a density of 5.3-5.8 g/cm³. The hardness on the Mohs scale is 1.5-2. The crystal structure is orthorhombic dipyramidal and the point group is 2/m 2/m 2/m. Each Pb has a coordination number of 9. The composition is 74.50% Pb and 25.50% Cl. Cotunnite occurs near volcanoes: Vesuvius, Italy; Tarapaca, Chile; and Tolbachik, Russia.^[15]

Toxicity

Like other lead containing compounds, exposure to PbCl₂ may cause lead poisoning.

References

1. ^ Brown, Lemay, Burnsten. "Chemistry The Central Science". Solubility-Product Constants for Compounds at 25^oC. (ed 6, 1994). p 1017
2. ^ Dictionary of Inorganic and Organometallic Compounds. Lead(II) Chloride. [1]
3. ^ Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 524
4. ^ Aboujalil, A. et al., "Molten salt synthesis of the lead titanate PbTiO3, investigation of the reactivity of various titanium and lead salts with molten alkali-metal nitrites", J. Mater. Chem., 1998, 8, 1601-1606
5. ^ Aboujalil, A. et al., "Molten salt synthesis of the lead titanate PbTiO3, investigation of the reactivity of various titanium and lead salts with molten alkali-metal nitrites", J. Mater. Chem., 1998, 8, 1601-1606
6. ^ Lowack, R.H. et al., "Decasubstituted decaphenylmetallocenes", J. Organomet. Chem., 1994, 476, 25-32
7. ^ Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 524
8. ^ Dictionary of Inorganic and Organometallic Compounds. Lead(II) Chloride. [2]
9. ^ Stained Glass Terms and Definitions. aurene glass. [3]
10. ^ Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 365
11. ^ Kirk-Othmer. "Encyclopedia of Chemical Technology". (ed 4). p 913
12. ^ Perry & Phillips. "Handbook of Inorganic Compounds". (1995). p 213
13. ^ Kirk-Othmer. "Encyclopedia of Chemical Technology". (ed 4). p 241
14. ^ Hargittai, I. et al.,"Two independent gas electron diffraction investigations of the structure of plumbous chloride", J. Mol. Struct., 1977, 42, 147
15. ^ Cotunnite [4]