Lead(II) chloride

Overview
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.

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 (Ksp) 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 Ksp = 1.8 x 10-10 and mercury (I) chloride (Hg2Cl2) with Ksp = 1.3 x 10-18.

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) → PbCl2(s) + 2 NaNO3(aq)
 * Pb(CH3COO)2(aq) + HCl(aq) → PbCl2(s) + 2 CH3COOH(aq)
 * basic PbCO3 + 2 HCl(aq) → PbCl2(s) + CO2(g) + H2O

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 PbCl2(s).
 * PbCl2(s) + Cl- →  [PbCl3]-(aq)

[PbCl3]-(aq) is isoelectronic with PCl3.
 * PbCl2(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

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:
 * PbCl2(l) + 3 NaNO2 → PbO + NaNO3 + 2NO + 2 NaCl

PbCl2 is also used to prepare ceramics by cation replacement reactions:
 * xPbCl2(l) + BaTiO3(s) → Ba1-xPbxTiO3 +  xBaCl2

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:
 * PbO2(s)|undefined + 4 HCl → PbCl2(s) + Cl2 + 2 H2O

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

PbCl2(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).


 * PbCl2 is used in organometallic synthesis to make metallocenes, known as plumbocenes. It is also used to make compounds of the formula R4Pb from Grignard reagents or organo-lithium compounds (see organometallic reactions above).
 * PbCl2 is used in production of infrared transmitting glass.
 * 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.
 * 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.
 * 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.
 * A basic chloride of lead, PbCl2·Pb(OH)2, is known as Patteson's white lead and is used as pigment in white paint.
 * 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.

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. PbCl2 is emitted from internal combustion engines that use ethylene chloride-tetraethyllead additives for antiknock purposes.

The mineral 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 volcanos: Vesuvius, Italy; Tarapaca, Chile; and Tolbachik, Russia.

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