Lead Dichloride

Lead(II) chloride (PbCl₂) is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead(II) chloride is one of the most important lead-based reagents. It also occurs naturally in the form of the mineral cotunnite.

Structure and properties

In solid PbCl₂, each lead ion is coordinated by nine chloride ions in a tricapped triangular prism formation — six lie at the vertices of a triangular prism and three lie beyond the centers of each rectangular prism face. The 9 chloride ions are not equidistant from the central lead atom, 7 lie at 280–309 pm and 2 at 370 pm. PbCl₂ forms white orthorhombic needles.

File:Cotunnite-3D-balls.png, Ball-and-stick model of part of the crystal structure of cotunnite
File:Cotunnite-3D-ionic.png, Space-filling model
File:Cotunnite-Pb-coordination-geometry-3D-balls.png, Coordination geometry of Pb²⁺
File:Cotunnite-Cl-coordination-geometry-3D-balls.png, Coordination geometry of Cl⁻
File:Cotunnite-Pb-coordination-CM-3D-balls.png, Coordination polyhedron of Pb²⁺

In the gas phase, PbCl₂ molecules have a bent structure with the Cl–Pb–Cl angle being 98° and each Pb–-Cl bond distance being 2.44 Å. Such PbCl₂ is emitted from internal combustion engines that use ethylene chloride- tetraethyllead additives for antiknock purposes.

PbCl₂ is sparingly soluble in water, solubility product ''K''_sp = at 20 °C. It is one of only 5 commonly water-insoluble chlorides, the other 4 being thallium(I) chloride, silver chloride (AgCl) with ''K''_sp = , copper(I) chloride (CuCl) with ''K''_sp = and mercury(I) chloride (Hg₂Cl₂) with ''K''_sp = .

Occurrence

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. Cotunnite occurs near volcanoes: Vesuvius, Italy; Tarapacá, Chile; and Tolbachik, Russia.

Synthesis

Double displacement/Metathesis

Lead(II) chloride precipitates upon addition of aqueous chloride sources (HCl, NaCl, KCl) to lead(II) compounds
* Soluble lead compounds
** Lead(II) nitrate
**:Pb(NO₃)₂ + 2 NaCl_(aq) → PbCl_2(s) + NaNO_3(aq)
** Lead(II) acetate
**:Pb(CH₃COO)_2(aq) + HCl_(aq) → PbCl_2(s) + 2 CH₃COOH
* Insoluble lead compounds
** Lead(II) carbonate
**:PbCO₃ + 2 HCl_(aq) → PbCl_2(s) + CO_2(g) + H₂ODictionary of Inorganic and Organometallic Compounds. ''Lead(II) Chloride.''http://www.chemnetbase.com]
** Lead dioxide
**:PbO_2(s) + 4 HCl → PbCl_2(s) + Cl₂ + 2 H₂O
** Lead(II) oxide
**:PbO_(s) + 2 HCl → PbCl_2(s) + H₂O

Direct Reduction

PbCl₂ can be formed by the reduction of copper(II) chloride by lead metal:
*: Pb + CuCl₂ → PbCl₂ + Cu

Direct chlorination

PbCl₂ also forms by the action of chlorine gas on lead metal:
: Pb + Cl₂ → PbCl₂

Reactions

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⁻ → bCl₃sup>−_(aq)
:PbCl_2(s) + 2 Cl⁻ → bCl₄sup>2−_(aq)

PbCl₂ reacts with molten NaNO₂ to give PbO:
:PbCl_2(l) + 3 NaNO₂ → PbO + NaNO₃ + 2 NO + 2 NaCl

PbCl₂ is used in synthesis of lead(IV) chloride (PbCl₄): Cl₂ is bubbled through a saturated solution of PbCl₂ in aqueous NH₄Cl forming H₄sub>2 bCl₆ The latter is reacted with cold concentrated sulfuric acid (H₂SO₄) forming PbCl₄ as an oil.

Lead(II) chloride is the main precursor for organometallic derivatives of lead, such as plumbocenes. 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

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

PbCl₂ can be used to produce PbO₂ by treating it with sodium hypochlorite (NaClO), forming a reddish-brown precipitate of PbO₂.

Uses

*Molten PbCl₂ is used in the synthesis of lead titanate and barium lead titanate ceramics by cation replacement reactions:
*:''x'' PbCl_2(l) + BaTiO_3(s) → Ba_1−''x''Pb_''x''TiO₃ + ''x'' BaCl₂
*PbCl₂ is used in production of infrared transmitting glass, and ornamental glass called aurene glass. Aurene glass has an iridescent surface formed by spraying with PbCl₂ and reheating under controlled conditions. Stannous chloride (SnCl₂) is used for the same purpose.
*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 Pattinson's white lead and is used as pigment in white paint. Lead paint is now banned as a health hazard in many countries by the White Lead (Painting) Convention, 1921.
*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 arsenic and tellurium. This is followed by the Parkes 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. BiCl₃ would form last.Kirk-Othmer. ''Encyclopedia of Chemical Technology''. (ed 4). p. 241

Toxicity

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

References

External links

IARC Monograph: "Lead and Lead Compounds"
* ttp://www.atsdr.cdc.gov/HEC/CSEM/lead/ Case Studies in Environmental Medicine – Lead Toxicitybr>ToxFAQs: Lead

{{Chlorides

Lead(II) compounds
Chlorides
Metal halides
IARC Group 2A carcinogens