Zinc iodide explained

Zinc iodide is the inorganic compound with the formula ZnI₂. It exists both in anhydrous form and as a dihydrate. Both are white and readily absorb water from the atmosphere. It has no major application.

Preparation

It can be prepared by the direct reaction of zinc and iodine in water^[1] or in refluxing ether.^[2] or by treating zinc with iodine in aqueous solution:^[3]

Zn + I₂ → ZnI₂

Structure as solid, gas, and in solution

The structure of solid ZnI₂ is unusual relative to the dichloride. While zinc centers are tetrahedrally coordinated, as in ZnCl₂, groups of four of these tetrahedra share three vertices to form “super-tetrahedra” of composition, which are linked by their vertices to form a three-dimensional structure.^[4] These "super-tetrahedra" are similar to the P₄O₁₀ structure.^{[4] [5]}

Molecular ZnI₂ is linear as predicted by VSEPR theory with a Zn-I bond length of 238 pm.^[4]

In aqueous solution the following have been detected: Zn(H₂O)₆²⁺, [ZnI(H₂O)₅]⁺, tetrahedral ZnI₂(H₂O)₂, ZnI₃(H₂O)⁻, and ZnI₄²⁻.^[6]

Applications

• Zinc iodide is often used as an x-ray opaque penetrant in industrial radiography to improve the contrast between the damage and intact composite.^{[7] [8]}
• United States patent 4,109,065 ^[9] describes a rechargeable aqueous zinc-halogen cell that includes an aqueous electrolytic solution containing a zinc salt selected from the class consisting of zinc bromide, zinc iodide, and mixtures thereof, in both positive and negative electrode compartments.
• In combination with osmium tetroxide, ZnI₂ is used as a stain in electron microscopy.^[10]
• As a Lewis acid, zinc iodide catalyzes for the conversion of methanol to triptane and hexamethylbenzene.^[11]

Notes and References

1. Book: F. Wagenknecht. R. Juza. Zinc iodide. Handbook of Preparative Inorganic Chemistry, 2nd Ed. . G. Brauer. Academic Press. 1963. NY, NY. 1. 1073.
2. Book: Eagleson, M. . 1994 . Concise Encyclopedia Chemistry . registration . Walter de Gruyter . 3-11-011451-8 .
3. DeMeo, S. . Synthesis and Decomposition of Zinc Iodide: Model Reactions for Investigating Chemical Change in the Introductory Laboratory . Journal of Chemical Education . 1995 . 72 . 9 . 836 . 10.1021/ed072p836 . 1995JChEd..72..836D .
4. Book: Wells, A. F. . 1984 . Structural Inorganic Chemistry . 5th . Oxford Science Publications . 0-19-855370-6 .
5. 10.1107/S0567740878010390. Structure Cristalline de l'Iodure de Zinc ZnI₂. 1978. Fourcroy. P. H.. Carré. D.. Rivet. J.. Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry. 34. 11. 3160–3162. 1978AcCrB..34.3160F .
6. Wakita, H. . Johansson, G. . Sandström, M. . Goggin, P. L. . Ohtaki, H. . Structure determination of zinc iodide complexes formed in aqueous solution . Journal of Solution Chemistry . 1991 . 20 . 7 . 643–668 . 10.1007/BF00650714 . 97496242 .
7. Book: Baker, A. . Dutton, S. . Kelly, D. . Composite Materials for Aircraft Structures . 2nd . 2004 . AIAA (American Institute of Aeronautics & Astronautics) . 1-56347-540-5 .
8. Book: Plastics Failure Guide . Ezrin, M. . Hanser Gardner Publications . 1996 . 1-56990-184-8 .
9. US . patent . 4109065 . Rechargeable aqueous zinc-halogen cell . 1978-08-22 . Will, F. G.; Secor, F. W. . General Electric .
10. Book: Hayat, M. A. . Principles and Techniques of Electron Microscopy: Biological Applications . 2000 . 4th . Cambridge University Press . 0-521-63287-0 .
11. Bercaw. John E.. Diaconescu. Paula L.. Paula Diaconescu. Grubbs. Robert H.. Kay. Richard D.. Kitching. Sarah. Labinger. Jay A.. Li. Xingwei. Mehrkhodavandi. Parisa. Morris. George E.. 2006-11-01. On the Mechanism of the Conversion of Methanol to 2,2,3-Trimethylbutane (Triptane) over Zinc Iodide. The Journal of Organic Chemistry. 71. 23. 8907–8917. 10.1021/jo0617823. 17081022. 0022-3263.