Chemistry informtion about Diiodoacetylene (CAS NO.624-74-8) is:
IUPAC Name: 1,2-Diiodoethyne
Synonyms: Diiodoacetylene ; Diiodoethyne
MF: C₂I₂
MW: 277.830340 g/mol 
Density: 3.367 g/cm³
Flash Point: 94.1 °C
Boiling Point: 190.4 °C at 760 mmHg
Vapour Pressure: 0.75 mmHg at 25°C 
Enthalpy of Vaporization: 40.92 kJ/mol
Following is the molecular structure of Diiodoacetylene (CAS NO.624-74-8) is:

An explosive sensitive to impact, crushing, or heating to 84°C. When heated to decomposition it emits toxic fumes of I⁻. See also IODIDES and ACETYLENE COMPOUNDS and ALKYNES.

DOT Classification:  Forbidden

Preperation of Diiodoacetylene (CAS NO.624-74-8) is:
This preperation is a slightly modified version of that of Dehn (ref. 1). 3g of potassium iodide was dissolved into 40ml of distilled water in a 100ml measuring cylinder. A steady stream of acetylene produced by calcium carbide in water was bubbled through the potassium iodide solution. A 12.5% solution of sodium hypochlorite was slowly dripped into the bubbling potassium iodide solution where by the solution turned reddish amber, then turning pale yellow. The slow addition of hypochlorite was continued until a floculent white precipitate of diiodoacetylene fills the measuring cylinder and the hypochlorite addition no longer turns the solution yellow (figure 1).
The precipitate was filtered, flushed with cold water and dried in a cardboard box since diiodoacetylene is light sensitive. Its also volatile and will be lost through sublimation if left uncovered.
The slow addition of the hypochlorite is used to produce the unstable hypoiodite, which reacts with the acetylene to produce diiodoacetylene. The equations for the reactions are as follows.
CaC₂ + 2 H₂O --> Ca(OH)₂ + C₂H₂
KI + NaOCl --> KCl + NaOI
2 NaOI + C₂H₂ --> 2 NaOH + C₂I₂
The yield of diiodoacetylene melting at 81°C from this procedure is about 88%. Losses are due to the instability of the hypoiodite which decomposes through the following equations:
3 NaOI --> 2 NaI + NaIO₃
NaOI + 3 NaOCl + NaOH + H₂O --> 3NaCl + H₃Na₂IO₆