Why benzenediazonium bromide and iodide are not important? When we think of aryldiazonium compounds, the first compound that comes to mind is benzenediazonium chloride. Sure thing, the compound is versatile and has been used to make a vast array of organic compounds and thus serves its purpose as a useful reagent. But what about other halides like benzendiazonium fluoride, bromide and iodide? Chemists and scientist do not take them noteworthy as there is little to no literature evidence of their synthesis. Are the compounds so unstable that chemists neglect them? Nobody has attempted to make the compounds in the way benzenediazonium chloride is made. Why is it so? Is it because of unfavourable reaction conditions? Can't we use hydrogen bromide or hydrogen iodide to make benzenediazonium bromide/iodide respectively? Like the following reaction:

$$\ce{C6H5NH2 + HNO2 + HX -&gt; C6H5N2+X- + 2H2O  ~~~~~~~~ X = F, Br, I}$$

What are the reasons for the non-existance of/not preparing other benzenediazonium halides? Is it just because of their instability or feasibility or any other reasons? Or is there is any (other) way to (theoretically) make these compounds (if they exist)?
There is indeed very little to be found in the literature. 

A recent Russian paper (Ref.1) mentions benzenediazonium bromide as a reactant. This thesis from 1975 (Ref.2) contains a preparation of benzenediazonium bromide and an attempted preparation of benzenediazonium iodide; it notes that the iodide is unstable as does this reference here.  The discussion section of this Organic Syntheses' preparation of fluorobenzene (Ref.3) mentions an alternative preparation by formation of benzenediazonium fluoride and its decomposition referencing work from the 1900s. 

My overall thought is that they are probably less stable than the well-known chlorides and undergo reaction in lower yields so they are of little interest.



References:


Yu. V. Ostapiuk, V. S. Matiichuk, N. I. Pidlypnyi, N. D. Obushak, "Convenient synthesis of $\alpha$-bromo ketones by the Meerwein reaction," Russian Journal of Organic Chemistry 2012, 48, 519–522 (https://doi.org/10.1134/S1070428012040094).
Geoffrey D. Picard, "Sulfonium Salts, Aliphatic and Aromatic Diazonium Salts, Formation and Isolation," Honors Theses 1975, 2119 (https://digitalworks.union.edu/theses/2119)(PDF).
D. T. Flood (Checked by W. W. Hartman and J. R. Byers), "Fluorobenzene," Org. Synth. 1933, 13, 46 (DOI: 10.15227/orgsyn.013.0046) and Organic Syntheses, Coll. Vol. 1943, 2, 295 (PDF).