MgBr2: It's formed of Magnesium Mg, and dibromide Br2. And since Magnesium is a metal and Dibromide a no-metal, it's more appropriate while naming the substance to remove the "di" from "dibromide". Following the rules for naming compounds, the prefixes "mono", "di", etc... should only be used between 2 nonmetals.
So, the name for MgBr2 is Magnesium Bromide.
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