Cyclic diaryliodonium salts are widely employed as linchpins for constructing functionalized biaryls and as precursors to arynes under basic conditions. However, their synthetically practical engagement with strongly nucleophilic organometallic reagents has remained largely unexplored. Here we disclose a reaction between these salts and aryl Grignard reagents that delivers iodine-capped teraryls with predominant meta-selectivity. In contrast to ligand coupling at iodine(III) or direct base-induced aryne formation, the present transformation is initiated by the formation of a cyclic triaryliodane, which serves as both an aryne precursor and a dynamic aryl reservoir within a self-propagating carbomagnesiation sequence. Spectroscopic analyses and control experiments support the involvement of the triaryliodane and reversible aryl exchange with aryl Grignard reagents, while trapping experiments intercept both aryne and terarylmagnesium intermediates. The reaction tolerates sterically and electronically diverse aryl Grignard reagents with consistent meta-selectivity, whereas strongly perturbing substituents on the iodonium salts can modulate the regioselectivity. The resulting iodo-teraryls retain a versatile C─I bond amenable to further π-extension or recyclization into cyclic iodonium architectures.