<p>Grignard reagents—cornerstones of synthetic chemistry—are hindered by enduring limitations in accessing complex architectures, which poses a persistent synthetic bottleneck. Meanwhile, quaternary carbon (stereo)centres, ubiquitous in bioactive molecules and natural products, remain formidable synthetic targets despite decades of research. Here we introduce a nickel-catalysed carbomagnesiation strategy that simultaneously overcomes these challenges through a rare contra-electronegativity transmetallation (Ni to Mg). This approach enables the efficient and modular synthesis of β-quaternary Grignard reagents via carbomagnesiation of 1,1-disubstituted alkenes and 1,3-dienes, employing aryl triflate and PhMgBr as carbon and magnesium sources, respectively. The resulting organomagnesium reagents undergo one-pot reactions with diverse electrophiles, delivering stereochemically complex quaternary centres with high precision. Mechanistically, bulky N-heterocyclic carbene (NHC)-based catalysts divert classical cross-coupling pathways, enforcing a counterintuitive Ni-to-Mg transmetallation that defies conventional electronegativity trends while achieving exceptional regio- and enantiocontrol. This contra-electronegativity transmetallation demonstrates substantial potential to advance carbometallation reactions and open new avenues for cross-coupling chemistry.</p><p></p>