The way I see it, the -R (alkyl part) obtains a negative charge and the MgX obtains a net positive charge on bond dissociation. So a nucleophile and an electrophile both are obtained simultaneously. So why is Grignard reagent a nucleophile and not an ambiphile? Where am I wrong?
In fact a Grignard reagent reacting with, let us say, a ketone, is ambiphilic. However, the electrophilic component of the reaction, in which the positively charged magnesium combines with the carbonyl oxygen, does not figure into the organic product on workup. See this answer.