(a) _MgBr + O=C=O

In the first reaction, Phenyl magnesium bromide is reacting with carbon dioxide. In carbon dioxide, a partial positive charge is developed on carbon while a partial negative charge will be developed on oxygen due to electronegativity difference. Electronegativity of oxygen is more than carbon so it will attract shared pair of electrons towards itself and therefore partial negative charge develops on it.

           In the case of Phenyl magnesium bromide, magnesium is electropositive in nature so its electronegativity is less than carbon, therefore a partial negative charge will develop on carbon which further going to attack partial positive carbon of carbon dioxide.