During link reaction pyruvic acid is converted into acetyl CoA with the help of pyruvate dehydrogenase complex. For this conversion
pyruvate dehydrogenase requires magnesium, CoA, ${\mathrm{NAD}}^{+}$etc.

$\mathrm{Pyruvic} \mathrm{acid} + \mathrm{CoA} + {\mathrm{NAD}}^{+} \underset{\mathrm{Pyruvate} \mathrm{dehydrogenase}}{\overset{{\mathrm{Mg}}^{2+}}{\to }} \mathrm{Acetyl} \mathrm{CoA} + {\mathrm{CO}}_2 + \mathrm{NADH} + {\mathrm{H}}^{+}$