Wohler synthesis: magnesium cyanate + ammonium chloride?
The reason Wohler used silver cyanate is because silver chloride precipitates out, leaving ammonium cyanate, which (in a separate step) was thermally converted to urea. If you used magnesium cyanate, there would be no precipitate since magnesium chloride is very soluble in water. However, if you evaporated the water and heated the mixture, you probably would end up making some urea. But it would not be pure.