Simple solution: Don't shade the middle of the coin, it's going to be animated anyway and it doesn't have much topography anyway ' n ';,
If the light source is in he same place as the eyes of the viewer, it's this simple - when the face of the coin is facing the viewer, it's going to be at its brightest, whenever it faces away, it gets a little darker. Get some more colors in there and get that fading effect down and it's going to do lots.
Remember that the same will apply for the sides of the coin, so that you make sure shading varies in the spin animation. Fun, I've been making spinning coin tokens for a personal project and thinking about the same kind of things, haha : D