No, baking soda is not a leaven. It is often used in baking as a leavening agent, but it is not the same thing as yeast or other types of leavens.  Baking soda is a chemical leavening agent that causes baked goods to rise by creating carbon dioxide gas.  Yeast and other leavens are biological leavening agents that cause baked goods to rise by producing alcohol and carbon dioxide gas.