Disclosed are metal oxide sols made by mixing at least one metal alkoxide, wherein the metal is a transition metal, a post-transition metal, an alkali metal, or alkaline earth metal; at least one organosilane; at least one boron oxide compound selected from the group consisting of boric acid, alkoxy boron compounds, hydrocarbyl boric acids, hydrocarbyl hydroxy boron alkoxides, and hydroxy boron alkoxides; and a liquid. Also disclosed are composites containing a polymer and the metal oxide sols.