The compounds according to the invention have the formula I ##STR1## in which R.sup.1, R.sup.2 and R.sup.3 are monovalent organic radicals or chlorine and Z is a tetravalent organic radical and A is a divalent organic grouping. These products are to be used as adhesion promoters, especially between inorganic solids and organic resins. 4 processes of preparation are given.