. "A curable composition containing a) a material which is polymerizable by free radicals or cations, a mixture of said materials or a metal which is polymerizable by free radicals and cations, b) at least one iron compound of the formula I ##EQU3## in which a is 1 or 2 and q is 1, 2 or 3, L is a divalent to heptavalent metal or non-metal, Q is a halogen atom, m is an integer corresponding to the sum" . . .