oup or a halogen atom;R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5 denote monovalent inorganic groups chosen from hydrogen, the hydroxyl group and the halogens such as chlorine and fluorine; or organic groups consisting of aliphatic radicals, of arylated aliphaticradicals and of aromatic radicals, including heterocyclic aromatic radicals, it being possible for all these radicals to comprise line