Molybdenum and tungsten compounds, their manufacture and their use as catalysts in the stereospecific polymerization of unsaturated organic compounds

A process for polymerizing at least one monomer selected from the group consisting of straight-chain and cyclic ethylenic unsaturated organic compounds of 2 to 8 carbon atoms, comprising contacting, at a temperature of from - 20.degree. to 100.degree. C, said at least one monomer substantially in the liquid phase with a catalytic amount of at least one compound of the general formula (H.sub.3-n X.sub.n) C-CO.sub.2 M(CO).sub.m (R).sub.p (L).sub.q in which each X is fluorine, bromine or chlorine with the provision that at least one X is fluorine, n is an integer selected from 1, 2 and 3, M is a metal selected from the group consisting of molybdenum and tungsten, R is selected from the group consisting of a hydride ion, or a methyl, ethyl, allyl, methallyl, crotyl, phenyl or benzyl radical, L is a Lewis base selected from the group consisting of aliphatic mono-, di- and tri-ethers, heterocyclic mono- and diethers, aliphatic monoether-monoalcohols, and aliphatic mono- and diketones, m is an integer from 1 to 3 inclusive, p is 1 and q is an integer selected from the values 1 to 2, with the proviso that the values of m, p, and q are such that, in the valence shell of said metal M, the sum of the number of electrons contributed by said metal M and the number of electrons contributed by the ligands is 18.