Catalysts, which are free of tin, are described which are useful in the reaction between an isocyanate and an alcohol, which is a key step for preparing polyurethane polymers. The catalyst is a metal complex or salt of formula (1): [Zn(L1)l1(L2)l2(Y)x]  (1) in which: l1≧1 and l2=0 or 1; with l1+l2=2, x=1 or 2, the symbol L1 represents a ligand which is a β-dicarbonylato anion or the enolate anion of a β-dicarbonyl compound or an acetylacetato anion derived from a β-ketoester, when l1=2, the symbols L1 are identical or different, the symbol L2 represents an anionic ligand which is different from L1, and the symbol Y represents a neutral ligand; where the metal complex or salt C of formula (1) is not zinc diacetylacetonate.