Abstract: Use of at least one heteropolyacid H.sub.n A.sub.a D.sub.c O.sub.y.xH.sub.2 O where A is phosphorus, boron, silicon, germanium, tin, arsenic, antimony, copper, nickel, cobalt, iron, cerium, thorium, chromium and mixtures thereof; D is molybdenum, tungsten, vanadium and combinations thereof; a is 0.1-10; c is 6-18; n is the number of acidic hydrogens in the heteropolyacid and is a number above 1; y is the number of oxygens in the heteropolyacid and is a number of 10-70; and x is the number of moles of water of crystal-lisation and is a number of 0-40, as catalyst in the preparation of esters of unsaturated carboxylic acids by direct esterification of acids with alcohols or by transesterification, these reactions being carried out in the liquid phase.

Abstract: The invention relates to platinum hydrides, a process for their preparation and their use as components of catalytic systems. The platinum hydrides have a bridged bimetallic structure represented by the general formula: ##STR1## wherein: M is a metal of valence n, at least equal to 2,X is a halogen atom, andP P is a schematic representation of a ligand of the general formula: ##STR2## in which: R.sub.1, R.sub.2, R.sub.5, and R.sub.6, identical or different, are chosen from among aliphatic or cycloaliphatic hydrocarbon radicals with 1 to 8 carbon atoms and aromatic hydrocarbon radicals with 6 to 10 carbon atoms,R.sub.3 and R.sub.4, identical or different, are chosen from among a hydrogen atom and aliphatic hydrocarbon radicals with 1 to 8 carbon atoms, possibly functionalized and/or together forming a ring, andm is an integer higher than or equal to 4.

Abstract: The catalyst system is characterized in that it comprises:at least one platinum complex of formula LPtX.sub.2 in which L is an organic compound containing at least two phosphorus atoms capable of coordinating platinum and X is a halogen atom, andat least one combination formed between iron and an alkene carbonate, the alkene group preferably containing from 2 to 6 carbon atoms.The process for its preparation is characterized in that at least one combination formed between iron adn the alkene carbonate is reacted with at least one platinum complex of formula LPtX.sub.2 in at least one solvent for the said complex, at a temperature of between 10.degree. C. and the boiling point of the said solvent. The combination of iron and of an alkene carbonate may be formed by electrochemical reduction.

Abstract: The catalytic system comprises:at least one platinum complex of formula LPtX.sub.2 in which L is an organic compound containing at least two phosphorus atoms capable of coordinating platinum and X is a halogen atom, andat least organic tin compound comprising at least one Sn--O bond and at least one C--O bond.Application to the production of aldehydes by the hydroformlation of an ethylenically unsaturated organic compound, by reacting the said organic compound, at a temperature of between 10.degree. C. and 300.degree. C. and under a pressure of between 1 and 350 bars, with a mixture of carbon monoxide and hydrogen in the presence of an effective amount of the said catalytic system.