Abstract: A process is disclosed for preparing carboxylic acid anhydrides by reacting certain esters or ethers with carbon monoxide in the presence of a nickel catalyst, a source of iodide and/or bromide and conventional promoters, which process is improved by the use as co-promoters of a compound selected from compounds of the formula ##STR1## in which X represents phosphorus, arsenic or antimony and Y represents oxygen, sulphur or selenium, and a and b are O or 1 and R.sup.5 represents hydrogen or an optionally substituted hydrocarbon group and R.sup.6 and R.sup.7 represent an optionally substituted hydrocarbon group; compounds wherein a and b are O and R.sup.6 and R.sup.7 form a heterocyclic group with X and R.sup.5 represents hydrogen or an optionally substituted hydrocarbon group; and complexes of compounds of formula I with any of a hydrocarbon iodide or bromide, an acyl iodide or bromide, or hydrogen iodide or hydrogen bromide.

Abstract: Compounds having the formula: ##STR1## wherein X.sub.1, X.sub.2, R, R.sub.1, R.sub.2 and R.sub.3 are as set forth hereinafter, their preparation and uses as ganglionic blocking agents are disclosed.