A novel bidentate catalytic ligand of general formula (I) is described. R represents a hydrocarbyl aromatic structure having at least one aromatic ring to which Q1 and Q2 are each linked, via the respective linking group, if present, on available adjacent atoms of the at least one aromatic ring. The groups X3 and X4 represent radicals joined via tertiary carbon atoms to the respective atom Q1 and the groups X1 and X2 represent radicals joined via primary, or substituted aromatic ring carbon atom(s) to the respective atom Q2. A and B represent an optional lower alkylene linking group. Q1 and Q2 each represent phosphorus, arsenic or antimony. A process for the carbonylation of ethylenically unsaturated compounds comprising reacting the compound with carbon monoxide in the presence of a source of hydroxyl groups, optionally, a source of anions and catalyst system obtainable by combining a metal of Group 8, 9 or 10 or a compound thereof and the bidentate ligand of general formula (I) is also described.