A novel bidentate ligand of general formula (I) is described together with a process for the carbonylation of ethylenically unsaturated compounds. The group X1 may be defined as a univalent hydrocarbyl radical of up to 30 atoms containing at least one nitrogen atom having a pKb in dilute aqueous solution at 18° C. of between 4 and 14 wherein the said at least one nitrogen atom is separated from the Q2 atom by between 1 and 3 carbon atoms. The group X2 is defined as X1, X3 or X4 or represents a univalent radical of up to 30 atoms having at least one primary, secondary or aromatic ring carbon atom wherein each said univalent radical is joined via said at least one primary, secondary or aromatic ring carbon atom(s) respectively to the respective atom Q2. Q1 and Q2 each independently represent phosphorus, arsenic or antimony.