The invention provides a method of forming a phosphonate diester compound from a ligand hydrolysis product (LHP) of a phosphite ligand used in a nickel-phosphite hydrocyanation catalyst, such as for conversion of 3-pentenenitrile to adiponitrile, which serves to eliminate acidic LHP compound for a hydrocyanation reaction milieu where the acidic LHP can catalyze further catalyst ligand destruction. The invention further provides phosphonate disester compounds prepared by alkylation of diarylphosphite LHP in the presence of a nickel-phosphite catalyst comprising a bidentate ligand, and a continuous hydrocyanation process for production of adiponitrile wherein catalyst ligand breakdown is inhibited through inactivation of ligand hydrolysis products towards further breakdown. A method of stabilizing a hydrocyanation catalyst is provided.