A hydrocyanation catalyst system for conversion of butadiene to pentenenitrile and for isomerization of 2-methyl-3-butenenitrile to a pentenenitrile, comprising a soluble complex of nickel and a bidentate ligand of formula, wherein X 12, X 13, X 14, X 22, X 23, and X 24 each independently is oxygen; R 12, R 13, R 22, and R 23, each independently is (C1-C10)alkyl, (C3-C10)cycloalkyl, (C3-C10)cycloalkyl(C1-C10)alkyl, (C6-C20)aryl, or (C6-C20)aryl(C1-C10)alkyl, wherein for any (C6-C20)aryl or (C6-C20)aryl(C1-C10)alkyl of R 12, R 13, R 22, or R 23, each aryl ring thereof is unsubstituted or is substituted with 1-4 substituents independently selected from the group consisting of (C1-C10)alkyl, (C3-C10)cycloalkyl, (C1-C10)alkoxy, (C3-C10)cycloalkoxy, (C3-C10)cycloalkyl(C1-C10)alkyl, (C3-C10)cycloalkoxy(C1-C10)alkyl, (C3-C10)cycloalkyl(C1-C10)alkoxy, (C3-C10)cycloalkoxy(C1-C10)alkoxy, (C6-C20)aryl, and (C6-C20)aryl(C1-C10)alkyl; or, optionally, one or more of pairs R 12 and R 13 or R 22 and R 23 are mutually directly bonded, such that the R 12 X 12 and R 13 X 13 groups, or the R 22 X 22 and R 23 X 23 groups, or both, together with the respective phosphorus atom to which each pair of groups is bonded, forms a respective ring; Y is an (C6-C20)arylene group, wherein each ring thereof is independently unsubstituted or is substituted with 1-4 (C1-C10)alkyl, (C3-C10)cycloalkyl, (C1-C10)alkoxy, (C3-C10)cycloalkoxy, (C3-C10)cycloalkyl(C1-C10)alkyl, (C3-C10)cycloalkoxy(C1-C10)alkyl, (C3-C10)cycloalkyl(C1-C10)alkoxy, (C3-C10)cycloalkoxy(C1-C10)alkoxy, (C6-C20)aryl, (C6-C20)aryl(C1-C10)alkyl, fluorine, chlorine, bromine, or (C1-C10)haloalkyl; the complex being dissolved in a solvent milieu consisting essentially of cis -2-pentenenitrile, trans -2-pentenenitrile, or a mixture thereof; wherein the catalyst system is free of a Lewis acid promoter. wherein X 12, X 13, X 14, X 22, X 23, and X 24 each independently is oxygen; R 12, R 13, R 22, and R 23, each independently is (C1-C10)alkyl, (C3-C10)cycloalkyl, (C3-C10)cycloalkyl(C1-C10)alkyl, (C6-C20)aryl, or (C6-C20)aryl(C1-C10)alkyl, wherein for any (C6-C20)aryl or (C6-C20)aryl(C1-C10)alkyl of R 12, R 13, R 22, or R 23, each aryl ring thereof is unsubstituted or is substituted with 1-4 substituents independently selected from the group consisting of (C1-C10)alkyl, (C3-C10)cycloalkyl, (C1-C10)alkoxy, (C3-C10)cycloalkoxy, (C3-C10)cycloalkyl(C1-C10)alkyl, (C3-C10)cycloalkoxy(C1-C10)alkyl, (C3-C10)cycloalkyl(C1-C10)alkoxy, (C3-C10)cycloalkoxy(C1-C10)alkoxy, (C6-C20)aryl, and (C6-C20)aryl(C1-C10)alkyl; or, optionally, one or more of pairs R 12 and R 13 or R 22 and R 23 are mutually directly bonded, such that the R 12 X 12 and R 13 X 13 groups, or the R 22 X 22 and R 23 X 23 groups, or both, together with the respective phosphorus atom to which each pair of groups is bonded, forms a respective ring; Y is an (C6-C20)arylene group, wherein each ring thereof is independently unsubstituted or is substituted with 1-4 (C1-C10)alkyl, (C3-C10)cycloalkyl, (C1-C10)alkoxy, (C3-C10)cycloalkoxy, (C3-C10)cycloalkyl(C1-C10)alkyl, (C3-C10)cycloalkoxy(C1-C10)alkyl, (C3-C10)cycloalkyl(C1-C10)alkoxy, (C3-C10)cycloalkoxy(C1-C10)alkoxy, (C6-C20)aryl, (C6-C20)aryl(C1-C10)alkyl, fluorine, chlorine, bromine, or (C1-C10)haloalkyl; the complex being dissolved in a solvent milieu consisting essentially of cis -2-pentenenitrile, trans -2-pentenenitrile, or a mixture thereof; wherein the catalyst system is free of a Lewis acid promoter.