A system comprising a) Ni(0) b) from 4 to 10 mol per mol of Ni(0) in a) of a compound (I) of the formula P(X1R1)(X2R2)(X3R3)   (I) where X1, X2, X3 are each, independently of one another, oxygen or a single bond, R1, R2, R3 are, independently of one another, identical or different organic radicals and c) from 1 to 4 mol per mol of Ni(0) in a) of a compound (II) of the formula where X11, X12, X13 X21, X22, X23 are each, independently of one another, oxygen or a single bond, R11, R12 are identical or different, individual or bridged organic radicals, R21, R22 are identical or different, individual or bridged organic radicals and Y is a bridging group is suitable as catalyst for preparing mixtures of monoolefinic C5 mononitriles having nonconjugated C═C and C═N bonds by hydrocyanation of a 1,3-butadiene-containing hydrocarbon mixture and for preparing a dinitrile by hydrocyanation of a mixture of monoolefinic C5 mononitriles having nonconjugated C═C and C═N bonds.