The compound in which cation is isoelectronic with anion is

K2S formed by K+ and S2- ion. We know that atomic number of K is 19 and in K+ ion its electronic number would be 18 similarly atomic number of S is 16 and in form S2- ion its electronic number would be 18 so the K+ and S2 are isoelectronic with each other in K2S.