To provide a transition metal ion complex to become a catalyst capable of efficiently and selectively producing 1-hexene by a trimerization reaction of ethylene.

The transition metal ion complex is represented, for example, by formula (1-1) wherein M is a group 4 transition metal atom of Periodic Table of the Elements; A is a counter anion; R¹ to R¹⁰ are H or the like; and X¹ and X² are halogen or the like.