A complex, and a process for producing the same, represented by any of the following formulae (I), (I′), (II), (II′), (III), (III′), (IV), and (IV′): where: R1s are independently an alkyl group or an aryl group; R2s are independently a hydrogen atom, an alkyl group, or an aryl group; R3s are independently an alkyl group or an aryl group, provided that two R3s are optionally bonded with each other to form a ring; R4s are independently a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group, a nitro group, or a cyano group; and M is TiY2, where Y is Cl or alkoxide or the complex forms a di-μ-oxo dimer where Y is a μ-oxo ligand.