However, when the borate anion of the complex of the formula (1) has a structure in which R5 is an optionally substituted alkyl group and R6, R7 and R8 are optionally substituted aryls, it is considered that R5 on the borate anion, i.e., the optionally substituted alkyl group, undergoes preferential and effective cleavage from the borate anjou to improve the sensitivity.