When high concentrations of thiocyanate are present, the higher order complexes are predominant in the solution.  However, if the molarity of thiocyanate is very low, the only complex formed in any appreciable amount is the monothio-cyanatoiron (III) ion.
In TodayÕs Experiment
Fe 3+ + SCN-  =  FeSCN 2+ 
Keq =     [FeSCN 2+ ]__        [Fe 3+ ] [SCN- ]