Reduction potential of a complex

I am given the reduction potential of $\ce{Fe^{3+}}$ to $\ce{Fe^{2+}}$ as well as the dissociation constants $\ce{K_D}$ for a $\ce{Fe^{3+}}$ complex and a $\ce{Fe^{2+}}$ complex (effectively, the ratio of concentrations of the $\ce{Fe^{3+}}$ complex and $\ce{Fe^{2+}}$ complex, if I divide one constant by the other).
I need to find the reduction potential of the $\ce{Fe^{3+}}$ complex.
Is it possible that I could use the reduction potential of $\ce{Fe^{3+}}$ to $\ce{Fe^{2+}}$ as an estimate for that of the $\ce{Fe^{3+}}$ complex, and then use the Nernst equation to plug in the concentrations of the two complexes? Or is there no guarantee that these two potentials would be close?