Which metal complex ion is expected to be subject to a Jahn-Teller distortion? \text { a. }\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+} \text { b. }\left[\mathrm{Cr}(\mathrm{en})_{3}\right]^{3+} \text { C. }\left[\operatorname{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+} \text { d. }\left[\mathrm{Cr}(\mathrm{CN})_{6}\right]^{3-}

\text { e. }\left[\mathrm{Cr}(\mathrm{bpy})_{3}\right]^{3+}