21. A metal ion in a weak-field octahedral complex has two more unpaired electrons than the same ion does in a strong-field octahedral complex. Which of the following could the metal ion be?
(A) Cr³⁺
(B) Mn²⁺
(C) Co²⁺
(D) Ti²⁺
(E) Cu²⁺

考試觀念本題測驗晶場理論（Cryst...