The reason this an exception is because it turns out that half filled d orbitals are more stable than we could even predict.

这个例外的原因是因为,半填充d轨道比,我们预期的填充方式更加稳定。

麻省理工公开课 - 化学原理课程节选

You wouldn't be expected to be able to guess that this would happen, because using any kind of simple theory, we would, in fact, predict that this would not be the case, but what we find experimentally is that it's more stable to have half filled d orbital than to have a 4 s 2, and a 3 d 4.

你们不用预期有能力,猜出这个会发生，因为使用任一种简单的理论，我们事实上可以预测不会是那种情况，但是我们在实验中发现的,是半填充d轨道比,4s23d4更加稳定。

麻省理工公开课 - 化学原理课程节选