According to the Aufbau Principle, electrons prefer to occupy the lowest available energy level. (Order of lowest to highest energy levels: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, etc.)

In the Classroom Companion, it says that the electron configuration for:

Co = [Ar] 4s^2 3d^7
Co2+ = [Ar] 3d^7

Electrons are removed from the highest energy level first. I am confused as to why two electrons were removed from the s subshell instead of the d subshell. According to the Aufbau principle, 4s is lower in energy than 3d. Also if two electrons were removed from the d-subshell, it would create the configuration: 4s^2 3d^5, where the d-subshell is half full (wouldn't we want that).

Thank you!