"(...) more recent calculations (see Frenking and Fröhlich, Chem. Rev.
, (2000), 100
, 717) indicate that 3d is always lower in energy than 4s. The reason why 4s is populated before 3d is fully filled (in neutral atoms) is then traced to the fact that inter-electron repulsions are significantly less between electrons in the 4s orbital due to its greater size. In cations, however, the 3d orbitals are sufficiently stabilised relative to 4s that any advantage of reduced inter-electron repulsions in 4s is outweighted by the lower energy of 3d."