In bond formation in a coordination complex, the original d electrons of the transition metal go into nonhybridized d orbitals. Where do the electron pairs donated by the ligand go? Appropriately hybridized orbitals of higher energy (four sp2, four dsp2, six d2sp3, or six sp3d2) are then filled by the electron pairs donated by the ligand.

Don't sweat this stuff too much. Now you have a bit of familiarity with the molecular orbital picture of coordination complexes. This is the kind of conceptual underpinning that makes understanding concrete.
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