D electron count

The d electron count is a chemistry formalism used to describe the electron configuration of the valence electrons of a transition metal center in a coordination complex. The d electron count is an effective way to understand the geometry and reactivity of transition metal complexes. The formalism has been incorporated into the two major models used to describe coordination complexes; crystal field theory and ligand field theory, which is a more advanced version based on molecular orbital theory.

The electron configuration for transition metals predicted by the simple Aufbau principle and Madelung's rule has serious conflicts with experimental observations for transition metal centers under most ambient conditions. Under most conditions all of the valence electrons of a transition metal center are located in d orbitals while the standard model of electron configuration would predict some of them to be in the pertinent s orbital.

The valence of a transition metal center can be described by standard quantum numbers. The Aufbau principle and Madelung's rule would predict for period n that the ns orbitals fill prior to the (n − 1)d orbitals. For example, the 4s fills before the 3d in period 4. In general chemistry textbooks, a few exceptions are acknowledged with only one electron in the ns orbital in favor of completing a half or whole d shell. The usual explanation is that "half-filled or completely filled subshells are particularly stable arrangements of electrons". An example is chromium whose electron configuration is [Ar]4s¹3d⁵ with a half-filled d subshell, although Madelung's rule would predict [Ar]4s²3d⁴. Similarly copper is [Ar]4s¹3d¹⁰ with a full d subshell, and not [Ar]4s²3d⁹.

Matters are further complicated when metal centers are oxidized. Since the (n − 1)d shell is predicted to have higher energy than the ns shell, it might be expected that electrons would be removed from the (n − 1)d shell first. Experimentally it has been observed that not only are the ns electrons removed first, even for unionized complexes all of the valence electrons are located in the (n − 1)d orbitals.

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