= Solution
The <metal> is Cr(II), $d^4$. A <chloride ligand> produces a relatively weak <ligand field>, so the octahedral center is a <high-spin complex> with $t_{2g}^3e_g^1$, $S=2$. The singly occupied $e_g$ pair is orbitally degenerate: $d_{z^2}$ and $d_{x^2-y^2}$ cannot retain identical occupancies once the distortion selects one. This is a strong setting for the <Jahn-Teller effect>.
On tetragonal elongation, the two axial Cr–Cl distances increase. Repulsion is reduced most for $d_{z^2}$, so it falls relative to $d_{x^2-y^2}$; $d_{xz},d_{yz}$ also fall relative to $d_{xy}$. The single $e_g$ <electron> occupies the lower $d_{z^2}$-derived level. The electronic stabilization offsets an elastic cost until a distorted equilibrium geometry is reached. \b[The high-<spin> $d^4$ occupancy drives an elongated, rather than regular, octahedron.] The three lower-set <electrons> remain one per orbital in this simple diagram; it is primarily the unequal $e_g$ occupancy that produces the strong distortion.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-2-fields.png]
{title=Qualitative d-orbital correlations for axial elongation and axial-ligand removal}
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