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orthocresol
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What prevents an octahedral complex from always assuming the quadratic planar limiting case, due to the jahn-tellerJahn–Teller distortion?

When a given high-spin complex is jahn-tellerJahn–Teller distorted, this does not change the relative position of the total energy of the complex (assuming d10$\mathrm{d^{10}}$ configuration).

Indeed, the distortion only causes the degenerate triplet to split into a doublet and a singlet, and the degenerate doublet splits into two singlets.

In the case of a d4$\mathrm{d^4}$ high-spin complex, however, the total energy of the system is lowered, since the one ex-e_g$\mathrm{e_g}$ electron is now in a singlet, which has a lower energy relative to the doublet.

What stops this process, which is lowering the total energy more and more? Why isn't every possible complex for a jahn-tellerJahn–Teller distortion automatically distorted in such a way, that we end up with a quadratic planar boundary case, as it can be observed e.g. in [AuCl_4]^-$\ce{[AuCl4]-}$?

What prevents an octahedral complex from always assuming the quadratic planar limiting case, due to the jahn-teller distortion?

When a given high-spin complex is jahn-teller distorted, this does not change the relative position of the total energy of the complex (assuming d10 configuration).

Indeed, the distortion only causes the degenerate triplet to split into a doublet and a singlet, and the degenerate doublet splits into two singlets.

In the case of a d4 high-spin complex, however, the total energy of the system is lowered, since the one ex-e_g electron is now in a singlet, which has a lower energy relative to the doublet.

What stops this process, which is lowering the total energy more and more? Why isn't every possible complex for a jahn-teller distortion automatically distorted in such a way, that we end up with a quadratic planar boundary case, as it can be observed e.g. in [AuCl_4]^-?

What prevents an octahedral complex from always assuming the quadratic planar limiting case, due to the Jahn–Teller distortion?

When a given high-spin complex is Jahn–Teller distorted, this does not change the relative position of the total energy of the complex (assuming $\mathrm{d^{10}}$ configuration).

Indeed, the distortion only causes the degenerate triplet to split into a doublet and a singlet, and the degenerate doublet splits into two singlets.

In the case of a $\mathrm{d^4}$ high-spin complex, however, the total energy of the system is lowered, since the one ex-$\mathrm{e_g}$ electron is now in a singlet, which has a lower energy relative to the doublet.

What stops this process, which is lowering the total energy more and more? Why isn't every possible complex for a Jahn–Teller distortion automatically distorted in such a way, that we end up with a quadratic planar boundary case, as it can be observed e.g. in $\ce{[AuCl4]-}$?

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Rivinius
Rivinius
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What prevents an octahedral complex from always assuming the quadratic planar limiting case, due to the jahn-teller distortion?

When a given high-spin complex is jahn-teller distorted, this does not change the relative position of the total energy of the complex (assuming d10 configuration).

Indeed, the distortion only causes the degenerate triplet to split into a doublet and a singlet, and the degenerate doublet splits into two singlets.

In the case of a d4 high-spin complex, however, the total energy of the system is lowered, since the one ex-e_g electron is now in a singlet, which has a lower energy relative to the doublet.

What stops this process, which is lowering the total energy more and more? Why isn't every possible complex for a jahn-teller distortion automatically distorted in such a way, that we end up with a quadratic planar boundary case, as it can be observed e.g. in [AuCl_4]^-?