In octahedral field, the baricenter remains conserved and there is lowering of energy of $\mathrm{t_{2g}}$ orbitals.

What is a baricenter and why must it be conserved? Why is there lowering of energy of $\mathrm{t_{2g}}$ as ligands approach?

Also when ligands are far there an increase in energy but not when they are nearer. What is going on?

I know energy must be conserved but they are many ways to release/utilise energy like sound, heat, light, kinetic energy.

  • $\begingroup$ @Melanie I disagree with this edit, thus I will roll it back in a second. Until OP states otherwise, I am willing to assume that baricenter was a misspelling in OP’s book. Searching for it on Wikipedia gives no result while using a y does. $\endgroup$
    – Jan
    Nov 20 '16 at 19:40

The baricentre should probably be spelt barycentre. It is something analogous to a centre of mass. Simple models such as some flavours of the crystal field theory will assume a destabilisation only of the metal orbitals. However, for an energetic destabilisation to occur, something else must be stabilised — this is the conservation of energy. Thus, the logic says that the raise in energy of the $\mathrm{e_g}$ orbitals must be offset by a lowering of the $\mathrm{t_{2g}}$ orbitals.

When constructing a proper MO scheme of an octahedral coordination complex, this argument is revealed as bogus. As can be seen in this answer, the $\mathrm{e_g}$ orbitals are actually $\mathrm{e_g^*}$ orbitals, i.e. antibonding with respect to the $\ce{M\bond{<-}L}$ bonds which is the reason why they are raised. Disregarding π interactions, the $\mathrm{t_{2g}}$ orbitals are nonbonding and thus their energy is not affected.

  • 3
    $\begingroup$ B-b-but the Wikipedia article spells it barycenter. $\endgroup$
    – orthocresol
    Nov 20 '16 at 18:58

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