With regards to this question, I have been unable to find a convincing answer on the Internet as it seems that not many people consider exceptions to the 18-electron rule as the 18-electron rule is commonly viewed as a shorthand for determination of structural stability instead of an actual tool.
I have therefore asked my chemistry teachers, but they have given me varying responses which may even be contradictory.
The basis of my question is this: the 18-electron rule has widely been used as the rule-of-thumb to determine stability of transition metal complexes. Hexaaquachromium (III) only has 15 electrons (2x6 from all aqua ligands and 3 from the 3d3 configuration of Cr3+ ion), so from this line of argument, shouldn't Cr3+ be a decently strong oxidizing agent in aqueous medium as it attempts to fill up its t2g nonbonding orbitals with d-electrons?
One of my teachers have attempted to explain the stability of Cr3+ complexes by referencing the Frost diagram and arguing that Cr3+ ions, at least in the acidic medium, is the most redox-stable out of all possible oxidation states due to its highest free energy of formation from Cr metal, such that it is unlikely for the Cr3+ ion to change its oxidation state in order to fulfil the 18-electron rule. He also further explained that another way for Cr3+ to fill up its t2g bonding orbitals is for more ligands to bind to it, which distorts the octahedral shape and thus destabilizing the complex instead.
However, I do not find the above explanation convincing as it does not explain why Cr3+ ions in water do not spontaneously form complexes with Cr=O bonds without a change in oxidation state in order to fulfil the 18-electron rule. Another of my chemistry teacher has even mentioned that Cr3+ octahedral complex should exhibit redox instability due to its inability to fulfil the 18-electron rule unless bound to a radical ligand, which seemingly contradicts the empirical observations.
My friend has proposed the suggestion that, given the t2g nonbonding orbitals of the ML6 complex contains 3 electrons, it cannot accommodate any more ligand group orbitals as donation of ligand group orbitals will come in pairs of electrons with opposite spin which means that
a) For each ligand, one of the electrons donated must change its spin in order to fill up the t2g nonbonding orbitals without violating Pauli's exclusion principle
b) For the case of a high-spin complex, to fill up the eg antibonding orbitals but a change in spin is also required without violating Hund's rule of spin multiplicity
Therefore, it is pretty much impossible for Cr3+ aqueous ions to fulfil the 18-electron rule
However, I still do not find the ligand field approach convincing as it seems to be conceptually false: it does not make sense that electrons from ligands fill up the t2g nonbonding orbitals of the chromium center as they are the 3d orbitals which are supposed to be left untouched.
On this note, may I ask if any of the aforementioned explanations are correct in explaining the relative stability hexaaquachromium (III) despite its violation of the 18-electron rule? If not, may I ask if there is an accurate explanation, or any papers/reviews published discussing this class of exceptions?
