2 added 1 character in body edited Jul 7 '15 at 18:35 bon 12.7k1010 gold badges4343 silver badges8080 bronze badges inIn water both $$\ce{Cr^{3+}}$$ and $$\ce{Fe^{3+}}$$ are in octahedral configurationconfigurations. This means, that $$d$$-orbitals become unequal in their energy,energy; specifically 3 of them are lower and 2 are higher. This consideration is known as 'crystallthe premise of 'crystal field theory'.   Depending on the nearest neighborhood, the splitting may be strong enough to force electron pairing or noit may not. For water it usually isn't. This means, that 'first half-filled shell' here is 3 electrons - $$\ce{V^{2+}}$$ $$\ce{Cr^{3+}}$$ or $$\ce{Mn^{4+}}$$. The second half-filled shell in low field (water ligand) is $$\ce{Mn^{2+}}$$ or $$\ce{Fe^{3+}}$$ (5 $$d$$-orbitals), both surprisingly stable. In strong field (say, $$\ce{CN^-}$$ ligand), it is 6 electrons (3 double occupied lower orbitals), like in $$\ce{[Co(NH3)_{6}]^{3+}}$$ and $$\ce{[Fe(CN)_{6}]^{4-}}$$. The next "subshell" is, 8 electrons, (6 on 3 lower orbitals and 2 on higher), like in $$\ce{Ni^{2+}}$$. Depending on the strength of the ligands, an unusual square planar coordination may become preferable, with two higher orbitals also splitting. It is typical for $$\ce{Ni}$$ subgroup in +2 oxidation state and $$\ce{Cu}$$ subgroup in +3 oxidation state. TL;DR : invest some time into reading about crystal field theory. in water both $$\ce{Cr^{3+}}$$ $$\ce{Fe^{3+}}$$ are in octahedral configuration. This means, that $$d$$-orbitals become unequal in their energy, specifically 3 of them are lower and 2 are higher. This consideration is known as 'crystall field theory'.   Depending on the nearest neighborhood, the splitting may be strong enough to force electron pairing or no. For water it usually isn't. This means, that 'first half-filled shell' here is 3 electrons - $$\ce{V^{2+}}$$ $$\ce{Cr^{3+}}$$ or $$\ce{Mn^{4+}}$$. The second half-filled shell in low field (water ligand) is $$\ce{Mn^{2+}}$$ or $$\ce{Fe^{3+}}$$ (5 $$d$$-orbitals), both surprisingly stable. In strong field (say, $$\ce{CN^-}$$ ligand), it is 6 electrons (3 double occupied lower orbitals), like in $$\ce{[Co(NH3)_{6}]^{3+}}$$ and $$\ce{[Fe(CN)_{6}]^{4-}}$$. The next "subshell" is, 8 electrons, (6 on 3 lower orbitals and 2 on higher), like in $$\ce{Ni^{2+}}$$. Depending on the strength of the ligands, an unusual square planar coordination may become preferable, with two higher orbitals also splitting. It is typical for $$\ce{Ni}$$ subgroup in +2 oxidation state and $$\ce{Cu}$$ subgroup in +3 oxidation state. TL;DR : invest some time into reading about crystal field theory. In water both $$\ce{Cr^{3+}}$$ and $$\ce{Fe^{3+}}$$ are in octahedral configurations. This means that $$d$$-orbitals become unequal in their energy; specifically 3 of them are lower and 2 are higher. This is the premise of 'crystal field theory'. Depending on the nearest neighborhood, the splitting may be strong enough to force electron pairing or it may not. For water it usually isn't. This means that 'first half-filled shell' here is 3 electrons - $$\ce{V^{2+}}$$ $$\ce{Cr^{3+}}$$ or $$\ce{Mn^{4+}}$$. The second half-filled shell in low field (water ligand) is $$\ce{Mn^{2+}}$$ or $$\ce{Fe^{3+}}$$ (5 $$d$$-orbitals), both surprisingly stable. In strong field (say, $$\ce{CN^-}$$ ligand), it is 6 electrons (3 double occupied lower orbitals), like in $$\ce{[Co(NH3)_{6}]^{3+}}$$ and $$\ce{[Fe(CN)_{6}]^{4-}}$$. The next "subshell" is, 8 electrons, (6 on 3 lower orbitals and 2 on higher), like in $$\ce{Ni^{2+}}$$. Depending on the strength of the ligands, an unusual square planar coordination may become preferable, with two higher orbitals also splitting. It is typical for $$\ce{Ni}$$ subgroup in +2 oxidation state and $$\ce{Cu}$$ subgroup in +3 oxidation state. TL;DR : invest some time into reading about crystal field theory. 1 answered Jul 7 '15 at 18:32 permeakra 18.8k11 gold badge4141 silver badges8787 bronze badges in water both $$\ce{Cr^{3+}}$$ $$\ce{Fe^{3+}}$$ are in octahedral configuration. This means, that $$d$$-orbitals become unequal in their energy, specifically 3 of them are lower and 2 are higher. This consideration is known as 'crystall field theory'. Depending on the nearest neighborhood, the splitting may be strong enough to force electron pairing or no. For water it usually isn't. This means, that 'first half-filled shell' here is 3 electrons - $$\ce{V^{2+}}$$ $$\ce{Cr^{3+}}$$ or $$\ce{Mn^{4+}}$$. The second half-filled shell in low field (water ligand) is $$\ce{Mn^{2+}}$$ or $$\ce{Fe^{3+}}$$ (5 $$d$$-orbitals), both surprisingly stable. In strong field (say, $$\ce{CN^-}$$ ligand), it is 6 electrons (3 double occupied lower orbitals), like in $$\ce{[Co(NH3)_{6}]^{3+}}$$ and $$\ce{[Fe(CN)_{6}]^{4-}}$$. The next "subshell" is, 8 electrons, (6 on 3 lower orbitals and 2 on higher), like in $$\ce{Ni^{2+}}$$. Depending on the strength of the ligands, an unusual square planar coordination may become preferable, with two higher orbitals also splitting. It is typical for $$\ce{Ni}$$ subgroup in +2 oxidation state and $$\ce{Cu}$$ subgroup in +3 oxidation state. TL;DR : invest some time into reading about crystal field theory.