CrO₃ is d⁰, therefore the colour is not due to d→d transitions. Since Cr is in a high oxidation state (+6) and attached to electron-rich oxide ligands, the colour is caused by ligand-to-metal charge transfer (LMCT). The colour is dark because LMCT is not restricted by any selection rules.

PCy₃ is a stronger σ-donor and π-acceptor than PPh₃, which results in the PCy₃ complex to be square planar and the PPh₃ complex to be tetrahedral. The d→d transtion is higher in energy for a square planar complex compared to a tetrahedral because the d_x2_-y2 orbital used in the square planar geometry lies along the bond axis, resulting in the anti-bonding b_1g to be high in energy compared to the antibonding orbitals in a tetrahedral complex that result from poor orbital overlap (and are therefore lower) in energy. If red is observed (square planar), then green light is absorbed and if green is observed (tetrahedral) then red light is absorbed. Green light is higher energy than red light, corresponding to the energy differences between Δ_sp and Δ_t.

Cl⁻ and H₂O are weak field ligands, which will result in the d⁵ Mn complex to be high-spin. All d orbitals will be occupied by 1 unpaired electron, therefore the transitions are forbidden because ΔS = 0 (there can be no spin flips). With six of the same ligands and all d orbitals equally occupied, the molecular geometry will be a perfect octahedral, so transiitons will only occur during vibrational distortions. Both of these factors will result in pale colour.

CO is a strong field ligand, which will result in the d⁶ Mn complex to be low-spin. The non-bonding t_2g orbitals will each be occupied by 2 electrons, so electrons can jump to e_g* without flipping their spins. Since there are 5 CO ligands and 1 Br⁻ ligand, the complex will be permanently distorted from O_h symmetry, and so there will be a permanent mixing of p and d orbitals. Both of these factors will result in an intense colour.

Mn⁷⁺ is d⁰, therefore the colour is not due to d→d transitions. Since Mn is in a high oxidation state (+7) and attached to electron-rich water ligands, the colour is caused by ligand-to-metal charge transfer (LMCT). The colour is dark because LMCT is not restricted by any selection rules.

Mn²⁺ in H₂O is d⁵ high-spin. All d orbitals will be occupied by 1 unpaired electron and the geometry will be a perfect octahedral, so transiitons will only occur during vibrational distortions, resulting in a pale colour.

d⁷ high-spin octahedral

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CoCl₂ in water results in an octahedral [Co(H₂O)₆]²⁺ complex, which is d⁷ high-spin. The d→d transitions are forbidden and so the colour is pale. Green light (complementary to red) is absorbed during vibrational distortions.

d⁷ tetrahedral

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Heating evaporates the water, leaving the d⁷ tetrahedral [CoCl₄]²⁻ complex. Tetrahedral complexes have permanent p and d orbital mixing, so e→t₂ transitions are allowed (only d⁵ and d¹⁰ tetrahedral complexes should be pale due to the spin selection rule). Since Δ_t < Δ_o, the energy of the light absorbed is less. Orange light (complementary to blue) is absorbed, which is lower energy than green light.

bipy is a strong field ligand, which will result in the d⁶ Ru complex to be low-spin with perfect octahedral geometry, so the complex should be colourless. Since Ru is in a low oxidation state (+2) and attached to π-acceptor ligands, the colour is caused by metal-to-ligand charge transfer (MLCT). The colour is intense because MLCT is not restricted by any selection rules.

The metal complex is most likely a hydrate (e.g., CuSO₄•5H₂O) and water is released during heating. The resultant dehydrated metal salt does not have water ligands splitting the d orbitals (into non-bonding and antibonding sets), so there are no electronic transitons within the visible spectrum.

Ti³⁺ is d¹, with unequal occupation of the non-bonding t_2g, resulting in a permanently distorted octahedral geometry. This gives the complex permanent p and d orbital mixing so transitions are allowed resulting in the observed colour.

Ti⁴⁺ is d⁰, therefore there are no d→d transitions possible and no colour.

WCl₆ is d⁰, therefore the colour is not due to d→d transitions. Since W is in a high oxidation state (+6) and attached to electron-rich chloride ligands, the colour is caused by ligand-to-metal charge transfer (LMCT). The colour is dark because LMCT is not restricted by any selection rules.

Since NH₃ is a stronger σ donor than H₂O, Δ_o (and energy) will increase. When red is observed, green light is absorbed. So changing the ligand to NH₃ will result in either blue or violet light being absorbed and an orange or yellow metal complex.

Ir⁴⁺ is d⁵ high-spin (since weak field ligands are present). This results in a perfect octahedral geometry, so d→d transitions are forbidden. Since Ir is in a high oxidation state (+4) and attached to electron-rich ligands, the colour is caused by ligand-to-metal charge transfer (LMCT). The colour is intense because LMCT is not restricted by any selection rules.

Fe³⁺ is d⁵ and since the complex is dark red it must be low-spin (high-spin would be colourless). d⁵ low-spin has a distorted octahedral geometry with permanent p and d orbital mixing.

Co³⁺ is d⁶ and since the complex is pale pink it must be low-spin (high-spin would be coloured). d⁶ low-spin has a perfect octahedral geometry so d→d transitions are forbidden.