The color of the transition metal complex [Fe H2O 6]Cl3 is pale green. The color of a transition metal complex is determined by the electronic transitions between the energy levels of the d-orbitals, which are split into different energy levels due to the crystal field splitting effect.In the case of [Fe H2O 6], the central metal ion is Fe, which has a 3d5 electronic configuration. When the six water ligands approach the Fe ion, they create a crystal field that splits the d-orbitals into two sets of energy levels: the lower energy t2g orbitals dxy, dyz, and dxz and the higher energy eg orbitals dz and dx-y . The energy gap between these two sets of orbitals is called the crystal field splitting energy .The color of the complex is due to the absorption of light that promotes an electron from the t2g orbitals to the eg orbitals. The absorbed light's wavelength corresponds to the energy difference between these orbitals . The remaining transmitted or reflected light gives the complex its observed color.When the complex is heated, the crystal field splitting energy can change due to the change in the ligand's position or bond strength. This change in can cause a shift in the absorbed light's wavelength, leading to a change in the observed color of the complex.When a ligand is replaced, the crystal field splitting energy can also change depending on the new ligand's field strength. For example, if water ligands are replaced by stronger field ligands like CN, the will increase, and the absorbed light's wavelength will shift to a higher energy lower wavelength . This will result in a change in the observed color of the complex.In summary, the color of the transition metal complex [Fe H2O 6]Cl3 is pale green, and it can change when the complex is heated or when a ligand is replaced due to the changes in the crystal field splitting energy and the corresponding electronic transitions between the split d-orbitals.