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Colour in coordination compounds arises from d-d transitions: an electron absorbs visible light of energy Delta and transitions from t2g to eg (octahedral). The observed colour is the complement of the absorbed colour. [Ti(H2O)6]3+ (d1) absorbs green-yellow, appears purple.
Factors affecting colour: (1) Ligand field strength — changes Delta, hence absorbed wavelength. (2) Metal oxidation state — higher charge increases Delta. (3) Geometry — octahedral vs tetrahedral gives different splitting patterns. Colourless complexes: d0 (no electron for transition) and d10 (no vacancy to transition into). Exception: charge transfer transitions can give colour even in d0/d10 (e.g., KMnO4 is d0 but intensely purple via LMCT).
Magnetism is determined by unpaired electrons. Spin-only formula: mu = sqrt(n(n+2)) BM. Standard values: 1 unpaired = 1.73 BM, 2 = 2.83, 3 = 3.87, 4 = 4.90, 5 = 5.92 BM. Paramagnetic compounds have unpaired electrons and are attracted into a magnetic field. Diamagnetic compounds have all electrons paired and are weakly repelled.
Magnetic measurements distinguish spin states: [Co(NH3)6]3+ has mu = 0 (diamagnetic, low spin d6) while [CoF6]3- has mu = 4.90 BM (4 unpaired, high spin d6). This also helps determine geometry for d8: diamagnetic = square planar, paramagnetic = tetrahedral.