The story of the lanthanides begins in 1787 when a young Swedish artillery officer, Lieutenant Carl Axel Arrhenius, who was a keen amateur geologist, was exploring a quarry at a small town called Ytterby, near Stockholm. The lanthanide metals generally show hexagonal close packed or cubic close packed structures at room temperature, and body centred cubic structures at high temperatures. The filling of the 5f orbitals in the actinide series is not quite as regular as the filling of the lanthanide 4f orbitals. For actinide complexes, spin-orbit coupling is considerably greater, and the greater radial extension of the 5f orbitals, especially for the early actinides, results in larger crystal field splittings and there is often considerable overlap between levels of different states. The small energy difference between the 5f, 6d and 7s electrons in the actinides means that, especially for the early actinides, more than one oxidation state is available.