The projected reliance on nuclear energy worldwide in the coming decades demands a better understanding of the fundamental properties of the understudied late actinide elements from americium through californium. Many in situ high-pressure studies utilizing diamond-anvil cells were performed on metals, alloys, and binary compounds of these highly radioactive elements in the late 1900s and provided valuable insights into their electronic structures. At ambient conditions the $5f$ electrons of the late actinides are radially beneath the Rn core electrons, shielding them and making them unable to participate in bonding interactions. Under pressure, however, metals and alloys of Am–Cf undergo a Mott transition where the initially localized $5f$ electrons transition to an itinerant state and can contribute to bonding interactions. The area of high-pressure studies on Am–Cf has been reinvigorated over the past five years, expanding the scope to coordination complexes of these elements and employing spectroscopic techniques to probe $5f$ electronic structure. Complexes with ligands of various donor atoms have been synthesized and subjected to these high-pressure spectroscopic investigations to probe the influence of the hard-soft nature of the donor atom on the involvement of the $5f$ orbitals in bonding. The effects of donor atom as well as metal center identity on the degree of the $5f$ contribution to bonding are discussed.

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• Density of states
• Electronic structure of atoms & molecules
• High-pressure studies
• Kondo effect
• Peierls transition
• Photoabsorption
• Photoluminescence
• X-ray diffraction