The 5f-block elements, in particular the earlier ones, possess a particularly rich
comprising s, p, d and f atomic orbitals of competitive relevance.
Direct-Hamiltonian & self-consistent orbital effects of scalar relativity &
as well as the common core-shielding & centrifugal effects all play a role,
in addition to Coulomb correlation & angular momenta coupling.
The demands of reliable computational approaches are challenging.
Density functionals sometimes work, sometimes behave catastrophically
like in the first-row transition metal series.
Compounds of the first actinoid (An) octet of elements (Ac, Th, Pa, U, Np, Pu, Am, Cm) exhibit many astonishing bonding properties. Five examples are here discussed.
(1) Carbon in C(σππ&sigma)U(σπ)X may be more convincingly called quadruply bonded than in famous C2, because the rich valence shell of U allows C-2s,2p to form two σ-bonds in the same direction.
(2) Instead of a YC=AnX σπ-double-bond, the spin-uncoupled diradical YC•-•AnX may be more stable, owing to large two-electron Coulomb and one-electron spin-orbit interactions.
(3) Actinoid oxide molecules show exciting patterns of electronic-geometric structures, owing to various bonding patterns to oxygen in the form of O2-, O1-, O22-, O21-, O20, etc.
(4) Heavy noble gas atoms may change the electronic state of matrix encapsulated AnXn molecules because of the near-degeneracy of the multitude of different valence states.
(5) Remarkable self-organizing patterns are obtained for poly-actinoyls forming extended networks somewhat different from the poly-oxo-metallates of group-6 transition elements.
Some popular rules of atomic orbital occupation (p
must be applied with some caution.