Compounds featuring unsupported metal–metal bonds between actinide elements remain highly sought after yet confined experimentally to inert gas matrix studies. Notwithstanding this paucity, actinide–actinide bonding has been the subject of extensive computational research. In this contribution, high level quantum chemical calculations at both the scalar and spin-orbit levels are used
to probe the Th–Th bonding in a range of zero valent systems of general formula LThThL. Several of these compounds have very short Th–Th bonds arising from a new type of Th–Th quadruple bond with a previously unreported electronic configuration featuring two unpaired electrons in 6d-based δ bonding orbitals. H3AsThThAsH3 is found to have the shortest Th–Th bond yet reported (2.590 Å).
The Th2 unit is a highly sensitive probe of ligand electron donor/acceptor ability; we can tune the Th–Th bond from quadruple to triple, double and single by judicious choice of the L group, up to 2.888 Å for singly-bonded ONThThNO.