The high covalence of metal-ligand bonds as stability limiting factor: the case of Rh(IX)O and Rh(IX)NO.

Rhodium, a 4d transition metal and a lighter analogue of iridium, is known to exhibit its highest VIth oxidation state in RhF molecule. In this report, the stability and decomposition pathways of two species containing rhodium at a potentially formal +IX oxidation state, [RhO] and RhNO, have been investigated theoretically within the framework of the relativistic two-component Hamiltonian calculations. Possible rearrangement into isomers featuring lower formal oxidation numbers has been explored. We found that both species studied are metastable with respect to elimination of O or NO. However, the local minima containing Rh(IX) are protected by sufficient energy barriers on the decomposition pathway, and they could in principle be prepared. The analysis of a broader set of compounds containing group 8 and 9 metals in high formal oxidation states that correspond to the group number showed that, in contrast to a standard trend, the limits of formally attainable oxidation state correlate with high level of covalent bonding character in the complexes studied.