Similar to Fe (VIII)O 4, Ir (IX)NO 3 has been predicted to be mildly metastable with respect to (η 2-NO)IrO 2 and (η 1-NO)IrO 2 but elimination of NO is predicted to be protected by considerable barriers thus leaving some hope that Ir (IX)NO 3 might be observed experimentally. Moving to the right in the periodic table, in group 9, only the observed + cation and the recently predicted neutral molecule IrNO 3 (nitride trioxide) feature genuine nonavalent transition metal center. The well-known group 8 complexes are represented by stable yet largely covalent Os (VIII)O 4 and the less stable and highly reactive Ru (VIII)O 4 while related Fe (VIII)O 4 has never been observed but it has been predicted to be metastable. The theoretical prediction and experimental observation of Ir (XI) oxidation state have raised the questions of kinetic and thermochemical stabilities of transition metal systems with uncommonly high oxidation numbers of a metal (coinciding with the group number). In other words, substantial covalence is often taken as an indication of strength of the chemical bonding. A similar trend may be noticed for many other homo- and related heteronuclear systems, with some interesting exceptions. If one now performs “electronegativity perturbation” by substituting one C atom by less electronegative B and another C atom by more electronegative N, the bonding energy is still high (389 kJ/mol), but it markedly decreases as compared with C 2. Take C 2 molecule, with the dissociation energy of 607 kJ/mol. Obviously, in this simplified picture, the homonuclear diatomics best fulfil the criteria which favour strong bonding, albeit heteronuclear systems may still benefit to a certain degree from an increased Coulombic stabilization, i.e. The better the match of energy and spatial distribution of orbitals (overlap), the stronger the chemical bond. spatial decay of electronic density) of atomic orbitals involved in chemical bonding. The classical theory of 2-electron/2-center chemical bonding based on a molecular orbital picture, as taught to the chemistry students, puts accent on proper match of energy and “size” (i.e. 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. However, the local minima containing Rh(IX) are protected by sufficient energy barriers on the decomposition pathway, and they could in principle be prepared. We found that both species studied are metastable with respect to elimination of O 2 or NO. Possible rearrangement into isomers featuring lower formal oxidation numbers has been explored. In this report, the stability and decomposition pathways of two species containing rhodium at a potentially formal +IX oxidation state, + and RhNO 3, have been investigated theoretically within the framework of the relativistic two-component Hamiltonian calculations. Rhodium, a 4d transition metal and a lighter analogue of iridium, is known to exhibit its highest VIth oxidation state in RhF 6 molecule.
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