Recent Advances in Multinuclear Metal Nitrosyl Complexes

The coordination chemistry of metal nitrosyls has expanded rapidly in the past decades due to major advances of nitric oxide and its metal compounds in biology. This review article highlights advances made in the area of multinuclear metal nitrosyl complexes, including Roussin's salts and their ester derivatives from 2003 to present. The review article focuses on isolated multinuclear metal nitrosyl complexes and is organized into different sections by the number of metal centers and bridging ligands.

NO2 bond cleavage by MoL3 complexes

The cleavage of one N-O bond in NO2 by two equivalents of Mo(NRAr)3 has been shown to occur to form molybdenum oxide and nitrosyl complexes. The mechanism and electronic rearrangement of this reaction was investigated using density functional theory, using both a model Mo(NH2)3 system and the full [N((t)Bu)(3,5-dimethylphenyl)] experimental ligand. For the model ligand, several possible modes of coordination for the resulting complex were observed, along with isomerisation and bond breaking pathways. The lowest barrier for direct bond cleavage was found to be via the singlet η(2)-N,O complex (7 kJ mol(-1)). Formation of a bimetallic species was also possible, giving an overall decrease in energy and a lower barrier for reaction (3 kJ mol(-1)). Results for the full ligand showed similar trends in energies for both isomerisation between the different isomers, and for the mononuclear bond cleavage. The lowest calculated barrier for cleavage was only 21 kJ mol(-1)via the triplet η(1)-O isomer, with a strong thermodynamic driving force to the final products of the doublet metal oxide and a molecule of NO. Formation of the full ligand dinuclear complex was not accompanied by an equivalent decrease in energy seen with the model ligand. Direct bond cleavage via an η(1)-O complex is thus the likely mechanism for the experimental reaction that occurs at ambient temperature and pressure. Unlike the other known reactions between MoL3 complexes and small molecules, the second equivalent of the metal does not appear to be necessary, but instead irreversibly binds to the released nitric oxide.

On the selective cleavage of nitrous oxide by metal–amide complexes

The remarkable selective cleavage of nitrous oxide by metal–amide systems, involving a bimetallic mechanism, has been investigated using computational methodology and rationalized on the basis of the interplay of structural and bonding factors.

Dinitrogen metal complexes with a strongly activated N-N bond: a computational investigation of [(Cy2N)3Nb-(μ-NN)-Nb(NCy2)3] and related [Nb-(μ-NN)-Nb] systems

The structural and bonding properties of the dinitrogen-bridged diniobium [(Cy(2)N)(3)Nb(μ-NN)Nb(NCy(2))(3)] complex experimentally characterized by Berno and Gambarotta, which exhibits a strongly activated N-N bond of 134 pm, have been explored using density functional methods and compared with those of a series of related [(R(2)N)(3)Nb(μ-NN)Nb(NR(2))(3)] (R = H, Me, (i)Pr, (t)Bu, Cy) model species and other experimentally relevant [Nb(μ-NN)Nb] systems, in order to rationalize the unusually long N-N distance. Geometry optimizations of [(Cy(2)N)(3)Nb(μ-NN)Nb(NCy(2))(3)] and three other known systems indicate that the most favourable N-N distance lies within the range of commonly reported results for end-on bound dinitrogen-diniobium complexes, between 123 and 128 pm. However, structures exhibiting appreciably longer N-N distances, close to 134 pm, are found to be only weakly disfavoured, and may represent the preferred geometry in cases where lengthening of the N-N bond counteracts the effects of highly repulsive steric interactions between terminal fragments. Calculations on model complexes, in which small-sized [R = H, Me] terminal groups are involved, support the finding that N-N bond lengths within the 123-128 pm range are most favoured, whereas calculations on larger [R = (i)Pr, (t)Bu] model species indicate that the presence of excessively repulsive intramolecular interactions can lead to substantial changes in the geometric properties of the [Nb-NN-Nb] core, including significant increase in N-N bond length and activation. The preference for N-N distances ranging between 123 and 128 pm, irrespective of ligand size and identity, can be understood on the basis that the principal bonding mechanisms across the central [Nb-NN-Nb] core are largely unaffected by changes in the chemical composition and properties of terminal fragments. However, the balance between repulsive (steric) and attractive (electrostatic plus orbital) bonding contributions can be altered by the presence of geometrically rigid and oversized peripheral groups and, in these cases, the interplay between repulsive and attractive bonding effects is dominated by the former and can result in abnormally elongated N-N distances. The present calculations thus provide a rationale for the observed structural properties of the [(Cy(2)N)(3)Nb(μ-NN)Nb(NCy(2))(3)] system on the basis of the interplay between electronic and steric factors.