Géométrie moléculaire octaédrique

En chimie, une géométrie moléculaire octaédrique est la géométrie des molécules où un atome central, noté A, est lié à six atomes, groupe d'atomes ou ligands, notés X, formant un octaèdre régulier. Cette configuration est notée AX6E0 selon la théorie VSEPR. Les angles de liaison sont de 90° lorsque tous les substituants sont les mêmes, et la structure est alors un octaèdre parfait appartenant au groupe ponctuel de symétrie Oh. C'est par exemple le cas avec l'hexafluorure de soufre (SF6) ou avec l'hexacarbonyle de molybdène (Mo(CO)6), mais la plupart des molécules octaédriques ne possèdent pas une si forte symétrie. Le terme « octaédrique » est utilisé de façon peu rigoureuse par les chimistes qui se focalisent sur la géométrie des liaisons vers l'atome central, sans considérer les différences entre les ligands. Par exemple, le complexe d'hexaminecobalt(III) ([Co(NH3)6]3+) n'est pas octaédrique au sens mathématique du terme, du fait de l'orientation des liaisons N-H, mais il est to

Mécanismes d'isomérisation et d'échange de ligand de complexes octaédriques labiles de cobalt (II)

Fabrice Riblet

There are only few structural thermodynamic and kinetic studies on labile octahedral complexes available today. Octahedral paramagnetic compounds of cobalt(II) have an optimum electronic relaxation leading to NMR spectra with good resolution. Therefore, they are excellent candidates to follow substitution and isomerisation processes by multinuclear NMR at variable pressure and temperature. This work groups the study of nine labile complexes (three families of compounds with three examples each) which can exist in different isomeric forms. The first part of the work concerns octahedral complexes with bis-chelating ligands of the general formula [Co(LA)2Rpy2] with LA being a bidentate acetylacetonate ligand and Rpy a monodentate ligand of the γ-substituted pyridine type. The second part of the work concerns octahedral complexes with tris-chelating ligands of the general [Co(LA)2(LB)] with LA being a bidentate acetylacetonate ligand and LB a bidentate ligand of the γ-substituted bipyridine type. The third part of the work concerns trinuclear heterometallic complexes with the general formula [Fe2M(μ3-O)(μ2-Ac)6(Rpy)3] with M = CoII, NiII and Rpy a monodentate ligand of the γ-substituted pyridine type. Each type of the metal centres has an octahedral geometry. For each example chosen, the different species present in equilibrium in solution have been identified and the thermodynamic and kinetic parameters measured are discussed. Mechanisms for isomerisation and ligand exchange are proposed on the basis of variable temperature and pressure kinetics data. It has been shown that the bis-chelate compounds isomerise via a dissociative mechanism (with an intermediate of bipyramid geometry). The monodentate ligand participates more or less depending on its donor character. In the case of complexes with γ-methylpyridine, the mechanism consists essentially of an opening of one arm of the bidentate ligand (with little exchange of the pyridine). However, in the case of complexes with γ-trifluoromethylpyridine, a more frequent exchange of the monodentate ligand is observed competing with the opening of the arm of the bidentate ligand. In the case of tris-chelate compounds the isomerisation mechanism proposed on the bases of the exchange matrix used for spectral simulations is a twist with an intermediate of prismatic geometry. This observed rates (faster) and the obtained activation parameters (smaller) are very different of those found for complexes with bis-chelating ligands. The trinuclear heterometallic complexes maintain their structure in dichloromethane solution. Variable pressure experiments allowed assigning a dissociative mechanism (D) for the exchange of pyridine on both the divalent and the trivalent centres. Experiments at variable temperature indicated an increase of lability for the trivalent metals and a decrease in lability for the divalent metals compared to the homoleptic octahedral mononuclear complexes. The activation parameters for the trinuclear heterometallic complexes are bigger than those found for the bis- and tris-chelate compounds.

EPFL2007

Functional Nanostructures Based on Organometallic Half-Sandwich Complexes

Hexanuclear coordination cages of the formula [(C5Me4R)M(C7H3NO 4)]6 (M = Rh, Ir; R = Me, H) were obtained by stepwise reaction of [(C5Me4R)M(μ-Cl)Cl]2 with, first AgOAc and then pyridine-3,5-dicarboxylic acid. Crystallographic analyses show that the cages adopt a distorted octahedral geometry with the pyridine-3,5-dicarboxylates functioning as dianionic bridging ligands, each of which connects three different (C5Me4R)M fragments. The cages act as exoreceptors for large alkali metal cations K+ and Cs+, as well as NH4+, but show low affinity for Na+. Crystallographic and NMR spectroscopic analyses indicate that two metal ions can be coordinated to the surface of the cages. The respective binding sites comprise three carbonyl O-atoms from the bridging pyridine-3,5-dicarboxylate ligands. Novel supramolecular architectures were obtained by reacting the dinuclear macrocycle [(cymene)Ru(μ-C12H12O4)(H 2O)]2 with different N-donor ligands. A tetranuclear assembly was obtained with 1,2-di(4-pyridyl)ethylene, whereas hexanuclear complexes were formed with 2,4,6-tris(4-pyridyl)triazine and 1,3,5-tris(4-pyridylethynyl)benzene. The reaction with 5,10,15,20-tetra(4-pyridyl)porphyrine, on the other hand gave tetra- or octanuclear complexes, depending on the stoichiometry that was employed. The 2,4,6-tris(4-pyridyl)triazine complex was shown to act as a host for polycyclic aromatic compounds such as pyrene, triphenylene and coronene. A hexanuclear coordination cage has been formed by self-assembly of three elongated [(cymene)Ru(μ-C14H10O6)] 2 dinuclear macrocycles and two equivalents of 2,4,6-tris(4-pyridyl)triazine. The twisted coordination prism with a height of 3.5 Å (bottom to top) could be untwined by addition of coronene to – for half-sandwich complex-based cages – unprecedented size of 10.8 Å (bottom to top). Crystallographic and NMR spectroscopic analyses indicate that two coronene molecules can be encapsulated inside this cage.

EPFL2010

Mécanismes d'isomérisation et d'échange de ligand de complexes octaédriques labiles de cobalt (II)

Fabrice Riblet

EPFL2007

Functional Nanostructures Based on Organometallic Half-Sandwich Complexes

EPFL2010