Abstract
The determination of the molecular structures of metallacarboranes by X-ray diffraction remains critical to the development of the field, in some cases being the only viable way in which the overall architecture and the isomeric form of the molecule can be established. In such studies one problem frequently met is how to distinguish correctly {BH} and {CH} vertices, and this review begins by describing two relatively new methods, the Vertex-Centroid Distance (VCD) and Boron-Hydrogen Distance (BHD) methods, that have been developed to overcome the problem. Once the cage C atoms are located correctly, the resulting metallacarborane structure can frequently be analysed on the basis that cage B has a greater Structural Trans Effect (STE) than does cage C. In the absence of significant competing effects this gives rise to unequal M–L distances for a homogeneous ligand set and to a preferred Exopolyhedral Ligand Orientation (ELO) for a heterogeneous ligand set. ELO considerations can be used, amongst other things, to rank order the STEs of ligands and to identify suspect (in terms of cage C atom positions) metallacarborane structures.
Highlights
The first metallacarboranes, the sandwich complexes [3-Fe(1,2-C2 B9 H11 )2 ]2−, [3-Fe(1,2-C2 B9 H11 )2 ]−and their C-methyl derivatives, were reported in 1965 [1] and the first crystallographic study of a metallacarborane, [3-Cp-closo-3,1,2-FeC2 B9 H11 ], appeared later that year [2]
Metallacarboranes are 3-dimensional cluster compounds that in general can exist in a number of isomeric forms which cannot always be distinguished spectroscopically, in which case crystallographic study is the essential experimental technique [4]
Vertex 2 is directly connected to both Co atoms and all B atoms with Ueq values smaller than that of vertex 10 are bound to at least one Co, whilst vertex 10 is directly connected to neither, clearly illustrating the influence of an adjacent metal on the magnitude of Ueq. Both of the established methods for distinguishing between {BH} and {CH} fragments incarboranes generally, and metalla(hetero)carboranes have deficiencies and should be used with caution. Because of this we have developed two further approaches to the problem which we term the Vertex-Centroid
Summary
The first metallacarboranes, the sandwich complexes [3-Fe(1,2-C2 B9 H11 )2 ]2− , [3-Fe(1,2-C2 B9 H11 )2 ]−. Their C-methyl derivatives, were reported in 1965 [1] and the first crystallographic study of a metallacarborane, [3-Cp-closo-3,1,2-FeC2 B9 H11 ], appeared later that year [2]. Ever since those early days single-crystal X-ray diffraction studies have been a vital component of research into metallacarborane chemistry. A redox reaction, or by deboronation and oxidative closure of a 13-vertex precursor This makes it impossible to predict with confidence the correct isomeric form of the. 12-vertex cobaltacarborane closure of a 13-vertex precursor This makes it impossible to predict with confidence the correct by either chemical intuition or NMR spectroscopyby and means that recourse to NMR crystallographic isomeric form of the 12-vertex cobaltacarborane either chemical intuition or spectroscopystudy is necessary. Known isomers of [CpCoC2B9H11] have been characterised crystallographically [9,10,11,12,13,14]
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