Hexacoordination centre

One of the first electrochemically characterized models was the diiron(II) cation [Fe2(0H)(02CCH3)2(Me3TACN)2]+, (Me3TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane), the molecular structure of which is illustrated in Figure 28.35 In contrast to deoxyhaemerythin, both the two Fe(II) centres are hexacoordinate. 

Ab initio density functional calculations afford theoretical evidence of hexacoordinate main group atoms. Si, P, and As centred in planar, hexagonal hydrocopper complexes, CugHgX where X = S, P or As. ... 

The key features of this mechanism are the activation of the silyl ketene acetal initiator by the nucleophilic catalyst, the further coordination of the monomer to the nucleophilic silicon centre, rendering the silicon hexacoordinate, and the transfer of the silyl group to the incorporated monomer, thus regenerating the reactive silicon group to continue the polymerization. 

Examples of hexacoordinated q2 - BH4 complexes are known with octahedral (2.6a/T2.6) and distorted octahedral molecular geometries (2.6b). All of these structures present at least one hydride and two phosphino ligands. The metal centres are d4-Group 6 or Group 8 transition metals. The OsH3(BH4)(P Pr3)2 complex exhibits interesting dynamical features which have been computationally studied [14,15] and will be discussed with more detail in Sect. 4. 

View of the (M)-handed helix of the 3D [Li Cr (C204)3] anionic network, in which the hexacoordinated metallic Li(I) and Cr(III) centres are of (A) absolute configurations (reproduced with permission from reference, copyright 1999, American Chemical Society). 

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