A-bonded ligands

Fig. 7. A tethered cyclodextrin and the structure of P-cyclodextrin, the most common cyclodextrin used as a bonded ligand in Hquid chromatography.

The majority of ligands have a lone pair of electrons in a filled sp 1 type orbital that can overlap with a vacant metal dsp orbital, derived from the vacant d, p, and s orbitals of the metal, to form a conventional two-electron two-centre a bond. Ligands of this type increase the electron density on the central metal atom. This is the sort of bond that used to be called dative covalent and represented by an arrow. Nowadays it is more common to represent all bonding to metals of whatever kind by simple lines. 

Figure 3 Orbital correlation diagram between a main-group atom E and six a bonded ligands X in octahedral EXe...

The extensive chemistry of this oxidation state has two facets. The classical complexes containing a-bonded ligands have been known for nearly two centuries. At the beginning of the twentieth century, they were widely used by Alfred Werner in his fundamental research into the constitution, stereochemistry, and isomerism of coordination compounds. 

Zr and Hf complexes containing a-bonded ligands, without Cp, are also known. ... 

The significance of this example is that, whereas oobalt(III) forms several 6 -thiooyanato complexes in the presence of ligands capable of participating in back-bonding, this is an example where a conventional a-bonding ligand, such as NHg, coexists with the Co—SCN moiety. 

The formation of the binuclear complexes was shown to be affected both by the nature of the metal and by the a-bonded ligand. The Fe—Fe, 0,77 binuclear complex was the sole product 178) of reaction, while the Fe—W complex was isolated together with two side products, (XIII) and (XIV) 179). 

Many compounds are known containing CO in conjunction with other ligands, which may include tc acceptors such as phosphines, and/or a bonding ligands. For example, there is a series of compounds Mn(CO)5X, where X=H, halogen or an alkyl group. 

The preparation and bonding of complexes of the ) -allyl group, CH2=CH-CH2-, have already been discussed (p. 933). This group, and substituted derivatives of it, may act as a-bonded ligands, but it is as 3-electron rr-donor ligands that they are most important. Crudely ... 

The occurrence of this trans effect transmission is consistent with the molecular orbital scheme calculated for 12, which shows a strong mixing of the a orbitals of the H-Ir-H-Ir-NCCH3 backbone (Figure 3). [40] This could permit the transmission of a trans effect due to a a-bonding ligand from one extreme of the molecule to the other. 

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