Zero electron ligand

The addition of any zero-electron ligand to the metal is also an electrophilic addition AlMca, BF3, HgCl2, Cu+, and even r kC02, when it binds via carbon, all act in this way. Each of these reagents has an empty orbital by which it can accept a lone pair from the metal. 

Here, M is taken to be a member of the 3d series. The 3d, 4s and 4p orbitals are deemed to constitute its valence orbitals. L is a ligand which has available for bonding to M only a lone pair, oriented such that o overlap is possible with M orbitals having non-zero electron density along the M-L axes. Fig. 8.1 shows the labelling of the Cartesian axes, with respect to which the M atomic orbitals are labelled. The L lone pairs are... 

As a matter of classification, these single bond complexes are divided into three groups according to the number of electrons formally donated by the borane ligand to the metal. The zero-electron donor (or electron pair acceptor) complex is exemplified by the compound Na[(OC)sMn BH3] (J), the one-electron donor by the compound l,2-(CH3)2-3-[(C5H5)Fe(CO)2]-BioC2H9 (2), and the two-electron donor by the compound (CH3)4N[7,8-B9H,oCHPCr(CO)5] (3). 

A Z-function ligand also interacts with a metal center via a dative covalent bond, but differs from the L-function in that both electrons are donated by the metal rather than the ligand. As such, a Z-function ligand donates zero electrons to a metal center. Since the metal uses two electrons in forming the M-Z bond, a Z-function ligand raises... 

When the second capping ligand is introduced, it interacts with the remaining non-bonding orbitals of e symmetry. Therefore a frans-bicapped octahedral complex is unfavourable because the remaining available non-bonding orbitals have zero electron density along the z axis. 

Some rare ligands, such as boranes, donate zero electrons, and have been termed "Z-type" ligands. 

In complexes, the spin-forbidden intra-subshell transitions are zero-electron jumps as defined above, representing spin-pairing energy and sym-metryrtype-dependent interelectronic repulsion, and also the transitions between different subshells are both discussed in ligand field theory. However, for chemical purposes it is perhaps more interesting to study electron... 

Accurate electron density maps obtained from X-ray diffraction data, which show that there is no point of zero electron density on metal-ligand axes. It is therefore not possible to regard metal and ligand electrons as in any way separate. 

As a zero-electron reagent, an electrophile such as H" " or Me can attack the ligand, or the M-L bonds, or the metal—even in an 18e complex. Particularly in the case of the proton, initial attack may occur at one site, followed by rearrangement with transfer to a second site, so the location of the electrophile in the final product may be misleading. Electrophilic addition to metal complexes can therefore be mechanistically complexit is also less easily controllable and less often used than nucleophilic addition. 

Zero-electron neutral ligands are a growing class. For example, BR3, having a 6e boron, completes its octet by accepting lone pairs, as in H3N BR3 to become an 8e boron. If the lone pair comes from a metal, we have an L M BR3 bond in which BR3 provides Oe to the metal and thus leaves the metal electron count unaltered. The L M BR3 bond can alternatively be written with formal charges as L M+-B R3 (Eq. 2.11). 

In common with 2e nucleophiles, Oe electrophiles, such as or Me+, can attack a ligand. Unlike nucleophiles, however, they can also attack the M-L bond or the metal itself because, as zero electron reagents, wherever they attack, they do not alter the electron count of the complex. The resulting mechanistic complexity and unreliable selectivity makes electrophilic attack far less controllable and less useful than nucleophilic attack. Polysubstitution is also more common in the electrophilic case. " ... 

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

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