Sixteen-electron complexes, square planar

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Addition reactions. Certain square planar c/ (sixteen-electron) complexes such as [RhH(CO)L2] play a particularly important role in catalysis. These are sixteen-electron systems, having two less electrons than needed to complete a noble gas electronic configuration. It should not be surprising that such complexes add one ligand to become eighteen-electron complexes, reaction (18). 

Sixteen-electron square planar complexes are most commonly found for ds metals, in particular those metals having formal oxidation states of 2+ (Ni2+, Pd2+, Pt2+) and 1+ (Rh+, Ir+). Some of these complexes have important catalytic behavior, as discussed in Chapter 9. 

The metal atom in the square-planar complexes of Pd(II), Pt(II), Rh(I), Ir(I) has only sixteen electrons in its valence orbitals. These complexes are easily oxidized by the addition of oxygen or halogens to yield an octahedral... 

TT-acceptor characteristics, a 16-electron configuration is more stable than an 18-electron configuration. Sixteen-electron square-planar complexes may be able to accept one or two ligands at the vacant coordination sites (along the z axis) to achieve an 18-electron configuration. This is a common reaction of 16-electron square-planar complexes (Chapter 14). 

Sixteen-electron square-planar species are most commonly encountered for S metals, particularly for metals having formal oxidation states of 2-1- (Ni, Pd, and Pt " ) and 1-1- (Rh, lr ). Square-planar geometry is more common for second- and third-row transition-metal complexes than for first-row complexes. Two examples of square-planar (f complexes that are used as catalysts are Wilkinson s complex and Vaska s complex (Figure 13.10). 

Eighteen-electron complexes react more slowly than similar complexes with either more or less electrons. The eighteen-electron rule explains why some reactions are associative and others dissociative. Complexes in which the metal has sixteen or less valence electrons tend to react by associative mechanisms, since the metal has vacant low-energy orbitals which can be used to form a bond with the entering ligand. This orbital can accept an electron pair from an entering ligand and provide a path for associative substitution. Substitution reactions in square planar complexes illustrate this point, reaction (40). 

In a formal sense, these are oxidation reactions, because rhodium(l) is oxidized to rhodium(ni). These are also addition reactions because two ligands are added to square planar sixteen-electron systems which are transformed into octahedral eighteen-electron complexes. These reactions can also be viewed as an insertion reaction (see (4) below) in which a metal atom is inserted into a bond between two nonmetals. 

Another remarkable exception to the eighteen electron rule is found among the d transition-metal ions, such as Ni(ll), Pd(II), Pt(ll), Rh(l), lr(l), and Au(ni), which often appear as four-coordinate square planar complexes with only 16 valence electrons. These are said to comply with the sixteen electron rule. Finally, d ions such as Cu(l), Ag(l), and Au(l) can also form sixteen electron three-coordinate complexes, or two-coordinate linear complexes that obey the fourteen electron rule. 

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