Sixteen-electron complexes additions

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). 

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... 

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. 

The series of reactions above is a simple catalytic cycle for the water gas shift reaction, CO + H2O CO2 + H2. (a) What is the oxidation state of rhodium in each complex in the cycle (b) Indicate whether each complex is a sixteen- or eighteen-electron complex, (c) Which of these classes of reactions does each step of the cycle illustrate addition, elimination, oxidative addition, reductive elimination, insertion, or deinsertion ... 

The number of electrons in the valence shell of each complex or intermediate is shown. This illustrates the important rule that diamagnetic complexes of the transition metals from Groups IV—VIII have either sixteen or e hteen electrons in their outer shell. This rule is well known for stable complexes, being a modification of the inert gas rule. In addition Tolman 38) has recently pointed out that it works quite well for reactive intermediates or transition states. Exceptions are known, however. 

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