Electron Counting and Oxidation State

Common error alert Do not confuse total electron count, d electron count, and oxidation state with one another. All three characteristics are important to the reactivity of the metal. 

What do you think is the proper formulation for H2PtCl6 Why do you think the compound is commonly called chloroplatinic acid" Make sure your formulation gives a reasonable electron count and oxidation state. 

Give electron counts and oxidation states of all the complexes in Figures 4.1, 4.2, and 4.3. Based on the electron counts and oxidation states, what conclusions about similarities and differences between the three mechanisms can be drawn ... 

Carbenoid species such as CO and RNC also undergo insertions and eliminations (e.g., M-X + CO M-C(O)-X). These are 1,1-insertions, as opposed to the more common 1,2-insertions. Again, there is no change in total electron count or oxidation state of the metal, except transiently as CO coordinates to the metal before insertion. The insertion of CO into a M C bond is a key step in many important reactions. Again, the insertion is reversible. 

Give the electron counts, formal oxidation states, and d configurations of the following [Pt(NH3)4p, PtCl2(NH3)2, PtQJ", (Tf -C5Hs)2Ni,... 

There are two methods for counting electrons formal charge and oxidation state. These two counting methods actually represent two flipsides of the same coin. To calculate formal charge, we treat all bonds as covalent, regardless of whether they are or not ... 

To use either electron-counting procedure, it is necessary to know how many electrons each ligand in a complex donates to the metal. Table 15.1 gives electron contributions for a variety of ligands for both the neutral atom and oxidation state... 

The deprotonation of M-H to give M and H+ is one quite common dissociation reaction in which the pair of electrons of the bond goes to the metal. The metal doesn t change its total electron count, and its oxidation state decreases by 2. It s strange to think of a deprotonation as causing a reduction of the metal. The conundrum arises because of the oddities of the language that is used to describe metal complexes. The oxidation state of a complex with a M-H bond is calculated as if the bond were polarized toward H (i.e., M+ 11 ). Thus, when metal hydrides are deprotonated, it seems as if the metal is gaining electrons (from the hydride) that it did not have previously. 

The electron acceptor and donor properties of a metal center relative to a particular ligand depend, apart from the ligand itself, on a variety of factors associated with the metal (such as its position in the periodic table, its oxidation state and coordination number), to the co-ligands (e.g., their electronic donor/acceptor ability) and to the overall coordination entity (namely the electron count and the net charge). Naturally, all such factors also affect the reactivity of the ligand which is therefore determined by a complexity of combined effects whose relative weight is often not easy to predict. 

Oxidative addition and reductive elimination are the formal chemical processes involving oxidation and reduction of metal atoms accompanied by bond cleavage and formation between ligands A and B, respectively, as shown below. Thus, since A and B are one-electron ligands, the oxidation statte, electron count, and coordination number increase by two units in the oxidative addition. Oxidative addition to dinuclear or 17-electron complexes results in change of the oxidation state, electron count, and coordination number increase by one unit. The reductive elimination is the inverse process of oxidative addition and vice versa. 

So, in a fairly straightforward way, it is possible to predict the magnetic properties of many metal complexes based on symmetry and electron count. And, often, the orbital contribution is quenched by the ligand field. However, a very interesting example of a complex that behaves in the completely opposite way, that is in which there is almost no quenching of the orbital angular momentum is [Fe C(SiMe3)3 2]. The oxidation state of the iron is formally 2+, In the free ion, the electron... 

In the oxidative addition of an A-B bond to a metal, new M-A and M-B bonds are formed as the A-B bond is cleaved (Eq. 2.1). The reverse reaction, reductive elimination, leads to the extrusion of an A-B molecule from a precursor M(A)(B) complex this is often the product forming step in a catalytic reaction. In the oxidative direction, we break the A-B bond and form an M-A and an M-B. Since A and B are always considered as le X-type (anionic) ligands, the oxidation state, the electron count, and coordination number of the metal all increase by two units during the reaction. The change of +2 in the formal oxidation state gives the reaction the name oxidative addition. These terms as well as the conceptual basis of organometallic chemistry are discussed in a previous work [4]. 

Some metals prefer to change oxidation state, electron count, and coordination number by one unit instead of two. Eq. 2.8 shows an example of binuclear oxidative addition, a reaction that brings about these changes and therefore can be favored by such metals. This typically occurs for a paramagnetic first row 17e transition metal complex or an 18e M-M bonded dimer that can dissociate to give... 

---