Standard reduction potentials group 14 metals

The selectivity of RNH2 on M/A1203 and Raney catalysts decreased in the order Co Ni Ru>Rh>Pd>Pt. This order corresponds to the opposite sequence of reducibility of metal-oxides [8] and standard reduction potentials of metalions [9], The difference between Group VIII metals in selectivity to amines can probably been explained by the difference in the electronic properties of d-bands of metals [3], It is interacting to note that the formation of secondary amine, i.e. the nucleophilic addition of primary amine on the intermediate imine can also take place on the Group VIII metal itself. Therefore, the properties of the metal d-band could affect the reactivity of the imine and its interaction with the amine. One could expect that an electron enrichment of the metal d-band will decrease the electron donation from the unsaturated -C=NH system, and the nucleophilic attack at the C atom by the amine [3], Correlation between selectivity of metals in nitrile hydrogenation and their electronic properties will be published elsewhere. 

The two properties listed in Table 27-1 that suggest that Group 1A metals are unlikely to exist as free metals are (1) the low ionization energies, which show how easily the outermost electron can be removed and (2) very negative standard reduction potentials, which indicate that the aqueous ions are not easily reduced to metals and that the free metals are easily oxidized to 1+ cations. 

The standard reduction potential for Be2+ is the least negative of the elements in the group and by the same token beryllium is the least electropositive and has the greatest tendency to form covalent bonds. The bulk metal is relatively inert at room temperature and is not attacked by air or water at high temperatures. Beryllium powder is somewhat more reactive. The metal is passivated by cold concentrated nitric acid but dissolves in both dilute acid and alkaline solutions with the evolution of dihydrogen. The metal reacts with halogens at 600°C to form the corresponding dihalides. 

Table 6.4 The standard reduction potentials for the metallic elements of Groups 1 and 2 at pH = 0 ...

The accepted values for the standard reduction potentials for the Group 1 unipositive cations being reduced to the solid metal are given in Table 6.4, and some thermochemical data and the appropriate ionic radii are given in Table 6.5. 

The metals of Group 11 all form + l states that vary in their stability with respect to the metallic state. The standard reduction potentials for the couples Cu+/Cu and Ag + /Ag are +0.52 V and +0.8 V, respectively. That for Au + /Au has an estimated value of + 1.62 V. The thermodynamic data for the calculation of the reduction potentials are given in Table 7.18, which also contains the calculated potentials for Cu and Ag. 

These reactions are typically limited to the halogens. The procedure for predicting the outcome of these reactions is the same as for the metals. The halogens also appear on the table of standard reduction potentials, but for reasons we will discuss in Chapter 18, the halogens get more reactive as you go up the table of reduction potentials. An easy way to remember the reactivities of the halogens is they are less reactive going down the group (as atomic number increases). 

The ability of certain metals to donate electrons to electrophilic or unsaturated functional groups has proven useful in several reductive procedures. The facility with which these metals donate electrons is given by their standard reduction potentials. 

The high adsorption capacity of Ag+ ions by all the activated carbons was attributed to the reduction of Ag+ ions to metallic silver by the hydroquinone groups present on the carbon surface, which in turn are oxidized to quinone groups. This redox process is supported by the standard reduction potentials of Ag+ (Ag+ + e Ag, E = 0.7996 V) and quinhydrone electrode, = 0.6995 V. The increase in adsorption of Ag+ ions by the ammonia-treated sample was attributed to the formation of silver amino complexes which are quite stable under the conditions used in these studies. 

With the preceding as a brief review of redox reactions, we can now turn to a discussion of the standard reduction potentials of the alkali metals, which are listed in Table 12.1. Specifically, we want to know what information they can provide and how such information can be put to use to understand better the characteristics of not only the alkali metals but also other groups of the periodic table. Take lithium as an example. The half-equation for the reduction of aqueous lithium ions to lithium metal is shown in Equation (12.9) ... 

The metal-nonmetal line passes through the heart of the group, with carbon being a bona fide nonmetal and lead a bona fide metal. In between are two metalloids (silicon and germanium) and a borderline metal (tin). The progression in the acid-base character of the oxides of the elements further emphasizes the trend from nonmetal to metal. The formulas of the oxides and halides show the increasing importance of the +2 oxidation state down the group, and this is reinforced by a consideration of standard reduction potentials. 

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