Oxidation states enthalpy

There is a lack of information about b.e.cs involving 4d- and 5d transition metals. The detailed variation of b.e.cs with the formal oxidation state of the metal remains to be defined beyond the generalization that there is an approximate inverse relation between the two. However, the comparison of D (W-CO) withf) (W-CH3) indicates that the bond enthalpy/oxidation state relationship is complex. 

As in the preceding transition-metal groups, the refractory behaviour and the relative stabilities of the different oxidation states can be explained by the role of the (n — l)d electrons. Compared to vanadium, chromium has a lower mp, bp and enthalpy of atomization which implies that the 3d electrons are now just beginning to enter the inert electron core of the atom, and so are less readily delocalized by the formation of metal bonds. This is reflected too in the fact that the most stable oxidation state has dropped to +3, while chromium(VI) is strongly oxidizing ... 

However, like the mp, bp and enthalpy of atomization, it also reflects the weaker cohesive forces in the metallic lattice since for Tc and Re, which have much stronger metallic bonding, the -t-2 state is of little importance and the occurrence of cluster compounds with M-M bonds is a dominant feature of rhenium(III) chemistry. The almost uniform slope of the plot for Tc presages the facile interconversion between oxidation states, observed for this element. 

Bonds between many neutral ligands and transition metal atoms in low oxidation states, have mean bond disruption enthalpies in the range 80-200 kJ mol 1 (9). Bonds to charged ligands, e.g. 

The Frost diagrams for the first series of the d block elements in acidic solution, pH = 0, given in Figure 7.11(b) show many similarities with the variation of the enthalpy of formation of the oxides. Only the oxidation states observed for solid oxides are included. 

The relative stabilities of the dioxides, sesquioxides and monoxides for first period transition metals are given in Figure 7.11(c). The stability of the higher oxidation state oxides decreases across the period. As we will discuss later, higher oxidation states can be stabilized in a ternary oxide if the second metal is a basic oxide like an alkaline earth metal. The lines in Figure 7.11(c) can in such cases be used to estimate enthalpies of formation for unstable oxidation states in order to determine the enthalpy stabilization in the acid-base reactions see below. Finally, it should be noted that the relative stability of the oxides in the higher oxidation states increases from the 3d via 4d to the 5d elements, as illustrated for the Cr, Mo and W oxides in Figure 7.11(d). 

This assumption is crucial to the development of a body of thermochemical information on compounds of the transition metals. The assumption is made that b.e.c values are transferable from one compound to another in which the formal oxidation state of the metal is unchanged. This means that, for example, the value of Z (Cr-CO) in Cr(CO)6 is assumed unchanged11 in [Cr(r -C6H6)(CO)3], or that Z)(Cr-C6H6) in the same compound is assumed unchanged from its value in Cr(77-C6H6)2. In this example, A.Hf [Cr(T -C6H6)(CO)3, q] = —352.3 kJ mol-1, from which the enthalpy of disruption, A, for the process... 

These data for D (M-L) offer some basis for making predictions about the enthalpies of metal-carbon bonds involving other metals in these groups. It is important to bear in mind that all of the data in Table 6 concern the metals in their highest formal oxidation state. It is usually true that the mean bond enthalpy increases as the formal oxidation state of the metal decreases. This is exemplified by the values of >(M-Cl) (M = Nb, Ta, Mo, W) in various oxidation states of M (Table 7a), and... 

Table 7a. Variation of mean bond enthalpy, D (M-Cl) kJ mol 1 with formal oxidation state, (rf-electron configuration) of the metal. (Ref.23))...

Europium and ytterbium di-valence. The oxidation state II for Eu and Yb has already been considered when discussing the properties of a number of divalent metals (Ca, Sr, Ba in 5.4). This topic was put forward again here in order to give a more complete presentation of the lanthanide properties. The sum of the first three ionization enthalpies is relatively small the lanthanide metals are highly electropositive elements. They generally and easily form in solid oxides, complexes, etc., Ln+3 ions. Different ions may be formed by a few lanthanides such as Ce+4, Sm+2, Eu+2, Yb+2. According to Cotton and Wilkinson (1988) the existence of different oxidation states should be interpreted by considering the ionization... 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

Intermediate oxidation states of some transition metal oxides are insoluble in water because their lattice enthalpies are sufficiently large to cause the solution of the compounds in water to give ions to be endothermic. Examples are V202, Mo02, W02, Mn02 and Re02. 

The transition from positive ions with low oxidation states, via insoluble oxides with intermediate oxidation states, to oxoanions with high oxidation states, is caused by the competition between ionization energies, lattice enthalpies and enthalpies of hydration, similar to the discussion of the variations of ionic forms of the p-block elements given in Section 6.1. Further discussion occurs in Section 7.5.3. 

The maximum stability for vanadium as the V3 4 ion in acidic solution can be understood in terms of the maximization of the enthalpy of hydration for the +3 ion, above which hydrolysis alters the form and stability of the higher states. The + 4 and + 5 states are considerably electronegative compared to the lower oxidation states, and are able to engage in covalent bonding to ligand oxide ions to form V=0 bonds. 

This book offers no solutions to such severe problems. It consists of a review of the inorganic chemistry of the elements in all their oxidation states in an aqueous environment. Chapters 1 and 2 deal with the properties of liquid water and the hydration of ions. Acids and bases, hydrolysis and solubility are the main topics of Chapter 3. Chapters 4 and 5 deal with aspects of ionic form and stability in aqueous conditions. Chapters 6 (s- and p-block). 7 (d-block) and 8 (f-block) represent a survey of the aqueous chemistry of the elements of the Periodic Table. The chapters from 4 to 8 could form a separate course in the study of the periodicity of the chemistry of the elements in aqueous solution, chapters 4 and 5 giving the necessary thermodynamic background. A more extensive course, or possibly a second course, would include the very detailed treatment of enthalpies and entropies of hydration of ions, acids and bases, hydrolysis and solubility. 

The common oxidation states are -3, +3 and +5, but the simple ions As3-, As3+ and As5+ are not known. The krypton electronic structure could be attained by gain of three electrons, but there is a high energy requirement. Equally, the loss of five valence electrons to form As5+ is unrealizable because of the high ionization enthalpy. 

Finally, in Table I, Column Qq refers to the heat of dissociation of 02 from the oxide to the next lower oxidation state, calculated per half mole 02. The necessary data for the enthalpies of formation (Hf) were derived from Handbook of Chemistry and Physics (45th ed., 1964-1965). 

Thallium has two stable oxidation states, + f and +3. Use the Kapustmskii equation to predict the lattice energies of TIF and TIF. Predict the enthalpies of formation of these compounds. Discuss. 

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

A consideration of these relationships reveals8 that because E° is a thermodynamic parameter and represents an energy difference between two oxidation states and in many cases the spectroscopic or other parameter refers to only one half of the couple, then some special conditions must exist in order for these relationships to work. The special conditions under which these relationships work are that (a) steric effects are either unimportant or approximately the same in both halves of the redox couple and (b) changes in E° and the spectroscopic or other parameters arise mainly through electronic effects. The existence of many examples of these relationships for series of closely related complexes is perhaps not too unexpected as it is likely that, for such a series, the solvational contribution to the enthalpy change, and the total entropy change, for the redox reaction will remain constant, thus giving rise to the above necessary conditions. 

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