Mean bond dissociation enthalpy

The main purpose of these final comments is to show a few general trends in the thermochemistry of Group 14 organometallic compounds, helped by some (hopefully) reliable values. And one of the trends is revealed by a rather usual plot1,2, in which the mean bond dissociation enthalpies of the species MR4 (i.e. one-fourth of the enthalpy required to break all the M—R bonds) are represented as a function of the enthalpy of formation of M in the gaseous state. As observed in Figure 4, for R = H and Me, D(M—H) and D(M—Me) increase with the enthalpy of formation (or sublimation) of M. It is noted, on the other hand, that the differences D(M—H) — D(M—Me) vary from 47.7 kJmol-1... 

Table 6. Mean bond dissociation enthalpy D (M-L) kJ mol 1 for homoleptic metal compounds ML ...

Table 9. Valence m.o. electron configuration Ofg e"g e%g, enthalpy of sublimation, AHSU, and standard enthalpy of formation, (AHf, g), of metallocenes. Mean bond dissociation enthalpy, D (M-Cp) (Refs.68 69 ) and (Metal-cyclopentadienyl ring) bond length, r(M-Cp). (Ref.72 ).

In the previous exercise (whose outcome was not very successful), we used a new concept to assess the thermodynamic stability of chemical bonds the mean bond dissociation enthalpy (also known as mean bond disruption enthalpy). It is represented by DH° or by (DH°) (the symbol we adopt). As indicated, for... 

We conclude this section by giving, for the sake of clarity, the most general definition of mean bond dissociation enthalpy. For any molecule AYmXn, where A is a central atom and X and Y are any mono- or polyatomic groups, the A-X... 

Figure 5.4 Stepwise bond dissociation enthalpies in several first-row hydrides. For each compound, the light gray bars, starting on the left, represent DH°(A-H), DH A-H), etc. The dark gray bars represent the respective mean bond dissociation enthalpies, (DH°) (A—H). Data at T = 0 from [78],...

We have shown that the result of replacing stepwise bond dissociation enthalpies by mean bond dissociation enthalpies and transferring bond dissociation enthalpies from one molecule to another can be deceptive The assumption that in Cr(CO)3 (C6H6), DH° (Cr-CO) + DH° Cr-CO) + DH° Cr-CO) 3 (DH°)( Cr CO) led to an error of 72 kJ mol-1 in DH°[(CObCr-CeHe]. Yet this error cancels out if the same procedure is applied to derive relative Cr-arene bond dissociation enthalpies in a series of ( r 6-arcnc)chromium tricarbonyl complexes. 

We are now left with the evaluation of E s (Cr—CO), the Cr-CO bond enthalpy contribution in Cr(CO)6. The third thermochemical cycle in figure 5.6 shows how this bond enthalpy contribution can be evaluated from the Cr-CO mean bond dissociation enthalpy (107.0 0.8 kJ mol-1 see section 5.2) and the reorganization energy ER(CO ). 

Mortimer and co-workers extended these studies to many other CoL2X2 compounds and, using estimated or measured enthalpies of sublimation and heat capacities of the complexes and the ligands, were able to derive the corresponding Co-L mean bond dissociation enthalpies [238]. 

Table 1 Selected mean bond dissociation enthalpies (kJ mol ) for /7-block elements...

Table 6 Mean bond dissociation enthalpies (BDE) and calculated first ligand dissociation energies (FDE) of some binary metal carbonyls...

As far as equation (55a) is concerned, the enthalpy term is accurately known for most metals, corresponding to the transfer of the solid metal to its state of a monoatomic gas in its high-spin ground-state configuration. Equation (55b) contains the negative value of the bond enthalpy, which, divided by the number of M O bonds in the molecule, gives the mean bond dissociation enthalpy. M(CO) (g) differs from M(CO) (s) or M(CO) (l) by the heats of sublimation or vaporization, respectively. 

Table I. Mean Bond Dissociation Enthalpies and Differential Solvation Free...

The method used to derive metal-ligand bond enthalpy terms, E(M-L), and mean bond dissociation enthalpies, D(M-L), has been discussed in a recent review (7) and only a brief account is given here. The enthalpies of formation of the complexes M(Cp) L and M(Cp) Cl in the gas phase afford the enthalpy of Reaction which... 

The molybdenum-alkyl mean bond dissociation enthalpies are in the expected order and in good agreement e.g. with the trend reported for Th(Cp ) R complexes (Cp = rj -C Me ) (9a). On the other hand, it is interesting to note that the group additivity rule seems to apply to the Mo-alkyl family, as shown by the excellent linear relationships 7 and 8, where N is the number of carbon atoms in the alkyl chain. 

The Mo-alkyl mean bond dissociation enthalpies decrease with the increase of the n-alkyl chain length, in a way closely resembling the D(C-H) trend in the alkanes. The results suggest that, in the absence of important steric effects, further increases of the chain length will not significantly affect the Mo-alkyl bond dissociation enthalpy. 

The molybdenum-methyl mean bond dissociation enthalpy, (DH°) (Mo-Me), is related to the standard enthalpy of the reaction... 

Where Pr stands for C3H7. Proton NMR experiments at several temperatures have shown that the relative stability of the dihydride increases at higher temperatures. A thermodynamic analysis of the reaction led to A,7/°(59) = 5.0 2.5kJmor and 7 AA °(59)w 1.5 2.6kJmor. 2 As A,//°(59) = D7/ (W-H2)-fi)//°(H-H)-2 DH°)(W-H), the above value of Z)//°(W-H2) yields (Z)7/ ) (W-H) = 236kJ moP. This tungsten-hydrogen mean bond dissociation enthalpy in the W(ll) seven-coordinate complex is about lOOkJmoP lower than DH°(W-H) in W(Cp)(CO)3H, where the metal is in the same oxidation state ... 

The metal-methyl mean bond dissociation enthalpies in the main-group homoleptic molecules, MMe , vary from 131 kJmoF in ZnMc2 to 283kJmoF in AlMc3. Thus, the above value for Z) (M-Me) in a transition metal complex is not particularly high, suggesting that many cootdinatively unsaturated transition metal complexes would activate methane. 

Hill, J. O. Murray, J. P. Metal-Oxygen Thermochemical Bond Dissociation Enthalpies for Metal Pentane-2,4-dionate Complexes - A Review, Rev. Inorg. Chem. 1993, 13, 157-181. Tables with standard enthalpies of formation and vaporization, and mean bond dissociation enthalpies. 

Fig. 1.2 Mean bond dissociation enthalpies in methyl, ethyl and phenyl derivatives of Group IV elements at 298.15 K.

With respect to formation from the elements, metal carbonyls are exothermic compounds (Table 5.7). A measure of the strengths of the M—CO bonds is provided by the mean bond dissociation enthalpy D(M—CO), which is derived from the enthalpy of dissociation ... 

FIGURE 25 Mean bond dissociation enthalpies for neutral, monopositive, and dipositive actinide dioxide molecules. Reproduced with permission from Margalo and Gibson (2009). Copyright 2009 American Chemical Society. 

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