Are Bond Properties Additive?

Now, the next point that could have been made is that when we get to more common problems, the bond energies are not additive in that way. Or are they? One problem I see is the actual data are not really suitable for reaching a conclusion.

Let's consider the P –P bond energy, which is needed for considering the bond additivity of any phosphorous compounds. I made a quick calculation of the P – P bond energy in diphosphine, on the assumption that the P – H bond energy was the same as in phosphine, and I got the energy 242 kJ/mol. If you look up some bond energy tables, you find the energy is quoted as 201 kJ/mol. How did they get that? If you consider the heats of atomization of phosphorus, the bond energy is 221 kJ/mol, but if we assume that is in the P

The problem is obvious: to make any sense of this, we need more accurate data. We also need the data to involve energies of atomization, and not rely on the more easily obtained bond dissociation energies. But as far as I can see, the chemical community has given up trying to establish this data. Does it matter? I think it does. For me, a problem with modern chemical theory, which is essentially extremely complicated computations, is that it offers little assistance to the issues that matter for the chemist because there are no principles enunciated, but merely results and comments on various computational programs. The principles are needed, even if the calculations are not completely accurate, so that chemists can draw conclusions, and use these to formulate new plans of action. How many really think they understand why many synthetic reactions work that way? Do we care about the very fundamental component of our discipline? And, for that matter, does anyone care whether I write this blog?

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