Comparing energies of P−O, S−O, and Cl−O bonds

My text says that



> 
> Bond energies of P−O, S−O, and Cl−O follow the trend P−O < S−O < Cl−O.
> 
> 
> 


But I think it should be a *reverse* of what's given because *more the electronegativity difference, more will be the bond energy and lesser the bond length*. This what I believed so far before seeing P−O, S−O, Cl−O trend.


Moreover, after searching the web for the values of the same I was unable to find for P−O, S−O, Cl−O, but I found values for C−O and N−O and bond energies (B.E.) for it follows C−O > N−O (which supports my reasoning).


Then why the order is different for P−O, S−O, and Cl−O? Is it an exception, or could it be explained?




Your text doesn't seem to be in agreement with experimental data, and your suggested order is apparently the correct one.
I collected a set of bond dissociation energies (BDEs) $D^\circ\_{298}$ as well as bond distances for free molecules in gas phase $r$, all determined experimentally except $r$ for phosphorus dioxide radical $\ce{PO2}$:


$$
\begin{array}{l|ll|ll}
\hline
\ce{X} & D^\circ\_{298}(\ce{X-O})/\pu{kJ mol^-1} & r(\ce{X-O})/\pu{Å} & D^\circ\_{298}(\ce{X(O)-O})/\pu{kJ mol^-1} & r(\ce{X(O)-O})/\pu{Å} \\
\hline
\ce{P} & 589^\text{[1, p. 484]} & 1.4759^\text{[2, p. 9-27]} & 540^\text{[1, p. 484]} & 1.4665^{[3]} \\
\ce{S} & 517.90±0.05^\text{[1, p. 360]} & 1.484^\text{[2, p. 9-28]} & 551.1^\text{[1, p. 360]} & 1.4308^\text{[2, p. 9-28]} \\
\ce{Cl} & 267.47±0.08^\text{[1, p. 366]} & 1.5696 ^\text{[2, p. 9-22]} & 256.3±1.3^\text{[1, p. 366]} & 1.470^\text{[2, p. 9-22]} \\
\hline
\end{array}
$$


For diatomic molecules $\ce{X-O}$ there is a trend of *decreasing* BDEs


$$D^\circ\_{298}(\ce{P-O}) > D^\circ\_{298}(\ce{S-O}) > D^\circ\_{298}(\ce{Cl-O}),$$


also backed up by the *increase* in bond distances:


$$r(\ce{P-O}) < r(\ce{S-O}) < r(\ce{Cl-O}).$$


To tell the complete truth, comparing bond energies like that isn't entirely fair, as we are comparing bonds of various (fractional) order, e.g. in case of sulfur, it really should be a double bond $\ce{S=O}$ we are looking at.


For the sake of comparison and to demonstrate how introduction of a polyatomic system alters bonding, there is also data for $\ce{XO2}$ molecules where phosphorus and sulfur swap places and the BDEs order changes to


$$D^\circ\_{298}(\ce{S(O)-O}) > D^\circ\_{298}(\ce{P(O)-O}) > D^\circ\_{298}(\ce{Cl(O)-O}),$$


and the bond distances order changes accordingly:


$$r(\ce{S(O)-O}) < r(\ce{P(O)-O}) < r(\ce{Cl(O)-O}).$$


### References


1. Luo, Y.-R. *Comprehensive Handbook of Chemical Bond Energies*, 1st ed.; CRC press, **2007**. ISBN 978-0-8493-7366-4.
2. Haynes, W. M.; Lide, D. R.; Bruno, T. J. *CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data*, 97th ed.; Taylor & Francis Group (CRC Press): Boca Raton, FL, **2016**. ISBN 978-1-4987-5429-3.
3. [*Computational Chemistry Comparison and Benchmark DataBase*, Release 21 (August 2020) — Experimental data for PO2 (Phosphorus dioxide), Table "Atom - Atom Distances"](https://cccbdb.nist.gov/exp2x.asp?casno=12164975&charge=0). (accessed 2022-02-19)


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