If SOCl2 reacts with alcohols via SNi, why doesn't POCl3? A bit of a follow-up to Why is thionyl chloride preferred for preparing alkyl chlorides from alcohols?.



The reaction with $\ce{SOCl2}$ is also used instead of $\ce{PCl3}$ and $\ce{PCl5}$ when retention of stereochemistry is required. Phosphorus trichloride and phosphorus pentachloride both lead to inversion.



The mechanism proceeds via a $\mathrm{SN_i}$ pathway, or internal nucleophilic substitution. This step has been highlighed in red; an intimate ion pair$^{[\text{see below}]}$ is given in square brackets. Adding a nucleophilic solvent such as pyridine considerably increases inversion with $\ce{SOCl2}$ via an attack at the sulfur atom. Green marks blocking of the intramolecular substitution; blue is for inversion.






As far as I know, $\ce{POCl3}$ has not been seen following an $\mathrm{SN_i}$ mechanism (even without pyridine). Why?


If possible, please provide a qualitative mechanistic reason. Something that is readily explained by waving hands. Regardless of whether such an approach is achievable, quantitative (calculational) answers are also most welcome.



References discussing intimate ion pair formation


F. A. Carey, R. J. Sundberg. $(2007)$. Advanced Organic Chemistry
Part A: Structure and Mechanisms, $4$th edition, pp 269$-$276. ISBN: 0-306-46242-7
W. A. Hughes, E. D. Cowdrey, C. K. Ingold, S. Masterman, A. D. Scott. 'The Mechanism of Elimination Reactions. Part 1. Unimolecular Olefin Formation from Alkyl Halides in Bulphur Dioxide and Formic Acid'. Journal of the Chemical Society, $(1937)$, 1271$-$1277. DOI: 10.1039/JR9370001271
E. S. Lewis, C. E. Boozer. 'The Kinetics and Stereochemistry of the Decomposition of Secondary Alkyl Chlorosulfites'. Journal of the American Chemical Society, $(1952)$, 74, 308$-$311. DOI: 10.1021/ja01122a005
D. J. Cram. 'Studies in Stereochemistry. XVI. Ionic Intermediates in the Decomposition of Certain Alkyl Chlorosulfites'. Journal of the American Chemical Society, $(1953)$, 75, 332$-$338. DOI: 10.1021/ja01098a024
C. C. Lee, A. J. Finlayson. 'Rearrangement In The Reaction Between Thionyl Chloride And $3$-Methyl-$2$-Butanol. Canadian Journal of Chemistry, $(1961)$, 39(1): 260$-$261. DOI: 10.1139/v61-030
C. C. Lee, J. W. Clayton, D. G. Lee, A. J. Finlayson. 'Rearrangement Studies With $\ce{^14C-XIII}$: The Thermal Decomposition Of $1$-$\ce{^14C}$-$2$-Butyl Chlorosulfite'. Tetrahedron, $(1962)$, 18 1395$-$1402. DOI: 10.1021/ja01098a024
H. Patin, G. Mignani, C. Mahe, J-Y. Le Marouille, A. Benoit, D. Grandjean. 'Ferrocenyltrithiocarbonates: I. Direct access from α-ferrocenylcarbinols by a $\mathrm{SN_i}$ mechanism. Absolute x-ray structure determination of (R)-ferrocenylmethylmethane S-methyl-trithiocarbonate'. Journal of Organometallic Chemistry, $(1980)$, 193, 1, 93$-$103. DOI: 10.1016/S0022-328X(00)86079-9
J. L. Kice, G. C. Hanson. 'Mechanisms of SNi reactions. Effect of aralkyl group structure on ion-pair return in the decomposition of aralkyl thiocarbonates'. Journal of the American Chemical Society, $(1973)$, 38 (7), 1410$-$1415. DOI: 10.1021/jo00947a037
M. B. Smith. $(2013)$. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, $7$th edition, pp 311, 486$-$487, 490, 598, 1316. ISBN: 978-0-470-46259-1
 The $\mathrm{SN_i}$ (substitution nucleophilic internal) mechanism: retention of configuration. Powerpoint presentation, Università degli Studi di Napoli Federico II.
James. $\ce{SOCl2}$ and the $\mathrm{SN_i}$ Mechanism. Master Organic Chemistry, Alcohols. webpage
The step in the red box is wrong. You probably get dissociation to form an ion pair. The pair quickly collapses, leading to chloride attack on the cation before any dissociation or diffusion occurs so that you get the retention of stereochemistry.

For phosphorus based chlorination, this ion-pair mechanism is probably not favorable, though it's not clear to me why this is the way it is.