To determine the minimum energy pathway for the formation of pyrazolyl-phosphine oxide 3 from 1, density functional theory (DFT) calculations were carried out. We also sought to rationalise why the reaction is unique to H2O, and why the analogous pyrazolyl-phosphine oxide with R = t Bu substituents did not form. Our calculations showed addition of H2O across the N-P bond to be the first, and rate-limiting step for this reaction (Scheme 3). For 1 (R = i Pr), the lowest energy pathway occurs via cooperative action of two H2O molecules, with a six-membered transition state (TS1A) over a high, but accessible barrier of 26.6 kcal.mol -1 . For 2 (R = t Bu), an analogous sixmembered transition state could not be found, presumably due to the additional steric bulk of the t Bu groups. Instead, addition of one H2O molecule across the N-P bond proceeds over a significantly higher energy barrier of 41.8 kcal.mol -1 (TS1B), which we observed to be experimentally inaccessible even with heating to 120 °C. Heating higher than this led to decomposition of 2 and an intractable mixture of products (see ESI for details). Proceeding over TS1 leads to intermediate I1, with N-H and P-OH moieties, before cleavage of the N-P single bond and tautomerism to a phosphine oxide yields intermediate I2. Cummins and co-workers previously reported the reaction of an N-heterocyclic iminophosphorane, structurally similar to 1 and 2, to also react with H2O to give N-P bond cleavage and formation of N-NH2 and phosphine oxide functionalities, analogous to I2, although no further reactivity was reported. I2 exists initially as the Z-isomer, isomerises to the thermodynamically favoured E-isomer (I3) over an accessible energy barrier of 17.9 kcal.mol -1 for R = i Pr (TS2). The low energy barrier for this C=C isomerism can be attributed to delocalisation of the lone pair of the adjacent nitrogen centre into the C=C bond, and further into the -CO2Et and phosphine oxide functionalities. This was supported by natural bond orbital (NBO) analysis, which showed donation from the nitrogen lone pair into the C=C π* orbital of 65.3 kcal.mol -1 for I2 and 67.4 kcal.mol -1 for I3 (R = i Pr). Analysis of the Wiberg bond indices also showed reduced double bond character in the C=C bond (1.56 for I2; 1.65 for I3, both R = i Pr), and increased N-C double bond character (1.21 for I2; 1.16 for I3, both R = i Pr). Following this isomerism, ring-closing occurs via attack of the nucleophilic amine onto the carbonyl of the ester group, which are both on the same side of the E-isomer, to form I4, with concomitant release of EtOH. I4 exists as the keto form of the product; our calculations show that the enol form is clearly favoured over the keto form (energy difference of 18.3 kcal.mol -1 for 3). This preference for the enol form is in corroboration with both solid-state and solution phase experimental data.