Unravelling strong temperature-dependence of JHD in transition metal hydrides: solvation and non-covalent interactions versus temperature-elastic H–H bonds

A number of transition metal hydrides reveal intriguing temperature-dependent JHD in their deuterated derivatives and possibly the temperature dependent hydrogen–hydrogen distance (r(H–H)) as well. Previously, theoretical studies rationalized JHD and r(H–H) changes in such compounds through a “temperature-elastic” structure model with a significant population of vibrational states in an anharmonic potential. Based on the first variable temperature neutron diffraction study of a relevant complex, (p-H-POCOP)IrH2, observation of its elusive counterpart with longer r(H–H), crystallized as an adduct with C6F5I, and thorough spectroscopic and computational study, we argue that the model involving isomeric species in solution at least in some cases is more relevant. The existence of such isomers is enabled or enhanced by solvation and weak non-covalent interactions with solvent, such as halogen or dihydrogen bonds. “Non-classical” hydrides with r(H–H) ≈ 1.0–1.6 Å are especially sensitive to the above-mentioned factors.


Introduction
Transition-metal hydrides are involved in a countless number of reactions and catalytic cycles 1 and are of fundamental importance to organometallic chemistry. 2More specically, dihydrides, since they can be formed through direct reaction of metal centers with molecular hydrogen, are highly relevant to the processes of hydrogen activation and its extrusion from hydrogen-rich molecules, not the least for the purpose of hydrogen storage. 3Dihydrides also represent the simplest case of oxidative addition/reductive elimination 4 and thus serve as model compounds to study the interaction of transition metal centers with other molecules.In particular, some similarity of H-H and C-H bond activation should be mentioned, 5 and while C-H bond activation intermediates are oen elusive, the respective dihydrides usually have much higher stability. 6t is believed that dihydrides form an H-H bond activation continuum 2 as illustrated in Fig. 1. [7][8][9][10][11][12][13] It begins with classical dihydrogen complexes where the H-H bond acts as a Lewis base and remains comparatively intact and ends up with classical dihydrides where oxidative addition is nalized.2 In the middle are the so-called elongated dihydrogen complexes and compressed dihydrides.It follows from Fig. 1 that the H-H internuclear distance, r(H-H), is one of the key descriptors of dihydrides.The experimental determination of r(H-H) is challenging: X-ray diffraction frequently fails to locate hydrides nearby to a heavy metal atom, while neutron diffraction, which would be the ideal technique, requires growing relatively large crystals, which most of the time is difficult with these sensitive materials. Cmmon spectroscopic characterization methods include solution-state determination of T 1 (min) 14 and J HD , 15,16d both of which are correlated to r(H-H).A number of complexes were discovered, which reveal puzzling temperature-dependent J HD 11,16   (see Fig. 1 10,16a,b cis-Cp(CO) 2 -ReH 2 (5.8-6.5 Hz, toluene-d 8 ), 16f trans-[Os(H 2 )Cl(dppe) 2 ] + (13.6-14.2 Hz, D 2 Cl 2 ) 16g,h and a few more compounds.17 If one were to straightforwardly apply the J HD -r(H-H) correlation, it would seem that r(H-H) in such complexes is changing with temperature as well.Initially, this pattern was attributed to a rapid dihydrogen-dihydride equilibrium.16f, 18 However, over the years convincing spectroscopic or crystallographic evidence for the presence of two compounds was never obtained. A least one of the plausible isomers always remained elusive and highly uncertain, which, along with the limited datasets available, precluded quantitative analysis attempts.16c In an attempt to resolve this puzzling case, theoretical studies suggested the existence of unusual "temperature elastic" H-H bonds.16e,f, [19][20][21] The model involved a single structure with a highly anharmonic potential energy surface, which gives rise to excited vibrational states complementary to a ground state: if the ground state is a compressed dihydride, the excited vibrational states would be mainly of elongated dihydrogen complex nature with a shorter H-H distance, and vice versa.The population of those states leads to a change of r(H-H); elongated dihydrogen complexes were predicted to have a longer r(H-H) and lower J HD upon an increase in temperature, while compressed dihydrides should have shorter r(H-H) and higher J HD at higher temperature 20 which on a semiquantitative level is in line with the majority of experimental observations.
Both theoretical methods and J HD -r(H-H) correlations suggested that for certain compounds such as [Cp*Ir(dmpm)(H 2 )] ++ and [Cp*Ru(dppm)(H 2 )] + (Fig. 1) where J HD variation reaches 1-2 Hz, r(H-H) values may change up to 0.02-0.08Å over 100-300 Kthe value that potentially can be detected via crystallographic methods.However, all relevant neutron diffraction studies were conducted at a single temperature, and no accurate experimental verication of r(H-H) changes with temperature was so far obtained.Furthermore, both J HD -r(H-H) correlation 22,23 and the T 1 (min) method 14,24 could suffer from complications or data scattering.
Iridium pincer complexes of the type (X-POCOP)IrH 2 (POCOP = 2,6-( t Bu 2 PO) 2 C 6 H 3−x -X, where X = p-MeO-, p-H-, p-MeOOC and X 2 = m-bis-CF 3 in this work) arguably revealed the so far highest reported J HD variation of up to 3 Hz (ref.25-27)  (see also Table S4 †) over just 50-100 K temperature span.Hence, if r(H-H) indeed does change with temperature, the magnitude of such changes in (X-POCOP)IrH 2 type complexes, judging from J HD , makes them promising candidates for a crystallographic study.
Here we report the rst multi-temperature neutron diffraction study of the compressed dihydride (p-H-POCOP)IrH 2 , as well as extensive spectroscopic and theoretical studies of this and related compounds.In a solid state, a small lengthening of r(H-H) was observed, as opposed to a considerable shortening that was expected form solution-state data and previous theoretical studies on compressed dihydrides.We argue that the major component that contributes to the J HD change in solution is an equilibrium between "short" and "long" isomers of (X-POCOP)IrH 2 .Strong evidence for the existence of such isomers is presented.The solvent choice has a remarkable effect on the spectroscopic properties of (X-POCOP)IrH 2 , such as chemical shi of hydrides, isotope effect on chemical shis (Dd), J HD and T 1 (min).Due to a fairly at potential energy surface in the H-H bond stretching region, weak interactions with solvent can signicantly change the nature and equilibria between various hydride species in solution.A notable interaction is halogen bonding between hydrides and halogenated solvents.This bonding type was characterized including the rst neutron diffraction study of a "long" isomer exemplied by a (p-MeOOC-POCOP)IrH 2 /IC 6 F 5 adduct.Re-examination of literature data suggests that the model with isomers could be relevant to many hydride complexes.

Results and discussion
Solid-state structure of (p-H-POCOP)IrH 2 Within the family of (X-POCOP)IrH 2 compounds, (p-H-POCOP) IrH 2 was found to crystallize more readily than the other members and therefore was chosen for a neutron diffraction study.Data were collected at 10, 100 and 295 K.The analysis of the neutron diffraction results showed a raw crystallographic r(H-H) distance of 1.43(2) Å at 10 K, which remained virtually unchanged at 100 and 295 K (Fig. 2), in contrast to what is expected from solution-state measurements.At 295 K, the anisotropic displacement parameters (ADPs) for the dihydride hydrogen atoms were too big to be only due to intramolecular bending and stretching modes, and a normal coordinate analysis of the ADPs using the Bürgi-Capelli method 28 was performed (ESI 1.4 †).According to such analysis, an in-phase libration of the two dihydride hydrogens out of the ligand molecular plane, coupled with the rigid-body libration of the whole molecule about an axis passing through the P1-P2 atoms, was shown to have a frequency of 51(3) cm −1 , and this combined librational motion accounted for most of the motion of these hydrogens in the crystal.
It is well known that libration in the solid state can affect the interatomic distances determined in diffraction experiments, 29 and a correction of the bond lengths based on the librational parameters extracted from the ADP analysis was performed, and these corrected values show an elongation of the r(H-H) distance of 0.05 Å in the 10 to 295 K interval.Overall, the changes in the distance are small and in the opposite direction compared to what has been predicted for compressed dihydrides. 18ructure of (X-POCOP)IrH 2 and (PCP)IrH 2 in toluene solution.The effect of non-specic solvation Our neutron diffraction study seems to rule out a highmagnitude r(H-H)-T dependence in a single compound, and we therefore explored an alternative two-component model.Previous observations [25][26][27] on (X-POCOP)IrH 2 did not rationalize the complex spectral patterns correctly.One proposal, in order to explain the solvent and temperature dependence of J HD , involved an equilibrium between the putative elongated dihydrogen complex of Ir(I) and an Ir(III) dihydride with Ir-coordinated solvent (structures IV and V in Fig. 3). 23However, such a proposal suffers from multiple discrepancies (for example, it contradicts the data on the chemical shis of type V structures [30][31][32] ) and illustrates that without understanding the crucial role of solvation and non-covalent interactions, temperature-dependent J HD in transition metal hydrides cannot be properly addressed.The present comprehensive NMR data set (Table S4 †) for (X-POCOP)IrH 2 and the related complex (PCP) IrH 2 (PCP = 2,6-( t Bu 2 PCH 2 ) 2 C 6 H 3 ), together with crystallographic and computational data, allowed coming up with a quantitative model based on the discrete isomers with different r(H-H).The model involves two structures, symmetrical (S) and nonsymmetrical (NS) with respect to the position of hydride ligands (Fig. 3), and can be further augmented with specic interactions with solvent (NS-bound, see below).All solution spectra are hence treated as the weighted-average of S and NS.
A two-component (S and NS) t was found to well capture the temperature dependence of d( 1 H), d( 31 P), 2 J PH and J HD for X = mbis-CF 3and p-MeOOC-(Fig.4, ESI 3 †), and provided limiting chemical shis and coupling constants for S and NS, as well as thermodynamic parameters (Table S6 †).Pleasingly, the tted J HD -S of ca.9.6 Hz matched the one calculated for (p-H-POCOP) IrH 2 using the neutron diffraction distance (8.9 Hz; equation 16d ) very well.Hence, r(H-H) for S in solution is close to the crystallographically determined value of 1.43 Å.As for the NS structures, r(H-H) for (m-bis-CF 3 -POCOP)IrH 2 and (p-MeOOC-POCOP)IrH 2 can be estimated to be 1.7 < r(H-H) < 2.0 Å, based on the observed T 1 (min) (weighted-averaged) and tted J HD -NS.The limiting 2 J PH for S structures (e.g.7.6 Hz for X = m-bis-CF 3 -) are close to that for (p-H-POCOP)IrH 2 (8.3 Hz), pointing to a TBP geometry, while for NS structures 2 J PH (e.g.11.4 Hz for X = mbis-CF 3 -) approach that of square-pyramidal 33 complexes (X-POCOP)IrHCl (ca.13 Hz).S and NS differ by 1-2 kcal mol −1 with NS being a global minimum in solution.As seen from Fig. 4 one aspect of the model is huge isotope effects on chemical shis (dened as d(IrH 2 ) − d(IrHD); Dd) of up to −1.4 ppm or −5 ppm in other solvents.These are only consistent with the presence of strongly discriminated hydride sites in the molecule.We interpret it as a non-statistical distribution of deuterium between apical and equatorial positions in NS (isotope perturbation of equilibria).To correctly t Dd-T simultaneously with other parameters, one needs to balance between rening the limiting shis and assigning "intrinsic" Dd to S and NS.A possible, but not unequivocal solution is given in Fig. 4; t parameters and their comparison with DFT calculated values are provided in Table S8.† We would like to note that the accuracy of J PH and J HD measurement is strongly affected by the linewidth, which may in turn affect the t; more discussion is provided in ESI 3.1.† For (X-POCOP)IrH 2 with electron-donating groups (X = p-MeO-and p-H-), the temperature dependence of d( 1 H), 2 J PH and J HD is much smaller.Thus, the change of d(IrH 2 ) over −80, ., +25 °C is only 0.4 ppm.This likely reects smaller geometrical and energetic differences between S and NS.Therefore, S-NS equilibria changes are hard to differentiate from "non-specic" processes such as P-t Bu group rotations etc. T 1 (min) for X = p-MeO-and p-H-(129 and 120 ms) can be translated to an r(H-H) of 1.57 and 1.54 Å, respectively, using the established methodology. 14We therefore assume that the complexes with X = p-MeO-and p-H-in solution closely resemble the neutron diffraction structure, with the T 1 (min) based distances viewed as an upper limit for r(H-H) of S (for S-(p-H-POCOP)IrH 2 1.43 < r(H-H) < 1.54 Å). 2 J PH , and to some extent, J HD changes are obscured by line broadening.Yet, the tted J HD -NS is consistent with an r(H-H) of around 1.7 Å for NS.Isotope effects Dd do not exceed 0.22 ppm, providing further evidence of nearly symmetrical hydride sites (the geometry is closer to TBP rather than SP in both S and NS).Notably, Dd for complexes with X = p-MeO-and p-H-in toluene at certain temperatures reveals d(IrHD) up-eld versus d(IrH 2 ), which was not observed for X = m-bis-CF 3and p-MeOOC-(Table S4 †).This may be an observation of an "intrinsic" Dd in S. The temperature dependence of Dd then can be due to intermolecular isotope perturbation of S-NS equilibria with deuterium favoring NS or due to S this time being a global minimum (ESI 4 †).In other solvents, where NS has longer r(H-H) and is undoubtfully populated, isotope effects for X = p-MeO-and p-H-are in the same direction (although smaller) as for m-bis-CF 3and p-MeOOC-.
Finally, the complex (PCP)IrH 2 (ref.34) reveals NMR spectra with very minor temperature dependence of chemical shis and J HD (Fig. S2, Table S4 †).The J HD of 7.6 Hz corresponds to an r(H-H) of 1.49 Å, in good agreement with 1.49 Å obtained from a T 1 (min) of 94 ms.Supposedly, for (PCP)IrH 2 NS is higher in energy than S, and is not populated, giving rise to static spectra.

Computational study of (X-POCOP)IrH 2 and (PCP)IrH 2 in toluene solution
For LL ′ L ′′ MX 2 type d 6 complexes, the TBP geometry with a 120°M X 2 angle is disfavored and due to Jahn-Teller effects undergoes angle compression. 35Indeed, the potential energy surface (PES) of (p-H-POCOP)IrH 2 in the r(H-H) stretching region (D3BJ 36 -revPBE 37,38 level of theory, which was noted to perform well for Ir 39 ) in a vacuum reveals only an S structure (Fig. 5) with "distorted" TBP geometry.However, there is another distortion that can remove the degeneracy, which is the non-symmetrical movement of hydride ligands to give NS.When solvation is included, NS, which has a higher dipole moment, receives extra stabilization and appears as a separate minimum on the PES.Thus, with toluene set as a solvent (CPCM model) 40 two minima are found, corresponding to S and NS structures at 1.60 and 1.63 Å, respectively (Fig. 5).
A single-point correction of electronic energies using a highly accurate DLPNO-CCSD(T) 41 method resulted in a quite similar energy prole, with NS shied to 2.1 Å.We have also attempted other DFT functionals and basis sets (ESI 9.2 †), and the majority of methods argue for the distance in S between 1.48 and 1.65 Å in (p-H-POCOP)IrH 2 .That is slightly longer that the 10 K neutron diffraction distance of 1.43(2) Å. Possibly the difference reects uncaptured solvation/packing effects, but we cannot completely rule out other reasons.The direction of the least energetic cost upon deformation of r(H-H) in S is towards longer distances, which may explain the small elongation upon raising the temperature observed by neutron diffraction.
When it comes to locating NS on the PES, depending on the method, r(H-H) for NS in (p-H-POCOP)IrH 2 varies from 1.63 to 2.2 Å, while the energy gap between S and NS goes from +3.0 to −0.3 kcal mol −1 .For compounds with electron-withdrawing groups X = p-MeOOC-and m-bis-CF 3 -, the majority of methods indicate that NS-r(H-H) z 2.0-2.2Å and DH(NS-S) z −1, ., −2 kcal mol −1 , which is consistent with experimental observations.For X = p-MeO-, NS appears higher in energy than S by ca.0.3-2 kcal mol −1 .Since NMR spectra reveal small changes of d(IrH 2 ) and J HD in the same direction as for the more withdrawing groups, it could be that explicit solvation is needed for a correct description.For (PCP)IrH 2 NS is higher in energy than S (Fig. S31  Here we used the ReSpect program 42 to run much more trustworthy fully relativistic four-component DFT calculation of NMR properties, 42 required for systems with late transition metals. 43he evaluation of a test set of compounds revealed an underestimation of the hydride resonances beyond ca.−30 ppm.This was noted previously, 43 and we addressed it by applying a small empirical correction (ESI 10 †).The NMR parameters of (p-H-POCOP)IrH 2 as a function of r(H-H) are presented in Fig. 6.Upon an increase in the distance between the hydrides their chemical shis synchronously move up-eld.Beyond ca.1.6 Å, the de-symmetrization of the hydride environment causes drastic discrimination of H-apical and H-equatorial.Thus, Hapical eventually moves to ca. −40 ppm, just as in the SP (POCOP)IrHCl counterpart, while H-equatorial moves to ca. 10 ppm.Hence, the difference between the two hydrides may reach 50 ppm, which explains the large isotope effects on chemical shis in NS.The averaged d(IrH 2 ) goes through a minimum at ca. 1.8 A and then starts to increase upon further increase of r(H-H). 31P chemical shi also has an extreme point at a comparable distance (Fig. 6, middle).16d The predicted chemical shis are −16.8 ± 2 ppm for S (1.4 Å) and −19.7 ± 2 ppm for NS (1.7 Å) for the complex (p-H-POCOP)IrH 2 , which agrees well with the data from tting (−16.0 and −18.0 ppm).
Solvent effect on the structure of (X-POCOP)IrH 2 .Specic solvation While a non-specic solvation appears important for stabilization of NS isomers, the pronounced solvent dependence of (X-POCOP)IrH 2 spectra can nevertheless be linked to a specic solvation.In the study of a related aliphatic (PCyP)IrH 2 complex, 26 the authors based on DFT calculations suggested that it may be the solvent relative permittivity 3 that mainly affects the properties of the complex in solution.Here we designed an experiment to verify this hypothesis.Thus, by adding a soluble electrolyte to an organic solvent, it is possible to vary its relative permittivity with little effect on other properties. 44We prepared a solution of (p-H-POCOP)IrH 2 in CD 2 Cl 2 and THF-d 8 , as well as in the same solvents with 0.5 M NBu 4 PF 6 , which is supposed to triple the relative permittivity. 44Calculation using CPCM solvent CH 2 Cl 2 with natural 3 = 8.9 and with 3 set to 24.2 predicted that for (p-H-POCOP)IrH 2 NS should be favored by an extra 0.2 kcal mol −1 for higher 3; this is conceptually in line with previous data 26 that employed the Poisson-Boltzmann 45 reactive eld.However, unlike the control probe with a solvatochromic dye (see ESI 8 †), virtually no changes were observed in the NMR spectra of (p-H-POCOP)IrH 2 .Calculations thus somewhat overestimate the effect of 3. At the same time, a correlation of J HD and Dd with the Gutmann acceptor number of the solvents was observed (Table 1).With that in hand, we added C 6 F 5 I, which has a high acceptor number and low polarity, to a toluene-d 8 solution of (p-H-POCOP)IrH 2 , and observed a signicant decrease of J HD and increase of Dd.It thus follows that these are weak non-covalent interactions with solvent/dissolved compounds, which are mainly responsible for the spectral changes observed for (X-POCOP)IrH 2 .Notably, hydrides can form a halogen bond (XB) 48 with the strong XB donor C 6 F 5 I, something that has rarely been observed previously. 49n the presence of C (calculated d −5.4 ppm).While in the 1 H NMR spectrum a high eld-shi of the IrH 2 signal is observed, as it was observed for spectra in toluene, the 31 P NMR signal undergoes a low-eld shi upon addition of C 6 F 5 I.To rationalize this the NMR calculations can be re-called (Fig. 6), which predict a decrease of 31 P NMR shis upon passing a maximum near 1.7-1.9Å.
Halogen bond adducts have r(H-H) $ 2.1 Å and hence should reveal low-eld 31 P shis.Thus, for X = H-calculation gives 196.1 ppm for the 31  Computationally, there are several interaction modes of (p-H-POCOP)IrH 2 and C 6 F 5 I (Fig. 8).All energies are given at the DLPNO-CCSD(T) level, which we found necessary to obtain accurate values.This is in line with benchmarks on halogen bonds 50 that favored wavefunction methods.Structures a, b and c represent different variations of halogen bonding; c can be  ) and the very large difference between apical and equatorial hydrides.Thus, computational methods strongly support the conclusion that the formation of a is responsible for the observed spectral changes.The calculated XB energies for NS-bound-a with different X groups, measured versus S, are listed in Table 2. Somewhat counter-intuitively, the halogen bond is stronger for more electron-withdrawing X groups.This is rationalized through reducing a destabilizing interaction between aryl and hydride, which are trans to each other in NS and NS-bound (pincer aryl backbones with electron-withdrawing X groups have a smaller trans-effect).If the binding energy is measured against the most stable isomer in solution, then (PCP)IrH 2 with its −5.1 kcal mol −1 will provide nearly the strongest interaction.(PCP)IrH 2 exhibits the highest buildup of negative charge on hydride ligands, enhancing interaction with the s-hole on iodine atoms.
At the same time, such charge buildup disfavors the formation of NS and NS-bound.Thus, a more electron-rich metal center in (PCP)IrH 2 gives rise to counter-balancing effects.The calculated halogen bond energies for C 6 F 5 I are in agreement with experimental data taking possible uncertainties into account (ESI 6 †).
Ultimately, a neutron-diffraction study of a single-crystal of the (p-MeOOC-POCOP)IrH 2 /IC 6 F 5 adduct unambiguously conrmed that the dominating compound in the (p-MeOOC-POCOP)IrH 2 /IC 6 F 5 system is NS-bound-a (Fig. 9).The hydrides were clearly located in NS conguration, with r(H-H) was measured to be 2.22 Å and r(H-I) to be 2.51 Å. Fully in line with computational predictions, the equatorial Ir-H distance is elongated to 1.66(4) Å and the apical Ir-H distance is shortened to 1.52(9) Å, compared to almost identical Ir-H distances in S (1.60(1) and 1.615(8) Å raw; 1.62 Å for both aer libration correction).At the same time, D3BJ-revPBE seemingly overbinds the adduct (r(H-I) calc = 2.27 Å), and a very expensive DLPNO-SCS-MP2 method was needed to accurately reproduce the experimental r(H-I) (Table S12 †).Pleasingly, the DLPNO-CCSD(T) halogen bond energies of the two methods were comparable, which allowed examination of a series of complexes as discussed above.
The solid-state IR spectrum of (p-MeOOC-POCOP)IrH 2 / IC 6 F 5 revealed a broad band at 1870 cm −1 corresponding to a stretching vibration of a halogen-bound Ir-H unit trans to the   aryl moiety (see ESI 5 †).For comparison, a solution of (p-MeOOC-POCOP)IrH 2 in hexane exhibits a band at 2119 cm −1 in the hydride region (asymmetric IrH 2 vibration); a new very broad resonance at ca. 1860 cm −1 appeared when C 6 F 5 I was added.Thus, the same compounds are present in the solid state and in solution.
The quantication of the halogen bond strength experimentally is difficult, since there are complex equilibria existing in the (X-POCOP)IrH 2 /C 6 F 5 I/toluene system, and is further hampered by decomposition (ESI 6 †).The thermodynamic data in Table 2 should therefore be considered to be quite approximate.Variable temperature titration with C 6 F 5 I and tting to a 1/1 binding isotherm were performed for (p-H-POCOP)IrH We then investigated (X-POCOP)IrH 2 in CH 2 Cl 2 , which is a weaker halogen bond donor compared to C 6 F 5 I.The experimental NMR data are given in Table S4 and ESI 7. † A strong d(IrH 2 )-T dependence was observed, and isotope effects on chemical shi clearly indicated that X = p-MeOOC-, m-bis-CF 3and p-H-in CH 2 Cl 2 are de-symmetrized (Dd up to −2.7 ppm), while (p-MeO-POCOP)IrH 2 is not (Dd up to −0.12 ppm).It is important to note that the comparison of CH 2 Cl 2 with THF further supports the non-covalent nature of binding with solvent, as compared to coordination to the vacant, but strongly hindered site.Thus, THF is a well-established coordinating solvent, while CH 2 Cl 2 coordinates to metals rarely and weakly. 51MR spectra in CH 2 Cl 2 resemble observations with C 6 F 5 I, with both compounds having a high AN, while NMR spectra in THF resemble observations in toluene (Tables 1 and S4  There are numerous modes of interaction between CH 2 Cl 2 and (X-POCOP)IrH 2 (ESI 9.2 †).It appears that the structure with a dihydrogen bond between the acidic CH 2 Cl 2 hydrogen and Ir-H is favored (Fig. 10) over the halogen bond between CH 2 Cl 2 chlorine and Ir-H (DE −2.3 kcal mol −1 vs. −1.0kcal mol −1 , X = H).T 1 (min) data (see Table S4; † for example, 463 ms for (p-MeOOC-POCOP)IrH 2 in CH 2 Cl 2 ) require that both r(H-H) and r(IrH/H-CHCl 2 ) in NS-bound are above 2.0 Å.It implies that D3BJ-revPBE slightly over-binds the adduct with CH 2 Cl 2 , just as it was observed for C 6 F 5 I; alternatively, a high weight of free NS that satises the 2.0 Å criteria can be proposed.Fitting to the S/ NS-bound model well accounts for d( 1 H), 2 J PH and J HD data (ESI 7 †); free NS, if present in the system, thus cannot be evaluated.The experimental binding energy estimates are between −1.3 (X = p-MeO-) and −2.3 (X = m-bis-CF 3 ) kcal mol −1 (ESI 7 †), computational values span from −1.7 to −3.9 kcal mol −1 (Table S10 †).It thus follows that the dihydrogen bond with CH 2 Cl 2 is ca.2-4 kcal mol −1 weaker than the halogen bond with C 6 F 5 I, which is line with the AN of the solvents (Table 1).On a structural level, this difference is reected by the signicant desymmetrization of NS-bound in (X-POCOP)IrH 2 with X= p-MeO-by C 6 F 5 I, but not by CH 2 Cl 2 .This can be seen from the Dd values (−0.9 vs. −0.12ppm) and calculated r(H-H) (2.01 vs. 1.73 Å) for C 6 F 5 I/toluene and CH 2 Cl 2 , respectively.At the same time, the halogen bond with CH 2 Cl 2 is weaker than both XB with C 6 F 5 I and the dihydrogen bond with CH 2 Cl 2 ; this is in line with the smaller d-hole on Cl compared to I.
To our knowledge, dihydrogen bonds involving CH 2 Cl 2 and transition metal hydrides are scarce, if at all known.There is an example of an interaction with CH 2 Cl 2 that is transmitted to hydrides indirectly through binding of CH 2 Cl 2 with a counteranion (see below).
Model with isomers: re-examination of [Cp*Ru(dppm)(H 2 )] + Overall, the S/NS/NS-bound model provided satisfactory rationalization of experimental spectra in different solvents, in particular J HD values, without the use of vibrational averaging corrections.It could be that such corrections may further improve ts, but at least for (X-POCOP)IrH 2 they are not the primary reason of J HD change and, according to VT neutron diffraction data, might have an opposite direction (elongation of r(H-H) instead of shortening).It is noteworthy that the 1 J HD change with temperature reported for gaseous HD is an order of magnitude lower than the changes reported for hydrides. 52here remains some uncertainty regarding the value of vibrational corrections for hydride complexes, since relevant calculations exploited truncated models and were done without solvation.However, it could be that the largest J HD changes reported (>1-1.5 Hz over 50-80 K) are due to equilibrium between two or more species.Looking from that angle, the strongest temperature dependence of J HD was observed in [Cp*Ir(dmpm)(H 2 )] ++ and [Cp*Ru(dppm)(H 2 )] + .The complex [Cp*Ir(dmpm)(H 2 )] ++ actually has two minima on the PES; 16d,e,21 while initial studies exploited averaging over many vibrational states, 16d,e later it was suggested that a simple average over the two minima could possibly account for the majority of J HD change. 21The agreement with experimental data was at best semi-quantitative; however we suppose it could be improved by taking solvent effects into account.Another complex [Cp*Ru(dppm)(H 2 )] + , was deemed worth re-examination.When the non-truncated version of this compound was used, and the CH 2 Cl 2 solvation included, the two minima were present at 1.06 and 1.42 Å, respectively (ESI 11 †).The former distance is close to the one determined in the solid state by neutron diffraction (1.10 Å). 10 A reasonable J HD t can be constructed using that data, with the tted DH and DS being in excellent agreement with the values reported previously for a slow-regime dihydrogen-dihydride equilibrium. 53Remarkably, the chemical shi of the coordinated H 2 unit in [Cp*Ru(dppm)(H 2 )] + has little temperature dependence, which was viewed as an argument for the model with vibrational corrections.16b However, the chemical shis calculated for the 1.06 and 1. Explicit solvation attempts with CH 2 Cl 2 did not reveal well-dened non-covalent interactions between RuH 2 and CH 2 Cl 2 with the level of theory used (both Ru-H/Cl-CH 2 Cl and Ru-H/H-CHCl 2 interactions were considered, see ESI 11 †).It was found that instead, CH 2 Cl 2 could be bound to [Cp*Ru(dppm)(H 2 )] + through an interaction with the p-electron cloud of one of the Ph rings, and a hydrogen bond with the pP-CH 2 -P fragment.Such binding has a little effect on equilibria between the isomers.Ion pairing effects also seem to be of secondary importance since a non-nucleophilic counter-anion B(Ar F ) 4 − was used for solution measurements.16b The neutron diffraction structure 10 was obtained with a more nucleophilic BF 4 − ; however, unlike many other cases (see below), the latter did not reveal close contacts with RuH 2 .Instead, interaction between BF 4 − and pP-CH 2 -P fragment could be found in the solid state, and seemingly this interaction is preferred in solution as well, as shown by calculations (ESI 11 †).We thus suppose that our model in the rst approximation correctly reects the chemistry of [Cp*Ru(dppm)(H 2 )] + in solution.

Model with isomers and non-classical hydrides
Looking at a broader picture, a comparatively at PES, which is a feature typical of hydrides belonging to the non-classical region (r(H-H) of ca.1.0-1.6Å), makes such hydrides very sensitive to specic and non-specic solvation, as well as to other interactions.As a result, even weak interactions of 1-5 kcal mol −1 , which are perhaps numerous for highly polarizable M-H units, can trigger signicant changes in r(H-H).
Another way to present it is as follows.It is believed that there is a H-H bond activation continuum in transition metal hydrides (Fig. 1). 2,54This view is based on the existence of hydride complexes that cover the whole possible range of r(H-H) (ca.0.8-3.2A).However, little is known what this continuum may look like.To address this, we computationally varied the electronic properties of the model compounds ( Me4 X-PCP)IrH 2 in an incremental way by substituting the ligand H atoms with F, thus plotting r(H-H) vs. "electronrichness" of the ligands (Fig. 11).Instead of a straight line, the plot revealed three regions (see also ESI 12 †).Thus, regions of "classical" dihydride and dihydrogen complexes were observed, where r(H-H) exhibited a small-slope linear dependence on the number of F atoms.These regions were connected by an S-shaped "non-classical" region, where small changes in electron properties were accompanied by big changes in r(H-H).Remarkably, the increment of one F atom (corresponding to a 6 cm −1 n(CO) change using the popular organometallic metrics) was big enough for a ca.0.3 Å leap, bypassing the 1.3-1.0Å region.Dihydrides revealed Mayer bond orders of 0.7-0.9 for Ir-H and 0.1-0.2 for H-H, while dihydrogen complexes of 0.4-0.5 for Ir-H and 0.4-0.5 for H-H.A conceptually similar pattern was observed for another model system, [Os(H 2 )(en) 2 X] + (see ESI 12 †).
Hence, the pools of more rigid, resilient classical M(n)MH 2 and M(n+ 2)M(H 2 ) structures are connected via the pool of more so, fragile non-classical structures where r(H-H) distances are in fact in the region of transition states between dihydrogen complexes and dihydrides.If the pattern in Fig. 11 can be generalized beyond the compounds studied, then one would expect that (a) for the non-classical structures, external stimuli would likely produce considerable changes in r(H-H), (b) some of the apparent non-classical r(H-H) may be a weighted-average of isomeric structures and (c) the existence of such isomers enabled/enhanced by external stimuli is the primary reason for temperature-dependent J HD , d(MH 2 ) and other properties.
Literature data support the high sensitivity of "non-classical" structures to external stimuli/medium.Thus, for the "center of gravity" of the non-classical region, three neutron diffraction   S15 †).][57] The effects of vibrational averaging were always a competing explanation for a mismatch between neutron diffraction and DFT results; now it is made less likely.It is very reasonable to propose that in solution both free and bound complexes are present, in a manner similar to the one established for (X-POCOP)IrH 2 .Therefore, one could expect the NMR parameters to be temperature dependent.Indeed, temperature-dependent J HD was reported for [Os(H 2 )Cl(dppe)] + PF 6 − ; perhaps, it would have been observed for the other compounds as well, should the observation window be suitable to allow it. 58n the known cases, ion pairing interactions shorten r(H-H) compared to the non-bound forms (see for example Table S15 †).A remarkable example to further illustrate this is the complex [(PP 3 )Co(H 2 )] + , which in the solid state exists as a dihydrogen complex with a PF 6 − counter-anion, and as a dihydride complex with a less nucleophilic BPh 4 − counter-anion. 59In the dihydrogen form, PF The examination of X-ray diffraction structures in the 1.15-1.25 Å range revealed that almost all DFT-based distances are much shorter or much longer compared to the XRD ones (see ESI 13 †).This primarily reects the inability of X-ray diffraction to accurately locate hydrides, but in some cases could be a result of medium/packing effects.An interesting example is the complex [Os(C 6 H 4 pyOPh)(h 2 -H 2 )(P i Pr 3 ) 2 ] + BF

Conclusions
Overall, we have demonstrated that J HD changes in (X-POCOP) IrH 2 can be rationalized through the rapid equilibrium between S and NS isomers.Both of them were deduced from solutionstate spectra and unambiguously characterized by neutron diffraction in the solid state.The drastic geometric difference between the isomers (for example, 1.4 A for S vs. 2.2 A for NSbound) is responsible for the very high sensitivity of (X-POCOP)IrH 2 to the solvent environment.In particular, isotope effects on chemical shis proved a useful tool to probe the NS structure, due to the very high difference in chemical shis between the hydrides in NS.Thus, we established the rst example of a complex with temperature-dependent J HD , where two isomers were successfully isolated and characterized, and a good quality t of experimental data was obtained.A VT neutron diffraction study indicated only a minor r(H-H) change that could be associated with vibrational corrections.We also highlighted the role of specic solvation and noncovalent interactions for metal hydrides.The complexation of (X-POCOP)IrH 2 with C 6 F 5 I and CH 2 Cl 2 was characterized, including the rst neutron diffraction structure of a halogen bond involving hydrides.Although the binding energies are comparatively small (1-5kcal mol −1 ), they are responsible for the stabilization of the NS isomer through the formation of NSbound, which affects the span of d(IrH 2 ) and J HD in the presence of C 6 F 5 I and CH 2 Cl 2 , when compared to less interacting solvents such as toluene.
Computational methods supported the S/NS/NS-bound model chemistry.In the light of importance of solvation, we suppose that DFT calculations with the CPCM method provided overall satisfactory performance for non-specic solvation in the case of neutral complexes, even though the CPCM method minorly overestimated the effect of relative permittivity (and thus it may not in a perfectly precise way capture some of the ne features such as the geometry of NS and S-NS energy gaps).We note that the CPCM model is likely growing progressively less accurate upon charge buildup on the compounds studied, 64 so hydrides bearing multiple charges may be especially challenging.
As for specic solvation, such as noncovalent interactions with C 6 F 5 I and CH 2 Cl 2 , we found it essential to use DLPNO-CCSD(T) corrections to DFT energies, in order to obtain good agreement with the experimental data.Worth noting is that in addition to the thermodynamic data, the NMR parameters of some key structures were calculated with highly accurate methods and revealed good agreement with experiment.
Having successfully established the model with isomers for (X-POCOP)IrH 2 type hydrides with temperature-dependent J HD , we hypothesized that the most pronounced temperature dependence of J HD in other compressed dihydrides and elongated dihydrogen complexes is also explained by equilibria between two or more isomeric entities, which can be additionally discriminated by non-covalent interactions with solvent.In support of that hypothesis, a re-examination of the archetypical complex [Cp*Ru(dppm)(H 2 )] + found two minima on the PES with reasonable r(H-H) that allowed good t of the experimental data, especially given the limitations of the model for cationic complexes.Remarkably, unlike the calculated J HD , the calculated chemical shis for the two isomers almost coincide.
Looking at a broader picture, our attempt to access the "continuum" of H-H bond activation through incrementally decreasing the electron-deciency of the pincer ligand (Fig. 11) revealed that at least for some compounds, there are regions of "rigid" dihydrides and dihydrogen complexes, which are connected with an S-shaped region of "sensitive" non-classical hydride complexes.In this "sensitive" region, small external stimuli related to son-specic solvation, noncovalent interactions, packing effects, etc., would more likely produce noticeable changes in r(H-H), and as a consequence, in spectral parameters.The existence of isomers seems likely under these conditions.Since previous attempts to guess the nature of isomers were oen not precise, it worth listing why the isomers could be formed.This can occur due to: differential stabilization of two dihydride complexes by non-specic solvation due to e.g.different dipole moments (S and NS isomers), differential stabilization of the dihydrogen complex and a dihydride complex by non-specic solvation (dihydrogen and dihydride forms of [Cp*Ru(dppm)(H 2 )] + ), specic solvation/non-covalent interaction with solvent (formation of NS-bound via a dihydrogen bond with CH 2 Cl 2 and a halogen bond with IC 6 F 5 ), etc.To this one can add ion pairing, since several "non-classical" hydrides are known, which reveal different distances in the solid state, where contacts of between hydrides and a counter-anion could be found, and in solution (see above for discussion).Ion pairing must be to some extent present in solution as well, and such compounds might reveal temperature dependent spectra, should the combination of interaction strength and wideness of the observation window allow it.To conclude this part, it seems that "non-classical" hydrides that span approximately from 1.0 to 1.6 Å are especially sensitive to external stimuli.Due to various interactions with their environment and medium, such hydrides are likely to exhibit isomeric species and thus reveal temperature-dependent properties, including J HD .

Fig. 1
Fig. 1 Top: H-H bond activation continuum in transition metal hydrides.Bottom: a few examples of hydride complexes including those with temperature-dependent J HD , preferably where H-H bond distance determination via neutron diffraction has been done.

Fig. 2
Fig. 2 Crystal structure of the (p-H-POCOP)IrH 2 compound at 10, 100 and 295 K as measured with neutron diffraction, reporting also the H-H bond distance (top row: experimental and bottom row: corrected for libration).Ellipsoids are represented at the 50% probability level, and hydrogen atoms of the t Bu groups are omitted for clarity.

Fig. 3
Fig.3Structures that are involved in the description of (X-POCOP) IrH 2 in solution.TBP refers to trigonal-bipyramidal and SP to squarepyramidal.

Fig. 6
Fig.6The calculated 1 H and 31 P chemical shifts, as well as J HD and 2 J PH coupling constants in (p-H-POCOP)IrH 2 as a function of r(H-H).
P signal and −40.6 and 1.8 ppm for hydride signals (NS-bound-a).When a stronger halogen bond acceptor C 4 F 9 I is added in a larger amount, initially a low-eld shi is observed in 1 H NMR spectra upon cooling (Fig. 7).At ca. −30 °C the 1 H signals become so broad that they are nearly indistinguishable from the baseline.Upon further cooling signals reappear at −7.76 (IrH(IC 4 F 9 )H) and −22.06 ppm (IrH 2 /IC 4 F 9 ).Neither resonance shis below −60 °C, meaning that there is no free (p-MeOOC-POCOP)IrH 2 , and only adducts IrH(IC 4 F 9 )H and IrH 2 /IC 4 F 9 in a slow equilibrium are present.Based on changes in the ratio between these two, thermodynamic data can be extracted through the van't Hoff plot (DH = −5.7 ± 0.4 kcal mol −1 and DS = −25 ± 2 cal × mol −1 K −1 ) (Fig. S19 †).Also, following the IrHD signal in IrH 2 /IC 4 F 9 allows the measurement of the preference of deuterium to occupy the apical site (DH = −0.27± 0.01 kcal mol −1 and DS = −0.37 ± 0.06 cal × mol −1 K −1 ; Fig. S12 †), which agrees very well with both tted and DFT calculated values for C 6 F 5 I (Table S8 †).
2 and provided DH = −3.0kcal mol −1 and DS = −12.6 cal × mol −1 K −1 .Also, the tting of d(IrH 2 )-T dependence in the presence of a constant amount of C 6 F 5 I (S/NS/NS-bound model) was attempted for all X groups and provided comparable results, with the trend resembling the calculated one (see Fig. S16 † for d(IrH 2 )-T and van't Hoff plots).Measurements for (PCP)IrH 2 were precluded by a rapid reaction with C 6 F 5 I at rt and the formation of the IrH(C 6 F 5 I)H adduct at low temperatures (consistently, the calculated DH of formation of IrH(C 6 F 5 I) H is −7.4 kcal mol −1 for (p-H-POCOP)IrH 2 and −11.8 kcal mol −1 for (PCP)IrH 2 ).
†), with both compounds having a low AN.Therefore, we re-iterate that in the given examples the AN, not the DN linked to O and Cl lone pairs, is correlated to the appearance of spectra.Computationally, S structures are virtually unchanged in CH 2 Cl 2 , while NS has longer r(H-H) and lower energies, i.e. for NS-(p-H-POCOP)IrH 2 r(H-H) is 1.63 Å in toluene and 2.05 Å in CH 2 Cl 2 , and the S-NS gap is −0.06 and −0.4 kcal mol −1 , respectively (D3BJ-revPBE).
42 isomers almost coincide (calc.−7.2 and −7.6 ppm; exp.−6.7 ppm 16b ) and explain the lack of signicant d(H 2 )-T dependence.At the same time, the calculated J HD values reveal a considerable difference (18.8 and 7.2 Hz; values from correlations are 23 and 9.2 Hz).

Fig. 11 H
Fig. 11 H-H distance in ( Me4 X-PCP-F n )IrH 2 as a function of the number of F atoms in the ligand, probing the "continuum" of H-H activation.Dashed line represents classical regions where r(H-H) has linear low-slope dependence on the number of F atoms.Solid line represents S-shaped non-classical regions with r(H-H) being very sensitive to minor changes.

Table 1
Solvent effect on the NMR spectra of (p-H-POCOP)IrH 2 ; data at 0 °C for CH 2 Cl 2 and 25 °C for other solvents Poorly resolved.dDN refers to the donor number.eAN refers to the acceptor number.fRef.46.
a Ref. 44. b Not determined.c g Ref. 47.