though thousands of 
 transition - metal hydride complexes are known , 
 relatively few of them have unpaired electrons . even fewer have metals with a high - spin electronic configuration , 
 since hydride is a strong - field ligand and since hydride complexes 
 are often supported by strong - field ancillary ligands . in a series 
 of publications
, we described exceptional di(-hydrido)diiron(ii ) 
 complexes supported by bulky -diketiminate ligands ( chart 1 ) . in these complexes ,
the iron(ii ) ions had high - spin electronic configurations 
 because of the low metal coordination number and the -donor 
 character of the anionic -diketiminate .
all of the iron(ii ) 
 examples were dimers in the solid state , and the two pseudotetrahedral 
 metal centers were bridged by hydride ligands ( chart 1 , upper left ) . with the bulkiest -diketiminate ( l ) ,
the dimer dissociated in solution to 
 give a three - coordinate monomer , as shown by a combination of magnetic , 
 spectroscopic , and kinetics studies .
the high - spin electronic configuration of the complexes 
 presents 
 characterization challenges that are distinctive to paramagnetic species . 

namely , the resonances in the h nmr spectra are broadened 
 and highly shifted , and these chemical shifts do not correlate with 
 structure in the manner that is familiar from diamagnetic complexes .
the relaxation of h nuclei directly 
 bonded to the paramagnetic metal is particularly rapid , and to our 
 knowledge no metal - bound h nuclei have been detected in 
 nmr spectra of hydride complexes with a paramagnetic ground state .
another challenge is that the paramagnetic hydride complexes in 
 chart 1 are highly reactive : for example , they 
 cleave b 
c bonds and reductively 
 eliminate h2 with light or with added ligands including n2 . though 
 the high reactivity of the hydrides makes them difficult to handle , 
 their reactivity is also an opportunity to form new c  h bonds , 
 because the m 
h bond weakening 
 as a result of the partial population of fe  h  * orbitals 
 in the high - spin d electronic configuration .
one of the important reactions of coordinatively unsaturated metal - hydride 
 complexes is the exchange of the hydride hydrogens with free h2 .
this reaction has biological 
 relevance because of the h / d exchange of h2 protons with 
 solvent protons in nitrogenase enzymes , which occurs only in the presence of n2 .
this 
 specificity has been used to suggest that hydride species are key 
 intermediates during n2 reduction .
very recently , deuterium atoms from d2 were incorporated 
 into ethylene produced from acetylene reduction by nitrogenase .
however , the mechanisms are difficult to evaluate 
 without  model  studies on well - characterized synthetic 
 hydride complexes , particularly with iron .
unfortunately , the aforementioned inability to observe resonances 
 for h and d bound to a metal has so far prevented the monitoring of 
 h / d exchange in paramagnetic hydride complexes . in this article , 
 we describe synthetic and characterization advances 
 for two high - spin iron(ii)-hydride complexes , supported by the -diketiminate 
 ligands l and l ( chart 1 ) . in an interesting twist , these studies benefit 
 from unusually large ( up to 5.7 ppm ) isotope effects on the h nmr chemical shifts of the distant protons upon hydride deuteration , 
 a phenomenon that occurs only in the dimeric hydride 
 complexes in our system .
this tool for differentiating isotopologues 
 enables the revision of the h nmr assignments in two previously 
 reported hydride complexes .
in addition , this is the only way to distinguish 
 hydride from deuteride isotopologues , and this discovery enables us 
 to monitor h / d exchange in this system for the first time .
the results 
 show that the rates of h / d exchange between hydride complexes , and 
 between these hydride complexes and h2 , are greatly influenced 
 by the size of the supporting ligand .
mechanistic considerations lead 
 to new insights into the distinctive reactivity of high - spin hydride 
 complexes .
the 
 synthesis of [ lfe(-d)]2 ( 1-d2 ) was reported previously , 
 and initial characterization of 1-d2 by h nmr spectroscopy in c6d6 suggested that the deuteride and the hydride complexes had identical h nmr spectra .
however , closer 
 investigation has shown that the resonances have different chemical shifts in the different isotopologues .
a h 
 nmr spectrum of an equimolar mixture of 1 and 1-d2 in c6d6 showed that 
 the differences were not from temperature or medium effects ( figure 1 ) .
close examination of the h nmr spectrum 
 revealed several envelopes of nearby resonances , and the components 
 of each envelope had a 1:2:1 ratio of integrations .
for example , resonances 
 at  37.2 ( resonance assigned to 1 ) , 40.1 , 
 and 43.0 ( resonance assigned to 1-d2 ) ppm were observed in a 1:2:1 ratio , rather than the 
 single resonance at  37.2 ppm in 1 .
when 
 this experiment was repeated , starting from a different mixture of 
 isotopologues that contained more 1 than 1-d2 , the h nmr spectrum showed 
 the same number of resonances , but the integrations were no longer 
 1:2:1 and favored the hydride resonance at  37.2 ppm . 

the resonances located between 1 and 1-d2 are most reasonably assigned to { lfe}2(-h)(-d ) ( 1-d ) .
these experiments also indicate that mixing of 1 and 1-d2 rapidly yields 
 an equilibrium mixture of 1 , 1-d , and 1-d2 .
the difference between the chemical 
 shifts of distant protons in different isotopologues has been described 
 previously in a number of paramagnetic complexes , and theopold has termed this paramagnetic isotope effect 
 on chemical shift ( piecs ) .
this particular 
 sample has a 7% impurity of the oxo complex 
 { lfe}2(-o ) .
peaks 
 are marked as follows : dimer d , monomers m , oxo impurity i , and solvent 
 and solvent impurities s. ( b ) h nmr spectrum from mixing 
 equimolar amounts of [ lfe(-h)]2 ( 1 ) and [ lfe(-d)]2 ( 1-d2 ) in c6d6 for 45 min .
all three isotopologues of the dimer ( h / h , 
 h / d , and d / d ) are visible in ( b ) , as several groups of three nearby 
 peaks in a statistical 1:2:1 ratio .
only the parts of the spectra 
 from  45 to 70 ppm are shown for clarity . the discovery that 1 , 1-d , and 1-d2 exhibited piecs enabled 
 the use 
 of h nmr spectroscopy to accurately determine the amount 
 of deuterium incorporation into 1-d2 .
this was done by comparing the integrations of the three 
 isotopologues in the h nmr spectrum .
compound 1-d2 typically had greater than 90% deuterium 
 incorporation into the hydride positions , which is consistent with 
 the level of deuteration previously reported .
( in earlier studies 
 this determination was done indirectly , using mass spectrometric analysis 
 of 3-hexene - d1 generated from treating 1-d2 with 3-hexyne and then acid . )
note that piecs is not observed for some 
 of the peaks in the spectrum . 

seven of these peaks are assigned to monomeric lfeh , which is in equilibrium with 1 , as previously 
 shown .
this is the number of resonances 
 expected for l in an environment having c2v symmetry , when the n - aryl bonds have hindered rotation that makes the two methyl 
 groups of the isopropyl substituents inequivalent . by process of elimination
, 
 the piecs of the peaks of 1 enabled the assignment of 
 18 resonances each to the 1 , 1-d , and 1-d2 isotopologues of the dimers , 
 giving 54 resonances in total .
therefore , the h nmr spectrum 
 of isotopically pure 1 contains 25 resonances , where 
 7 peaks may be assigned to lfeh and 
 18 peaks may be assigned to the dimer .
it is notable that h nmr spectra of a mixture of the 
 three isotopologues always showed some additional resonances that 
 neither exhibit piecs nor can be assigned to the monomer lfeh .
comparison 
 with literature h nmr spectra indicated that these additional 
 resonances were associated with a persistent impurity , { lfe}2(-o ) , which can come from 
 the reaction of 1 with trace h2o to give { lfe}2(-o ) .
we were not able to completely avoid or eliminate 
 the oxo impurity , but careful handling gave samples of 1 that were below 10% oxo impurity , with typical samples between 4 
 and 8% .
the oxo impurity in 1-d2 was typically higher ( around 20% ) , due to the multiple d2 additions necessary for full deuterium incorporation .
the zero - field 
 mssbauer spectrum of solid 1 at 80 k was previously 
 reported to have  = 0.59 mm / s and |eq| = 1.58 mm / s .
however , the 
 published data were reexamined after the discovery of the persistent 
 oxo impurity .
the oxo impurity was modeled using parameters that were 
 fixed at literature values of  = 0.64 mm / s and |eq| = 1.42 mm / s , while the amount of the oxo impurity and the parameters of the major 
 component were refined to give the best fit to the data .
complex 1 was determined to be 71% of the earlier sample , and the 
 major component had  = 0.57 mm / s and |eq| = 1.63 mm / s . a new sample of 1 with < 5% 
 oxo impurity by nmr spectroscopy yielded a zero - field mssbauer 
 spectrum at 80 k with parameters  = 0.58 mm / s and |eq| = 1.62 mm / s , which are the same within the 
 experimental uncertainty of 0.02 mm / s .
the zero - field 
 mssbauer spectrum of solid 1-d2 at 80 k is shown in figure 2 .
the slightly 
 asymmetric two - line pattern was modeled with 
 a two - component fit , of which the major component was found to have 
  = 0.58 mm / s and |eq| = 
 1.74 mm / s , accounting for 79% of the sample .
we assign this subspectrum 
 to compound 1-d2 , whereas the 
 second doublet was fixed to the properties of the oxo impurity (  
 = 0.64 mm / s and |eq| = 1.42 mm / s ) . 

the isomer shifts of 1 and 1-d2 are identical , but there was a variation of 0.12 mm / s 
 in the quadrupole splitting values .
the reason for the difference 
 in the quadrupole splitting is unknown at this time .
unfortunately , 
 attempts to fit the variable - field mssbauer spectra of 1 and of 1-d2 did not 
 yield a comprehensible model that gave additional insight .
the isomer 
 shifts of 1 and 1-d2 lie in the range of  0.470.90 mm / s observed in other 
 high - spin fe(ii ) diketiminate complexes .
zero - field 
 mssbauer spectrum of [ lfe(-d)]2 ( 1-d2 ) recorded at 80 
 k. the signal with  = 0.58 mm / s and 
 |eq| = 1.74 mm / s accounted for 
 79% of the sample .
the blue line represents the contribution of the 
 oxo impurity , the green line represents the contribution of 1-d2 , the red line represents the 
 sum , and the black circles are the data .
we now shift to iron(ii ) complexes of the smaller 
 diketiminate ligand l , which we have previously derived 
 from the iron chloride precursors lfe(-cl)2li(thf ) ( thf = tetrahydrofuran ) and [ lfe(-cl)]2 .
however , the relative ease 
 of removing br salts encouraged us to prepare lfebr(thf ) 
 ( 2 ) in 82% yield from lk and febr2 . during the preparation of our work , compound 2
was 
 reported by tonzetich and lippard , using 
 a very similar prepatory method with lna .
our characterization 
 of 2 by nmr and x - ray diffraction ( supporting information ) is indistinguishable from the literature .
the zero - field mssbauer 
 spectrum of 2 at 80 k ( figure s-1 , supporting information ) has a doublet with  = 0.89 
 mm / s and |eq| = 2.36 mm / s , which 
 is nearly identical to that of lfe(-cl)2li(thf ) .
thf was removed from 2 by dissolving it in noncoordinating solvents ; this gave 
 [ lfe(-br)]2 , which precipitated from 
 solution as an orange powder .
tonzetich and lippard also reported this behavior , and they reported 
 that lfebr(thf ) had different electronic absorption spectra 
 in thf versus toluene , which resulted from the formation of [ lfe(-br)]2 .
here , the mssbauer spectrum of orange [ lfe(-br)]2 derived from 2 was recorded at 80 k ( figure 
 s-2 , supporting information ) .
one quadrupole 
 doublet with  = 0.91 mm / s and |eq| = 2.64 mm / s was observed . the isomer shift of [ lfe(-br)]2 is the same as 2 , but the 
 quadrupole splitting is larger , consistent with a slightly different 
 geometry at iron
. the successful 
 synthesis of 
 [ lfe(-h)]2 from addition 
 of h2 to an iron(i ) source prompted 
 us to use this synthetic method for an improved synthesis of [ lfe(-h)]2 and [ lfe(-d)]2 .
however , the order of addition of reagents was important , 
 as reduction of 2 with kc8 in et2o followed by h2 addition yielded many unidentified resonances 
 in the crude h nmr spectrum .
this suggests that the transient 
 iron(i ) species formed by kc8 reduction rapidly decomposes 
 in the absence of a trap .
therefore , 
 a degassed solution of 2 in thf was exposed to 14 equiv 
 of h2 gas prior to addition of kc8 .
after 3 h , volatile materials were removed from the brown 
 reaction mixture , and 3 was isolated in 56% yield following 
 workup .
the identity of 3 was established by comparing 
 its h nmr spectrum to the spectrum reported in the literature .
again , the iron - bound hydrogen atoms are not 
 visible by h nmr spectroscopy due to close proximity to 
 the paramagnetic iron atoms .
the deuterated isotopologue 3-d2 was synthesized using the above 
 method with d2 gas in 58% yield .
the h nmr 
 spectrum of 3-d2 ( figure 3 ) revealed that 3 and 3-d2 exhibited piecs as with the hydride dimers 
 described above .
thus , h nmr spectroscopy could similarly 
 be used to determine the amount of deuterium incorporated into the 
 hydride ligands .
the bridging 
 ligands in samples of 3-d2 were 
 typically greater than 90% deuterated , as judged by h 
 nmr spectroscopy .
h nmr spectra of ( bottom ) [ lfe(-h)]2 ( 3 ) and ( top ) [ lfe(-d)]2 ( 3-d2 ) in c6d6 . in the h 
 nmr spectrum of 3 at room temperature , 

only four paramagnetically shifted peaks were observed , a number that 
 is well short of the seven resonances expected for 3 in 
 a dimeric structure with d2d or d2h symmetry . 

in addition , the integrations of the peaks are 
 inconsistent with the original assignments for the h nmr 
 spectrum of 3 , so further 
 investigations were pursued .
h nmr spectra of 3 between 26 and 85 c in c6d6 are shown 
 in figure 4a .
the peak at  13 ppm at 
 26 c corresponds to the backbone methyl and meta - aryl protons , two resonances that are only distinct above 60 c 
 ( a close - up is shown in figure s-3 , supporting 
 information ) .
a previously undetected resonance for isopropyl 
 methyl groups integrating to 24 protons ( which had been hidden under 
 the residual benzene at room temperature ) became visible above 40 
 c .
the other resonance for isopropyl methyl protons , a broad 
 peak at  25 ppm in the 26 c spectrum , sharpened 
 at elevated temperatures . finally , a new broad resonance for the isopropyl 
 methine protons at  1.5 ppm was observed above 70 c . 

this resonance is broadened into the baseline at room temperature , 
 explaining why it had not been identified in previous studies .
the 
 remaining resonances at  24 and 56 ppm corresponded 
 to the para - aryl protons and the backbone protons , 
 respectively , completing the catalogue of resonances with the expected 
 integrations ( see experimental section ) .
( a ) variable - temperature h nmr spectra of [ lfe(-h)]2 ( 3 ) between 26 and 85 c 
 in c6d6 .
( b ) variable - temperature h nmr spectra of 3 between 90 and 20 c 
 in toluene - d8 .
the asterisks indicate 
 the resonance that splits with a coalescence temperature of 0 c .
the appearance and sharpening 
 of resonances in the high - temperature h nmr spectra of 3 suggested the possibility 
 of a fluxional process in solution .
therefore , low - temperature h nmr spectra were measured between 90 and 20 c 
 in toluene - d8 ( figure 4b ) .
the broad isopropyl methyl resonance that appeared at 
  25 ppm in the h nmr spectrum of 3 at 20 c split into two broad resonances at  
 27.6 and 48.3 ppm in the h nmr spectrum 
 at 75 c .
these resonances moved together and became 
 broader as the sample was warmed , with a coalescence temperature of 
 0 c .
the barrier for this fluxional process , assuming that the 
 resonances at 75 c are in the slow - exchange limit , is 
 g = 10.5 
 kcal / mol .
decoalescence of other peaks 
 was not observed within this temperature range , likely because there 
 was a smaller difference between the frequencies in the slow - exchange 
 limit .
the 
 purity and electronic structure of solid 3 and 3-d2 were evaluated using mssbauer 
 spectroscopy .
the zero - field mssbauer spectra of solid 3 and 3-d2 at 80 k are 
 shown in figure 5 .
compound 3 had 
  = 0.51 mm / s and |eq| = 
 2.05 mm / s .
the deuterated isotopologue 3-d2 had an indistinguishable mssbauer spectrum , 
 with  = 0.51 mm / s and |eq| = 2.10 mm / s .
there were impurities of 13 and 6% , respectively , 
 which are discussed in detail in the supporting 
 information .
one impurity doublet in each case has parameters 
 similar to those in the literature iron(i ) benzene compound lfe(-c6h6 ) , which has  = 0.70 mm / s and |eq| = 0.74 mm / s ( figure s-4 , supporting information ) .
we also note that this nearly nmr - silent 
 impurity was the major species in the mssbauer 
 spectrum previously attributed to 3 .
the 
 black circles are the data , and the red lines represent the sums of 
 a major doublet for 3 ( green ) and impurities ( blue , purple ) 
 that are discussed in the supporting information . as reported above , a 1:2:1 
 mixture of 1 , 1-d , and 1-d2 was obtained upon mixing an equimolar mixture 
 of 1 and 1-d2 in 
 c6d6 , and piecs enabled all three isotopologues 
 to be distinguished in the paramagnetic h nmr spectrum . 
 equilibrium was established within 45 min at room temperature .
an 
 analogous experiment was performed using an equimolar solution of 3 and 3-d2 maintained 
 at 30 c in c6d6 .
a 1:2:1 mixture of 3 , { lfe}2(-h)(-d ) ( 3-d ) , and 3-d2 was observed 
 after 2 h. this shows that the inability of 3 to form 
 monomeric lfeh in solution does not hinder hydride exchange between isotopologues .
the 
 exchange of hydride and deuteride ligands with d2 and h2 was also examined .
compound 3 was treated with 
 an excess of d2 ( 1 atm ) in c6d6 to 
 give a mixture of 3-d and 3-d2 .
when this mixture was treated 
 with fresh d2 , the equilibrium was pushed all the way to 
 the fully deuterated isotopologue , 3-d2 ( figure 6b ) .
two treatments with h2 caused the sample to 
 revert to 3 in quantitative yield ( figure 6c , d ) , showing that the exchange is reversible .
in contrast 
 to the immediate exchange in 3 , treatment of 1 in c6d6 with d2 ( 1 atm ) produced 
 the deuterated isotopologue only after much longer amounts of time . 

compound 1-d2 was finally observed 
 in quantitative yield after 50 h at room temperature .
hydride exchange 
 between h2 and 1-d2 is reversible , as treatment of 1-d2 with h2 ( 1 atm ) produced 1 in quantitative 
 yield under the same conditions and time .
treatment of 1 with 8 atm of d2 was faster but required 24 h to yield 1-d2 in quantitative yield .
h nmr spectra of [ lfe(-h)]2 ( 3 ) , { lfe}2(-h)(-d ) 
 ( 3-d ) , and [ lfe(-d)]2 ( 3-d2 ) isotopologues in c6d6 during gas exchange .
the columns on the right indicate 
 the order and type of gas that was added to give the observed spectrum .
this compound is much less reactive than the iron analogues described 
 above ; for example , it does not react with alkenes or lewis bases .
the low reactivity was attributed to the greater 
 stability of the dimer and/or to the decreased lability of the co  h 
 bonds .
therefore , it was interesting to evaluate the analogous cobalt 
 compounds for intermolecular h / d exchange . a sample of [ lco(-d)]2 was prepared 
 from [ lco(-f)]2 and 
 et3sid , using a method analogous to that used to synthesize 
 the protiated analogue .
several peaks 
 were shifted by 0.20.5 ppm from those in [ lco(-h)]2 , as verified by spiking the sample 
 with an equimolar amount of [ lco(-h)]2 ( figures s-6s-8 , supporting information ) .
thus , this dimer also exhibits piecs , though the shifts are not 
 as pronounced as in the iron species described above .
heating the 
 mixture of [ lco(-h)]2 and [ lco(-d)]2 to 
 80 c for 12 h gave no change in the nmr spectrum , indicating 
 that there is no significant exchange of hydrides between complexes 
 in this time frame .
this contrasts with 
 the iron analogues ( table 1 ) and is consistent 
 with the idea that the cobalt(ii ) hydride dimer does not break up 
 in solution .
the mssbauer spectra 
 of 1 and 3 have similar isomer shifts ,  
 = 0.59 and 0.51 mm / s , which are consistent with the values observed 
 in other high - spin iron(ii ) -diketiminate complexes .
high - spin , tetrahedral iron(ii ) sites in iron - sulfide clusters have 
 similar isomer shifts (  = 0.60.7 mm / s ) .
low - spin octahedral iron(ii ) sites have very 
 different isomer shifts in the range of  = 0.30.45 
 mm / s .
the intermediate - spin ( s = 1 ) iron(ii ) hydride complex [ fe(dppe)2h ] has  = 0.23 mm / s .
the 
 much higher isomer shifts in 1 and 3 strongly 
 support the assignment of 1 and 3 having high - spin fe(ii ) subsites and are consistent with the paramagnetic 
 shifts in the h nmr spectra .
we note that our assignment 
 of the mssbauer spectrum of 3 here replaces an 
 incorrect assignment we gave in an earlier paper ; the previous spectrum actually corresponds to lfe(arene ) , which can be formed during the synthesis of 3 when arenes are present .
the h nmr spectra of 1 and 3 were found to exhibit significant piecs , 
 as all the resonances shifted upon deuterium substitution .
the term 
 piecs was coined by heintz and theopold and has been reported in a number of complexes .
for example , deuteration of the hydrides in ( cp)4cr4(3h)4 ( cp 
 = -c5me4et ) gives changes in the chemical shifts of the cp 
 protons , though they are far removed from the bridging hydride ligands . 

most explanations for piecs are based on the shorter bond lengths 
 to d versus h. the heintz / theopold study 
 is particularly relevant to our complexes because of the presence 
 of bridging hydrides : shorter m 
m distance and the exchange coupling , which in turn would 
 influence the magnetic susceptibility and thus the chemical shift 
 of the protons .
however , other explanations 
 have been advanced for other cases of piecs : for example , differential 
 m 
h / m  d bond energies could influence the ligand - field 
 splitting slightly , which in turn could influence the paramagnetic 
 shift . however , we saw no evidence for any sizable 
 change in the ligand field of iron upon deuteration , because the quadrupole 
 splittings were the same within error between 3 and 3-d2 , and only slightly different 
 ( eq = 0.12 mm / s ) between 1 and 1-d2 .
the 
 piecs in the h nmr spectra of 1 and 3 varied from a negligible change in some resonances up to 
 5.7 ppm in others .
importantly , piecs was observed only for the dimeric form of 1 , as the monomer lfe(h / d ) had the same chemical shifts in the h nmr spectrum for both isotopologues .
this observation supports 
 the hypothesis that the piecs is connected to changes in the fe 
a significant finding of this study is that variable - temperature h nmr spectroscopy , together with piecs , was essential for 
 defining the correct h nmr assignments of 1 and 3 .
compound 3 was particularly vexing , because almost half of the seven 
 expected resonances were masked .
one resonance was hidden under the 
 residual solvent , another apparent resonance was actually two resonances 
 with the same chemical shift , and one was broadened into the baseline 
 at room temperature and was broad even at elevated temperatures .
however , 
 variable - temperature studies enabled us to finally assign the resonances 
 for 3 with confidence .
examination of the variable - temperature h nmr data 
 for 3 also revealed a fluxional process in solution with 
 a barrier of g = 10.5 kcal / mol at 0 c .
we tentatively assign the low - temperature 
 structure to be similar to the solid - state structure , in d2 symmetry with the diketiminate 
 planes perpendicular to one another . in this case
, the barrier would 
 correspond to the energy required to reach the d2h symmetric conformation in which the n2c3fe planes are coplanar ( scheme 1 ) and through which two d2 isomers 
 of different chirality can interconvert . meanwhile , piecs was vital to the assignment of all resonances 
 in the h nmr spectrum of 1 , which is complicated 
 by the presence of both monomer and dimer .
in addition , the crystallographic symmetry of the dimer in 1 is lower than that in 3 , because crowding gives 
 a boat conformation of the n2c3fe rings that 
 lowers the symmetry to idealized c2 .
we 
 observed 18 resonances in dimeric 1 , which is somewhat 
 less than the 21 predicted from c2 symmetry 
 in solution , implying that three of the resonances are lost to overlap 
 or broadness .
however , the number of peaks is significantly larger 
 than in 3 , indicating that the increased steric bulk 
 of the l ligand in 1 ( from 
 a buttressing effect ) prevented the fluxional process 
 that was observed in 3 .
the h nmr spectra 
 imply that the hydride ligands are oriented such that there is no 
 plane of symmetry relating the two l ligands .
overall , these studies show the usefulness of variable - temperature h nmr spectroscopy and piecs to decipher paramagnetic spectra 
 and solution structure despite multiple overlapping resonances .
compounds 1 and 3 undergo intermolecular hydride exchange with 
 their deuterated isotopologues . dissolving 1 and 1-d2 in c6d6 produced an equilibrium mixture containing dimeric 1 , 1-d , and 1-d2 , along with monomeric lfeh and lfed . in the iron(ii ) complexes , formation 
 of 1-d most likely results from equilibrium between monomeric 
 and dimeric 1 , which is slow on the nmr time scale but 
 rapid on the chemical manipulation time scale .
( the rate of dissociation 
 has been estimated to be 5  10 s at 288 k , based on kinetics studies of the reaction of 1 with alkynes . )
consistent with this 
 rate regime , equilibrium between the isotopologues of 3 is reached within 45 min at room temperature , as judged by h nmr spectroscopy . the hydride isotope exchange between 3 and 3-d2 can not follow 
 this process because 3 does not interconvert with monomer 
 in solution , as previously shown using kinetics studies on the reaction 
 with boranes
. two possible mechanisms 
 for hydride isotope exchange between 3 and 3-d2 are shown in scheme 2 .
the first is a concerted process , while the second requires rate - limiting 
 cleavage of one of the bridging hydrides to give a single terminal 
 hydride .
the hydride ligands in the terminal position could attack 
 another dimer to give exchange .
partial breaking of the dimer of 3 to give a terminal hydride , as in scheme 2b , was previously found to be the most reasonable mechanism 
 for the reaction of 3 with trialkylboranes , and so we consider this to be the most likely 
 possibility that is consistent with the combined studies on 3 .
it is also notable that the cobalt analogue , [ lco(-h)]2 , did not undergo h / d 
 exchange even with heating to 80 c .
its low reactivity in general 
 may be attributable to its inability to open one of the bridges , as 
 in scheme 2b for 3 , or both bridges , 
 as in 1 .
dissociation of 3 into monomers is inconsistent with earlier kinetics studies on the 
 reaction of boranes with 3 .
the exchange 
 of hydride complexes with h2 and d2 has been 
 studied in detail in the literature .
it 
 is well - established in mononuclear iron - hydride complexes . in each of these cases ,
an open coordination 
 site is required to bind h2 in an  binding 
 mode .
the oxidative addition of the h2 is not necessary , 
 because there can be direct h transfer from coordinated h2 to the hydride without changing the oxidation state at the metal .
it is interesting that the inability of 3 to form 
 monomeric lfeh in solution did not hinder hydride ligand 
 exchange with d2 .
equilibrium was established in less than 
 a minute after treatment of 3 with d2 . though 
 we can not determine the mechanism unambiguously , we can advance two 
 possible mechanisms .
first , the  opened  form of the 
 dimer has an open coordination site on one iron that could coordinate 
 h2 or d2 to make a transient side - on d2 complex that is well - situated to exchange with the hydride on the 
 other metal ( scheme 3a ) .
another potential 
 mechanism involves oxidative addition of h2 , either to 
 one metal ( giving a transient iron(iv ) on one side ) or to both metals 
 ( giving a diiron(iii ) species ) . the latter possibility is shown in 
 scheme 3b .
it is also relevant that compound 1 does 
 not undergo 
 facile hydride exchange with d2 , despite its ability to 
 form monomer in solution .
this required over 2 d at room temperature 
 under 1 atm of d2 and 1 d under 8 atm of d2 . 

the hindered reaction of 1 with d2 supports 
 the contention that h2/d2 exchange in these 
 hydride species does not proceed through a transient monomer .
it is possible that the reaction of 1 with d2 proceeded via the monomer at a significantly slower rate .
another 
 possibility is that the exchange goes through the dimeric form of 1 , but that the bulkier ligands hinder its ability to access 
 the reactive conformation .
dinuclear iron sites with bridging 
 hydride ligands have been studied 
 extensively as [ fefe]-hydrogenase models .
these diiron hydride complexes have been reported to undergo h / d 
 hydride ligand exchange with d2/h2 via photolysis . in these systems
, photolysis opens a coordination 
 site by dissociating co or cleaving a hydride bridge , and the h / d 
 hydride exchange requires days ( which contrasts with exchange in the 3/3-d2 system that occurs 
 in seconds ) .
h / d hydride ligand exchange 
 has also been reported using d2/h2o for hydrogenases , diiron complexes , other metal complexes , along with d as a deuterium source .
in addition , 
 nitrogenase can exchange d from d2o into h2 , 
 but only does so in the presence of n2 .
we suggest 
 that the mechanisms advanced above for hydride / d2 exchange 
 in 3 should be considered in nitrogenase : specifically , 
 bridging hydride species may play key roles in h2/d2 exchange in the femoco cluster .
d2 was utilized as a deuterium source to enable the 
 isolation and characterization of the low - coordinate iron deuteride 
 complexes [ lfe(-d)]2 and [ lfe(-d)]2 .
the mssbauer 
 spectra of these hydride complexes indicated that the metal centers 
 are high - spin iron(ii ) .
this effect was observed only in the 
 dimeric complexes , implicating the slightly smaller size of the m2(-d)2 core as the main cause of piecs .
piecs also enabled the correct h nmr assignments of 
 the hydride complexes , as well as the study of intermolecular hydride 
 exchange .
the exchange of hydrides between hydride complexes of the 
 largest supporting ligand is likely to occur through dissociation 
 of the dimers into monomers .
however , exchange of the hydrides with 
 added h2 occurs most rapidly with the smaller supporting 
 ligand , implicating diiron(ii ) hydrides as the key intermediates . 

more generally , these studies show that h nmr spectroscopy 
 can be a powerful tool for the study of paramagnetic iron hydride 
 complexes : not despite the paramagnetism , but because of the paramagnetism through the piecs effect .
all manipulations were performed 
 under a nitrogen atmosphere ( or argon atmosphere where specified ) 
 by schlenk techniques or in an m. braun glovebox maintained at or 
 below 1 ppm of o2 and h2o .
glassware was dried 
 at 150 c overnight , and celite was dried overnight at 200 c 
 under vacuum .
pentane , hexane , benzene , diethyl ether , and toluene 
 were purified by passage through activated alumina and q5 columns 
 from glass contour co. ( laguna beach , ca ) .
all solvents 
 were degassed by removing a small amount of solvent under reduced 
 pressure prior to argon glovebox entry .
toluene - d8 and thf - d8 were vacuum - transferred 
 from sodium benzophenone ketyl solutions and were stored over 3  
 molecular sieves . before use , an aliquot of each solvent was tested 
 with a drop of sodium benzophenone ketyl in thf solution .
ultrahigh - purity 
 h2 was purchased from air products , and d2 ( 99.8% 
 d ) was purchased from sigma - aldrich or cambridge isotope laboratories . 

lfecl , potassium graphite , [ lfe(-h)]2 , [ lco(-h)]2 , and lh were prepared by published procedures .
lk was prepared 
 using the published procedure , except 
 et2o was used as the solvent instead of toluene .
all resonances in the h nmr spectra are referenced 
 to residual protiated solvents : benzene ( 7.16 ppm ) , toluene ( 2.09 
 ppm ) , or thf ( 3.58 or 1.73 ppm ) . resonances were singlets unless otherwise 
 noted .
ir data were recorded 
 on a shimadzu ftir spectrophotometer ( ftir-8400s ) using a kbr pellet . 

uv  vis spectra were recorded on a cary 50 spectrophotometer 
 using schlenk - adapted quartz cuvettes with a 1 mm optical path length . 

gc - ms was performed using a shimadzu qp2010 system with electron impact 
 ionization .
elemental analyses 
 were obtained from the centc elemental analysis facility at the university 
 of rochester .
microanalysis samples were weighed with a perkinelmer 
 model ad-6 autobalance in a vac atmospheres glovebox under argon , 
 and their compositions were determined with a perkinelmer 2400 series 
 ii analyzer .
dimeric [ lfe(-h)]2 :  67.3 , 21.8 , 19.8 , 14.3 
 ( 18h , bu ) , 12.0 ( 18h , bu ) , 5.4 , 2.6 , 
 7.5 , 9.7 , 10.6 , 14.5 , 15.4 , 
 22.0 , 27.8 , 31.6 , 37.2 , 51.8 , 
 57.5 ppm . resonances in the dimers could not be assigned to 
 specific proton environments , because of overlap that prevented accurate 
 integration .
monomeric lfeh :  
 115 ( 1h , -h ) , 41.7 ( 18h , bu ) , 
 11.7 ( 4h , pr - ch or aryl m - h ) , 26.5 ( 12h , pr ch3 ) , 109 ( 4h , pr - ch or aryl m - h ) , 113 ( 2h , aryl p - h ) , 122 
 ( 12h , pr ch3 ) ppm .
the synthesis of [ lfe(-d)]2 relied on the same procedure as 
 the synthesis of 1 from h2 .
after 16 h , the headspace gases were removed , and fresh 
 d2 was added ; this process was repeated twice .
h nmr ( c6d6 , 25 c ) : [ lfe(-d)]2 :  73.0 , 23.0 , 21.0 , 14.3 
 ( 18h , bu ) , 13.0 ( 18h , bu ) , 6.4 , 1.7 , 3.9 , 8.0 , 10.6 , 
 16.9 , 17.5 , 25.1 , 29.6 , 33.0 , 
 43.0 , 50.2 , 55.0 ppm .
lfed :  115 ( 1h , -h ) , 41.7 ( 18h , bu ) , 11.7 ( 4h , pr - ch 
 or aryl m - h ) , 26.5 ( 12h , pr ch3 ) , 109 ( 4h , pr - ch or aryl m - h ) , 113 ( 2h , aryl p - h ) , 122 ( 12h , pr 
 ch3 ) ppm . the monodeuterated hydride dimer , 1-d , was also observed in solution as described above .
h 
 nmr ( c6d6 , 25 c ) :  70.2 , 22.4 , 
 20.5 , 14.3 ( 18h , bu ) , 12.5 ( 18h , bu ) , 5.9 , 2.1 , 6.6 , 7.7 , 
 10.6 , 15.7 , 16.5 , 23.6 , 28.7 , 
 32.3 , 40.1 , 51.0 , 56.2 ppm .
lk ( 3.215 g , 7.039 mmol ) was added to a flask with a 
 teflon pin closure and was dissolved in thf ( 75 ml ) to give a light 
 yellow solution .
anhydrous febr2 ( 1.532 g , 7.104 mmol , 
 1.01 equiv ) was added to the solution , which produced a red reaction 
 mixture .
the flask was sealed , and the mixture was heated at 70 c 
 for 16 h. the reaction mixture turned yellow in color upon heating . 

the yellow solution was concentrated to 20 ml , and pentane 
 ( 100 ml ) was added to precipitate additional insoluble material ( presumably 
 kbr ) , which was removed by filtration through celite .
the yellow solution 
 was concentrated to 5 ml , which resulted in the formation of a large 
 amount of yellow crystalline solid .
the supernatant was decanted , 
 and the crystalline yellow solid was washed with pentane ( 12 ml ) . 

additional product ( 1.328 g ) was collected from subsequent crystallizations 
 of the supernatant by layering with pentane and cooling to 45 
 c .
h nmr ( thf - d8 , 22 c ) :  18.6 ( 4h , aryl m - h ) , 4.9 ( 12h , pr ch3 ) , 8.7 ( 12h , pr ch3 ) , 12.3 ( br , 4h , pr - ch ) , 39.9 
 ( 2h , aryl p - h ) , 67.3 ( 6h , backbone ch3 ) , 78.7 ( 1h , -h ) ppm .
ir ( kbr ) : 3058 ( w ) , 2964 ( s ) , 2928 ( s ) , 1529 ( s ) , 1459 ( s ) , 
 1437 ( s ) , 1388 ( vs ) , 1316 ( s ) , 1261 ( m ) , 1176 ( m ) , 1100 ( m ) , 1057 
 ( w ) , 1022 ( m ) , 935 ( m ) , 899 ( w ) , 872 ( m ) , 855 ( m ) , 795 ( s ) , 758 ( s ) 
 cm .
vis ( thf ) : 333 ( 21.2 mmcm ) , 433 ( sh , 0.9 mmcm ) nm .
calcd for c33h49n2febro : c , 63.36 ; h , 7.91 ; n , 4.48 . found : c , 63.13 ; 
 h , 8.10 ; n , 4.31% . in an ar - filled 
 glovebox , lfebr(thf ) ( 703 mg , 1.12 mmol ) was dissolved 
 in thf ( 20 ml ) to give a yellow solution , which was added to a small 
 three - neck round - bottom flask with a stir bar . on a vacuum line ,
h2 ( 1 atm ) was added to a bulb ( 297.5 ml , 12.4 mmol , 11 equiv ) , 
 and the bulb was brought into the glovebox .
kc8 ( 182 mg , 
 1.35 mmol , 1.2 equiv ) was added to a solid addition tube .
the three - neck 
 flask was attached to the volume bulb , a vacuum adapter , and the solid 
 addition piece .
the reaction apparatus was degassed under reduced 
 pressure until a small amount of thf had been removed .
then , the apparatus 
 was backfilled with h2 by slowly opening the stopcock of 
 the volume bulb .
kc8 was added to the stirring solution 
 by inverting the solid addition piece , which immediately produced 
 a dark green reaction mixture .
after 3 h , the mixture was filtered through 
 celite , and the volatile components were removed under reduced pressure . 

the resulting brown residue was dissolved in toluene ( 35 ml ) and was 
 filtered through celite to remove additional insoluble material .
toluene 
 was removed under reduced pressure to give a brown powder , which was 
 washed with cold pentane ( 45 c , 10 ml ) .
the solid was 
 dried under reduced pressure to give 263 mg of brown powder .
the pentane 
 wash was concentrated to 3 ml and was layered with hexamethyldisiloxane 
 ( 4 ml ) . cooling to
h nmr 
 ( c6d6 , 25 c ) :  13.0 ( 12h + 8h , 
 backbone ch3 and aryl m - h ) , 7.1 ( 24h , pr ch3 ) , 24.0 ( 4h , aryl p - h ) , 24.8 ( br , 24h , pr ch3 ) , 55.9 ( 2h , -h ) ppm .
the pr - ch protons were not observed at this temperature 
 ( see text ) .
the deuterated isotopologue of 3 , 3-d2 , was synthesized using the same method 
 with d2 , in 58.1% yield .
a wilmad 
 522-pv-7 pressure nmr tube with a 5 mm outer diameter ( od ) and a maximum 
 pressure rating of 200 psi was used for all high - pressure gas experiments . 

the tube comes equipped with a wilmad pv - anv valve that is capable 
 of accepting a swagelok 1/8 tubing nut and ferrule .
poly(tetrafluoroethylene ) 
 ( ptfe ) tubing ( od = 1/8 ) and swagelok 1/8 tubing nuts 
 and ferrules were used for all the connections .
ptfe tubing was used 
 to connect the gas regulator to a t - shaped splitter , which provided 
 two paths .
one path connected through ptfe tubing a high - pressure 
 gas gauge and the pv - anv valve on the nmr tube .
the second path connected 
 the ptfe tubing to a swagelok valve , which was connected to a metal 
 o - ring joint .
the other part of the o - ring joint was equipped with 
 a metal - to - glass flange that had a glass 14/20 female joint .
the minimum experimental line width was 0.24 mm / s ( full width at half - height ) . 

isomer shifts are quoted relative to iron metal at 300 k. 
 the zero - field spectra were simulated by using lorentzian doublets .
 very 
 few hydride complexes are known in which the metals have a 
 high - spin electronic configuration . 
 we describe the characterization 
 of several high - spin iron(ii ) hydride / deuteride isotopologues and 
 their exchange reactions with one another and with h2/d2 . though the hydride / deuteride signal is not observable in nmr spectra , 
 the choice of isotope has an influence on the chemical shifts of distant 
 protons in the dimers through the paramagnetic isotope effect on chemical 
 shift . 
 this provides the first way to monitor the exchange of h and 
 d in the bridging positions of these hydride complexes . 
 the rate of 
 exchange depends on the size of the supporting ligand , and this is 
 consistent with the idea that h2/d2 exchange 
 into the hydrides occurs through the dimeric complexes rather than 
 through a transient monomer . 
 the understanding of h / d exchange mechanisms 
 in these high - spin iron hydride complexes may be relevant to postulated 
 nitrogenase mechanisms . 