unless otherwise stated , all reactions 
 were performed under a dinitrogen 
 atmosphere using either a controlled - atmosphere glovebox or schlenck 
 line techniques . 1,2-bis(diphenylphosphino)ethane ( dppe ) was purchased 
 from strem chemicals and used without further purification . crystalline 
 bis(trifluoro - methanesulfonyl)imide was purchased from acros organics 
 and used without further purification . all solvents were dried and deoxygenated 
 by purging with dry dinitrogen gas for 15 min before passing through 
 packed columns of activated alumina and activated copper . after synthesis , materials were stored under dry argon in an atmosphere - controlled 
 glovebox until their use in testing . solution - state nmr spectra 
 were recorded on a varian 300 mhz instrument 
 with h shifts reported relative to the residual solvent 
 peak , and p peaks reported relative to 85% h3po4 . deuterated benzene was purified by vacuum distillation 
 from a sodium / benzophenone solution before use . deuterated bromobenzene 
 was dried over cah2 and vacuum distilled before use . anhydrous fecl2 ( 319.8 
 mg , 2.523 mmol ) was suspended in 2 ml of tetrahydrofuran ( thf ) . a 
 solution of 1,2-bis(diphenylphosphino)ethane ( 2.0052 g , 5.033 mmol ) 
 in 6 ml thf was added , and the suspension was stirred until a milky 
 white precipitate formed . nabh4 ( 220.0 mg , 5.680 mmol ) 
 in 8 ml of dry ethanol was then added , producing strong outgassing 
 and causing all components to immediately dissolve to form a dark 
 red solution . the solution was stirred for 3 h during which outgassing 
 continued and a yellow powder precipitated . the remaining solution 
 was decanted from the precipitate , and the precipitate was dissolved 
 in benzene and filtered over celite to remove salt impurities . removal 
 of the solvent in vacuo produced the product as a fine yellow powder . h nmr ( 300 mhz ) in c6d6 :  12.76 ( quad . , 2h ) ; 1.80 ( s , 
 4h ) ; 2.14 ( s , 2h ) ; 2.40 ( s , 2h ) ; 6.44 ( s , 4h ) ; 6.587.12 ( m , 
 20h ) ; 7.147.80 ( m , 12h ) ; 8.67 ( s , 4h ) . p nmr 
 ( 300 mhz ) in c6d6 :  91.5 ( s ) ; 103.3 ( s ) . bis(trifluoromethylsulfonyl)imide 
 ( 327.8 mg , 1.166 mmol ) in 5 ml diethyl ether was added to a solution 
 of cis - feh2dppe2 ( 1.015 g , 
 1.188 mmol ) in 12 ml diethyl ether and allowed to stir for 1 h , forming 
 a light green precipitate . the precipitate was filtered over a medium 
 frit and washed twice with diethyl ether . removal of residual diethyl 
 ether in vacuo produced the product as a light green 
 powder . n2 coordination 
 products resulting from the reaction conditions were removed by exposure 
 of the product to 1 atm h2 gas at room temperature after 
 an initial round of thermal treatment as described below . h nmr ( 300 mhz ) in c6d5br :  12.61 
 ( s , broad , 1h ) ; 7.76 ( s , broad , 2h ) ; 2.13 ( s , 8h ) ; 7.167.51 
 ( m , 40h ) . p nmr ( 300 mhz ) in c6d5br :  92.0 ( s ) . [ feh(-h2)dppe2][ntf2 ] was placed under 
 vacuum and heated at 125 for 12 h. all as - synthesized samples 
 underwent one initial treatment under these conditions to remove n2 contaminants and the starting material was regenerated by 
 exposure of the product to 1 atm h2 gas at room temperature . the resulting blue - green powder was extremely reactive to 
 all available solvents , forming a red - purple solution on contact and 
 preventing nmr characterization . isotopically enriched 
 [ feh(-h2)dppe2][ntf2 ] and its thermal decomposition products were made 
 according to the procedures above . fecl2 was made by reacting 
 shavings of fe ( 0.13 mg , 0.228 mmol ) metal with excess 
 concentrated hydrochloric acid ( 1.0 ml , 12 m ) in 5 ml methanol . the 
 resulting yellow solution was reduced and dried at 160 under 
 vacuum for 8 h to produce the anhydrous starting material . dehydrated fecl22h2o and anhydrous fecl2 samples were 
 produced from this material by heating at 300 c under vacuum 
 for 6 h. organometallic samples were loaded into a custom - built , 
 o - ring sealed ptfe sample cell under an argon atmosphere . sealed samples 
 were stable for up to 24 h in air at room temperature . spectra were 
 recorded over six hours in the transmission configuration using a 
 gas - filled proportional counter and source radiation from the decay 
 of co in a rhodium matrix . as - received and dehydrated 
 fecl2 samples were sealed in a kapton sample cell with 
 kapton tape under an argon atmosphere and measured for 24 h in the 
 transmission configuration using a si solid - state detector . kinetic isotherms were recorded for 
 the thermal release of the h2 ligand from [ feh(-h2)dppe2 ] [ ntf2 ] using a 
 custom - built sieverts apparatus . solid [ feh(-h2)dppe2][ntf2 ] ( 257 mg ) was loaded into 
 a 14 ml stainless steel reactor under an argon atmosphere and sealed 
 before transfer to the instrument . swagelok vcr copper filter gaskets 
 with 2 m stainless steel filters were used to prevent loss 
 of the powdered sample from the reactor during measurements . after 
 transferring the reactor to the assembly , samples were allowed to 
 pump down overnight to 3.1  10 torr at the 
 pump inlet before measurements were performed . the rate of hydrogen 
 release was measured by placing the sample under vacuum conditions 
 for nominally 1 h at the desired temperature , followed by expansion 
 of hydrogen gas from a calibrated volume into the reactor . residual 
 gas mass spectroscopy was performed during the evacuation of the reactor 
 to identify the decomposition products . ideal gas mole balance was 
 used to determine the quantity reabsorbed after exposure of the degassed 
 sample to 0.5 atm hydrogen gas . time and pressure resolution of the 
 instrument were 500 ms and 5 torr , respectively . electronic structure calculations were 
 performed for the singlet state of the cationic fragments [ mh(-h2)dppe2 ] and [ mhdppe2 ] ( m = fe , ru , os ) , and the hydrogen molecule 
 using the gamess - us software package . geometry optimizations for the organometallic fragments were performed 
 using fully spin - restricted ( rhf ) , restricted open - shell ( rohf ) and 
 unrestricted ( uhf ) density functional theory calculations , with the 
 b3lyp exchange - correlation functional and the lanl2dz 
 basis set . an additional p polarization shell for light atoms 
 and d polarization shell for heavier atoms were added 
 to augment the basis set . starting structures for optimizations were 
 taken from x - ray diffraction ( xrd ) experiments reported previously 
 for [ feh(-h2)dppe2][bf4 ] and on calculated singlet structures for [ fehdppe2 ] . effective 
 core potentials representing the core 10 electrons for iron and phosphorus 
 atoms , the core 28 electrons for ruthenium and the core 60 electrons 
 for osmium were used . self - consistent field ( scf ) convergence was 
 set to 5.0  10 for all calculations . thermal corrections to the energy and normal - mode frequencies 
 were obtained from vibrational analysis of the hessian matrix for 
 each fragment calculated using the b3lyp / lanl2dz - ecp level of theory 
 with seminumerical methods , with contributions from positive and negative 
 displacements of 0.01 bohr . partition functions of the fragments , qtot at 1 atm pressure were also obtained from 
 vibrational analysis . a scaling factor of 0.96 was used in calculating 
 thermodynamic properties and normal - mode frequencies to correct known 
 errors in the lanl2dz basis set , consistent with common practice . symmetry of the rotational modes was determined 
 by analysis of the components of the mode eigenvector associated with 
 the two hydrogen atoms , and the mode was considered symmetric if the x , y , and z components 
 of the force constants for each atom were of opposite sign and deviated 
 in value by less than 0.1 millidyne / angstrom . ab - initio ground 
 state electronic energies were calculated using 
 the spin - component scaled mller plesset second order 
 perturbation ( scs - mp2 ) scheme with the 
 lanl2dz - ecp basis set from the rhf optimized geometries found from 
 density functional theory ( dft ) calculations . fock ( uhf ) 
 ground - state energy calculations were performed similarly using the 
 non - spin - component scaled mller plesset perturbation 
 with the lanl2dz - ecp basis set on spin - singlet and spin - triplet [ fehdppe2 ] and spin - singlet [ feh(-h2)dppe2 ] for comparison of the spin 
 state effects . scf convergence was set to 5.0  10 for these calculations , and the second - order gradient tolerance 
 was set to 0.15 au . the chemical potential of hydrogen gas was 
 calculated from the 
 partition function for the hydrogen molecule through the relationship1where k is boltzmann s 
 constant , t is the temperature in kelvin , p is the pressure of the system , and p 
 is the standard pressure of the system , taken to be 1 atm . rotational 
 degeneracy for the hydrogen molecule is included in the partition 
 function calculated with the gamess software . the energy for 
 each fragment , e , was obtained 
 as the sum of the ground - state energy , 0 , and the 
 thermal correction obtained from vibrational analysis , corr(t ) , which contains contributions from 
 the electronic , translational , vibrational , and rotational motions 
 of the molecule . the binding energy of the hydrogen molecule to the 
 organometallic site , e , was calculated as2where the subscripts m  h2 , h2 , and m represent the organometallic adduct 
 fragment , 
 hydrogen gas , and the bare organometallic fragment , respectively . to simulate the mssbauer parameters for the iron fragments , 
 fully ab initio geometry optimizations and total energy calculations 
 were performed using the tzvp level of theory for [ feh(-h2)dppe2 ] and [ fehdppe2 ] , with p and d polarization 
 shells as described above calculations were also performed on the 
 nitroprusside anion , [ fe(no)(cn)5 ] , 
 to be used as a reference state for isomer shifts . isomer shift and 
 electric quadrupole splitting values were obtained from the electronic 
 charge density and electric field gradient at the iron nucleus according 
 to the method described by blaha . isomer 
 shifts were converted from the nitroprusside scale to the bcc iron 
 scale by subtracting 0.257 mm / s . identification of the 
 decomposition pathway as well as the structure of the presumed five - coordinate 
 product species [ fehdppe2 ] has previously been 
 precluded by the extreme reactivity of the species with solvents and 
 with trace oxygen and moisture , preventing characterization by x - ray 
 diffraction or nmr . to investigate the 
 decomposition pathway further , we performed mssbauer spectroscopy 
 of thermally decomposed [ feh(-h2)dppe2][ntf2 ] before and after reabsorption 
 of hydrogen to help identify the cycling products . these spectra appear 
 in figure 1 along with the results of lorentzian 
 fits to the spectrum . the corresponding values for the isomer shifts 
 and electric quadrupole splittings , along with the literature values 
 for the directly synthesized [ feh(-h2)dppe2][bf4 ] and [ fehdppe2][bph4 ] , appear in table 1 . only partial 
 decomposition of the dihydrogen complex was observed upon cycling , 
 with the integrated intensities of the lorentzian fits giving a conversion 
 of approximately 68% . electric quadrupole splitting ( eqs ) values for 
 the major peaks in the thermally decomposed and reabsorbed samples 
 correspond well to literature values for the five - coordinate species 
 and the h2 adduct , respectively . eqs values are indicative 
 of perturbations of the electric field around the metal center due 
 to the attached ligands , and are a good indicator of the chemical 
 environment of the first coordination sphere . the similarity of the 
 eqs values to the literature values thus provides strong evidence 
 for the presence of the [ feh(-h2)dppe2 ] and [ fehdppe2 ] cations 
 in the starting and decomposed sample . isomer shift ( is ) values for 
 the cycled systems show deviations from the literature values , although 
 they remain within the range typically associated with low - spin fe complexes . these deviations likely arise from the different 
 choice of counterion used in this study and the associated change 
 in the charge density within the pair . the large eqs value for the 
 [ fehdppe2 ] complex is characteristic of nonbonding 
 electrons in a high - spin d configuration , 
 which correlates with previous mssbauer measurements and magnetic 
 susceptibility measurements of the [ fehdppe2 ] cation performed by franke , et al . revealing 
 paramagnetism in the material . the isomer shift of the complex remains 
 within the low - spin fe ( ii ) regime , suggesting that the typical reduction 
 in the s - electron density around the nucleus from 
 the high - spin state is compensated by donation from the attached ligands . with the crystal structures for [ fehdppe2 ] complexes unavailable from experiments , computational simulations 
 of the mssbauer parameters were used to help identify the 
 structure and electronic character of the molecule . ground - state structures 
 of [ fehdppe2 ] in the spin - singlet and spin - triplet 
 states have previously been performed by franke , et al . , and were reported to be nearly isoenergetic , 
 with the singlet state slightly more favorable . these states were 
 also reported to have different molecular geometries , with the singlet - state 
 molecule having an octahedral geometry while the triplet - state molecule 
 has quadratic - pyramidal geometry . these differing geometries are expected 
 to alter the eqs values due to the broken symmetry of the ligand arrangement 
 in the triplet - state molecule , and thus simulations of the eqs values 
 can allow for direct identification of the spin - state of [ fehdppe2 ] complexes observed in mssbauer experiments . dft geometry optimization of the singlet and triplet 
 five - coordinate states performed using ab initio calculations with 
 the tzvp basis set were successful in reproducing the structure symmetries 
 originally reported by franke , et al . structure visualizations and ground state energies from these calculations 
 are reported in the supporting information . for 
 eqs values the isomer shift of the 
 nitroprusside anion was used as a reference for the simulations and 
 is thus not reported . the 
 electric field gradient tensor at the iron nucleus and the 
 isomer shift relative to bcc iron were calculated from the electronic 
 structure of the optimized geometries . eqs values were derived from 
 the principal component of the tensor without consideration of the 
 asymmetry factor and is values were derived from the electron density 
 at the nucleus according to the method described by blaha . the principal component was found to be oriented primarily 
 along the iron - hydride axis in each of the simulations , with a slight 
 skew in the triplet state five - coordinate complex toward the empty 
 coordination site , which no longer aligns with the iron - hydride axis 
 in the quadratic - pyramidal geometry . the expected differentiation 
 in the eqs values between the singlet - state and triplet - state geometries 
 was observed , and the value for the triplet state was found to be 
 consistent with the experimental and literature mssbauer values 
 for [ fehdppe2 ] . simulations of the eqs values 
 for singlet [ feh(-h2)dppe2 ] and the nitroprusside anion were also performed and 
 were found to correspond well to their experimental values within 
 the expected range of error for simulations of mssbauer properties . these results provide strong evidence for predominance 
 of the triplet state - configuration of [ fehdppe2 ] at room temperature . isomer shift values for both simulations fell 
 within the range for low - spin fe ( ii ) organometallic complexes and 
 correlated well with the experimental values , but did not display 
 sufficient deviation to distinguish between the two spin configurations , 
 suggesting that there is not a significant difference in the electron 
 density at the iron nucleus between the two states . mssbauer spectra 
 for ( a ) as synthesized [ feh(-h2)dppe2][ntf2 ] after n2 contaminant removal 
 and rehydrogenation and ( b ) [ feh(-h2)dppe2][ntf2 ] thermally 
 decomposed under vacuum at 125 c for 12 h. experimental data 
 ( black ) overlaid with lorentzian doublet fits corresponding to [ feh(-h2)dppe2][ntf2 ] ( green ) , 
 [ fehdppe2][ntf2 ] ( purple ) , anhydrous fecl2 ( orange ) , and fecl22h2o ( blue ) . the sum of the fits , which minimized the sum of the absolute error 
 with the data , appears in red . fecl2 and its dihydrate , 
 as residual reactants from the synthesis , account for approximately 
 25% of the integrated intensity in both spectra . simulations of eqs values allow for determination of the 
 sign of 
 the principal component of the electric field gradient , and correspondingly 
 the sign of the eqs . these signs are presented with their values in 
 table 1 . with identification of the sign , the method of partial quadrupole splittings presented by bancroft can be used to determine the individual contribution 
 of the h2 ligand to the eqs from the experimental value . the expression for the eqs based on partial quadrupole splittings 
 ( q ) is3where q(li ) is the partial quadrupole splitting 
 of ligand i , and i is the angle the ligand  metal bond makes with the principal 
 component axis of the electric field gradient . assuming perfect octahedral 
 geometry and the principal axis aligned along the h  fe  h2 axis , the expression for the eqs becomes4using the values of q(h ) = 1.04 mm / s and q(dppe/2 ) 
 = 0.50 mm / s , this expression leads to a value of q(h2 ) = 0.245 mm / s , which correlates well with 
 the value of 0.22 mm / s presented by morris and schlaf for 
 [ feh(-h2)dppe2][bf4 ] . the low q value 
 of dihydrogen as a ligand , compared to the large value of the hydride 
 ligand , should be universal for fe ( ii ) organometallic complexes because 
 of the additive nature of the partial quadrupole splitting model . this distinct feature of dihydrogen as a ligand may be useful for 
 identifying dihydrogen complexes and kubas binding in mssbauer 
 active materials , as well as defining the onset of dihydrogen - dihydride 
 transitions . the mssbauer spectra also indicate that 
 significant quantities 
 of fecl2 remaining unreacted within the isotopically enriched 
 samples , with both anhydrous and dihydrate species accounting for 
 approximately 25% of the total integrated intensity . this is likely 
 due to incomplete mixing in the small scale vessels used to produce 
 the material , leading to colloidal fecl2 within the thf 
 mixture during complexation with the dppe ligand . this issue is a 
 known problem of this synthetic route , in the past leading to erroneous 
 identification of the species [ fecl2dppe2 ] . discussed this in detail , 
 and also identified the individual species through mssbauer 
 spectroscopy . in our experiments , the residual fecl2 within 
 the sample appears to have been beneficial , preferentially scavenging 
 trace water to protect the dehydrogenated species during measurements , 
 as the anhydrous material is completely converted during the thermal 
 decomposition cycle . this effect also places a bound on the hygroscopic 
 affinity of the decomposition product , since fecl2 was 
 preferentially hydrated . thermodynamic isotherms 
 using the sieverts 
 technique , such as those measured for the isoelectronic manganese 
 complexes reported previously , were attempted 
 but could not be obtained for [ feh(-h2)dppe2][ntf2 ] because reversible equilibrium 
 pressures from the complex were below the experimentally detectable 
 level of 5 torr for all temperatures up to 420 k. at 420 k , the complex 
 decomposed irreversibly , releasing a large quantity of hydrogen gas 
 and producing a black solid containing multiple phosphorus peaks and 
 a complex proton spectrum in nmr measurements , indicating multiple 
 decomposition products . reversible release of dihydrogen from 
 the complex was observable below 420 k by residual gas mass spectroscopy 
 ( see supporting information ) and by reabsorption 
 of hydrogen gas onto the sample as determined by mole balance in the 
 sieverts apparatus . based on these observations , kinetic rates of 
 desorption of dihydrogen from the hydrogenated complex were measured 
 between 350 and 410 k , and the results are shown in figure 2 . an arrhenius fit to the kinetic data gives an 
 activation energy of ea = 32.3 kj / mol 
 for the release process . assuming a simple decomposition scheme and 
 extrapolating the kinetic data to 298 k gives a value of 2.98  
 10 s for the decomposition 
 rate constant , which indicates a significant barrier to hydrogen release 
 and a significantly lower release rate than reported for w(-h2)(co)3(pcy3)2 in solution . reabsorption at 
 all temperatures took place from 0.5 atm h2 overpressure 
 in less than the 500 ms time resolution of the 
 instrument , indicating the absence of diffusional barriers and a strong 
 driving force for reabsorption in the sample , but otherwise preventing 
 direct measurement of the absorption rates . this absence of diffusional 
 barriers in the reabsorption suggests that the slow release rate is 
 intrinsic to the decomposition process , which along with the previously 
 reported results for w(co)3(pcy3)2 , indicates a wide range of activation energies are possible for 
 hydrogen release in dihydrogen complexes . during the course 
 of kinetic testing , the samples underwent multiple 
 absorption / desorption cycles without observed loss of capacity , as 
 shown in figure 2 . this consistency supports 
 a simple decomposition and reabsorption scheme and a lack of formation 
 of stable side products or multiple reaction pathways , in accordance 
 with the mssbauer results . kinetic measurements for hydrogen release 
 from the reversible thermal 
 decomposition of [ feh(-h2)dppe2][ntf2 ] . the sample was heated to the temperature shown 
 under 0.5 atm hydrogen gas , then exposed to dynamic vacuum for nominally 
 1 h before re - exposure to a known quantity of hydrogen gas to establish 
 the quantity released . all data points were acquired on the same sample 
 through multiple thermal decomposition cycles . for the nth decomposition cycle , blue crosses (  ) correspond to tn > tn1 , and red diamonds (  ) correspond 
 to tn < tn1 . an arrhenius fit gives ea = 32.3 kj / mol , r = 0.8659 . to investigate the thermodynamic 
 properties of the absorption reaction , 
 simulations of the langmuir absorption behavior onto the singlet - state 
 complexes [ mhdppe2 ] ( m = fe , 
 ru , os ) were performed , based on the previously observed behavior 
 for the isothermal manganese complexes . however , an additional complication to the absorption behavior arises 
 in complexes containing a hydride trans to the dihydrogen 
 binding site . dissociation of the bound dihydrogen molecule and recombination 
 in the resulting trihydride [ mh3dppe2 ] can lead to inversion of the binding site within the molecule . bautista 
 et al . found through solution - state nmr 
 experiments that this behavior is present and occurs rapidly in the 
 iron cation [ feh(-h2)dppe2 ] and its osmium analogue , but did not observe the behavior 
 significantly in the ruthenium analogue . this inversion would have 
 no effect on the bulk binding properties of h2 on the molecular 
 level or on the system in solution , where the orientation of the molecules 
 is randomized . however , in the solid - state , where the relative orientation 
 of the molecules is fixed , the configuration of molecules with original 
 and inverted binding site geometries can lead to additional entropic 
 effects that must be taken into consideration in modeling the binding . to address the entropy arising from the inversion potential of 
 the molecules , an adjustment to the formulation of the partition function 
 used in the statistical langmuir isotherm can be performed . because the unbound states are unable to alter 
 their configuration , this inversion only affects the entropy of the 
 bound states . the partition function describing this behavior can 
 be formulated by assigning an orientation to the hydrogen molecules , 
 such that  up  or  down the partition function 
 for the system can then be expressed as5where the subscript arrows denote the orientation 
 of the binding molecule , with the total number of bound dihydrogen 
 molecules n = n + n. from this partition function , 
 a new langmuir isotherm can be derived , giving6 the modified isotherm , 
 given in eq 6 was 
 used for iron and osmium complex absorption simulations , whereas a 
 standard statistical langmuir isotherm was used for the ruthenium 
 complex . the numerical prefactor of two before the exponentials in 
 eq 6 is the only difference between the equation 
 for the invertible iron and osmium complexes and the noninvertible 
 ruthenium complex , and represents the increase in the available system 
 configurations from invertibility . analysis of the rotational mode 
 eigenvector components indicated that the rotation of the h2 molecule about its center of mass was asymmetric for all of the 
 complexes , and a value of qs = 1 was used 
 for all simulations . energy and enthalpy values are 
 given in kj / mol , and entropy values are given in j / molk . as with the group 7 complexes , the 
 vibrational analysis revealed 
 that occupied normal modes of the hydrogen molecule in the group 8 
 h2 adducts were dominated by the rotational modes , with 
 frequencies of 385.7 , 385.5 , and 202.1 cm for 
 fe , ru , and os species , respectively . the rotational mode at 385.7 
 cm for the iron complex was previously assigned 
 by eckert et al . as a translation mode 
 based on the presence of additional peaks at 225 and 255 cm in the vibration spectra from inelastic neutron scattering experiments . these lower energy peaks were also reproduced by the gamess simulations 
 at 237.6 and 254.3 cm , and were found to be associated 
 with strong translation modes of the iron center toward the hydride , 
 which couple to both the hydride and the dihydrogen atoms . the broad 
 peak at 400 cm in the neutron scattering experiment 
 thus likely corresponds to the primary rotational mode of the dihydrogen 
 molecule . although evidence for square - pyramidal geometries 
 of five - coordinate 
 ruthenium(ii ) hydride complexes with tetraphosphine ligands exists , 
 the strong nmr signals observed in these studies suggests that these 
 molecules are diamagnetic and thus in the singlet state , and that the square - pyramidal geometries reported are a result of 
 the electronic and steric effects of the ligands . unfortunately , geometric 
 investigations of species with the specific ligands in our study are 
 unavailable from nmr studies because of the high affinity of the uncoordinated 
 site for solvent molecules . for the purpose of 
 investigating periodic trends , we therefore make the assumption that 
 the ruthenium and osmium complexes being studied show analogous geometries 
 to their iron analogue . simulations of singlet - state complexes with 
 octahedral geometries were first performed to establish trends down 
 the periodic table in the absence of spin effects . the standard enthalpy 
 and entropy obtained from the keq values 
 of the simulated langmuir isotherms are given in table 2 . as with the group 6 complexes reported by gonzalez and hoff and the group 7 complexes reported by abrecht 
 and fultz , the binding enthalpy in the 
 singlet state group 8 complexes increases as 5d > 3d > 4d . interestingly , 
 the difference between the enthalpies and entropies of the osmium 
 species and the other group 8 complexes is much higher than the corresponding 
 differences in the group 6 and 7 complexes . this result suggests stronger 
 back - donation from the osmium center and stabilization of the trihydride 
 species , which is consistent with the reported nmr studies in solution . simulated singlet - state thermodynamic 
 values and isotherms for 
 the osmium and ruthenium complexes correlate well with qualitative 
 observations of the stabilities of the dihydrogen complexes . [ osh(-h2)dppe2 ] was found to be extremely stable to loss of 
 the dihydrogen ligand at all temperatures , with greater than 97% coverage 
 down to 0.25 torr at 373 k. this behavior is also consistent with 
 observations of exchange kinetics in the complex by bautista et al . , 
 which indicate that [ osh(-h2)dppe2][bf4 ] is highly stable to exchange of the h2 ligand with n2 , d2 , or ch3cn in the solid state or in solution . simulated isotherms for absorption 
 over the ruthenium complex are presented in figure 3 . as shown in the figure , the ruthenium complex has only 90% 
 coverage at room temperature and 1.0 atm h2 pressure , and 
 rapidly loses coordination of the dihydrogen complex at increasing 
 temperatures . this behavior indicates significant overpressure is 
 required to maintain the dihydrogen complex , consistent with experimental 
 reports of the instability of the complex over time at room temperature . simulated 
 langmuir isotherms for hydrogen absorption onto [ ruhdppe2 ] up to 2280 torr ( 3 bar ) hydrogen pressure . the absorption enthalpy for the singlet iron fragment 
 [ fehdppe2 ] is lower than that of the manganese fragment [ mn(co)dppe2 ] , which significantly underestimates the binding strength in the 
 iron complex relative to our experimental observations and reported 
 behavior in the solid state , suggesting a strong effect on the binding 
 enthalpy due to spin effects despite previous reports of the nearly 
 isoenergetic nature of the singlet and triplet states for the iron 
 complex . to investigate this effect further , 
 mp2 calculations were performed on [ fehdppe2 ] in both the singlet and triplet states using the lanl2dz - ecp basis 
 set using the rohf and uhf shell approximations . the results are given 
 in table 3 along with the uncorrected mp2 values 
 from the initial rhf calculations . as shown , uhf calculations greatly 
 overestimate the magnitude of the binding enthalpy , suggesting complete 
 irreversibility of the h2 absorption until decomposition 
 of the complex . the uhf calculations also resulted in a singlet ground 
 state for the five - coordinate complex , which contradicts the results 
 of the mssbauer experiments . we thus discounted the uhf results 
 as likely suffering from errors due to significant spin contamination 
 within such a large molecule . rohf calculations , despite not reproducing 
 the original rhf ground - state energy for the singlet state , provide 
 both the correct triplet ground - state of the five - coordinate species 
 and a negative hydrogen absorption enthalpy and free energy . these 
 values , however , do not predict the increased binding strength for 
 the direct binding of hydrogen to triplet - state five - coordinate species . we thus conclude that the hydrogen release must occur between the 
 hydrogenated complex to the singlet - state , which then rapidly converts 
 to the more favorable triplet - state configuration , accounting for 
 the differences in observed behavior between the iron and ruthenium 
 complexes . this transition must also occur more rapidly than the mssbauer 
 time scale for the iron complex , since the singlet species was not 
 observed . however , because of the issues associated with calculating 
 reaction thermodynamics from dft for nonisopiestic reactions , these 
 values should not be taken as quantitative . such spin state 
 transitions may be responsible for other complications 
 known to exist selectively in iron dihydrogen complexes . for instance , 
 bianchini et al . found that during the catalysis 
 of alkyne hydrogenation , the tetraphoshine complex [ ( pp3)feh(h2)][bph4 ] ( pp3 = p(ch2ch2pph2 ) ) preferentially removes a phosphine 
 arm before releasing the dihydrogen ligand , while the ruthenium analogue 
 [ ( pp3)ruh(h2)][bph4 ] first releases 
 the dihydrogen molecule for the same catalytic process . if present 
 in the tetraphosphine complexes , the loss of hydrogen to form a high - energy 
 singlet state may be less favorable than the removal of a phosphine 
 arm , and this effect would not be present in the analogous ruthenium 
 complex . recently coronado , et al . also 
 detailed the spin - crossover effects in the iron - based metal organic 
 framework [ fe(btzx)3]clo4 ( btzx = 1,4-bis(tetrazol-1-ylmethyl)benzene ) 
 upon co2 adsorption , which showed a unique magnetic transition 
 for co2 that was not present for n2 adsorption . such unique spin - dependent reactions may be useful for engineered 
 materials for gas absorption reactions . singlet / triplet 
 designation indicates 
 the state of the five - coordinate species . energy and enthalpy values 
 were calculated from the same vibrational analysis , and are given 
 in kj / mol , and entropy values are given in j / molk .
mssbauer 
 spectroscopy , experimental thermodynamic measurements , 
 and computational studies were performed to investigate the properties 
 of molecular hydrogen binding to the organometallic fragments [ mhdppe2]+ ( m = fe , ru , os ; dppe = 1,2-bis(diphenylphosphino)ethane ) 
 to form the dihydrogen complex fragments [ mh(2-h2)dppe2]+ . 
 mssbauer spectroscopy 
 showed that the dehydrogenated complex [ fehdppe2]+ adopts a geometry consistent with the triplet spin state , transitioning 
 to a singlet state complex upon addition of the dihydrogen molecule 
 in a manner similar to the previously studied dinitrogen complexes . 
 from simulations , 
 this spin transition behavior was found to be responsible 
 for the strong binding behavior experimentally observed in the iron 
 complex . 
 spin - singlet to spin - singlet transitions were found to exhibit 
 thermodynamics consistent with the 5d > 3d > 4d binding trend 
 observed 
 for other transition metal dihydrogen complexes . finally , the method 
 for distinguishing between dihydrogen and dihydride complexes based 
 on partial quadrupole splittings observed in mssbauer spectra 
 was confirmed , providing a tool for further characterization of these 
 unique species for mssbauer active compounds .