the protonation of 
 metal complexes and clusters continues to attract 
 much attention because of its relevance to catalysis involving proton 
 and hydride transfer reactions . in organometallic and heterogeneous 
 catalysis , protonation is a step in substrate hydrogenation and hydrogen 
 evolution . in biology , metal protonation is implicated 
 in the reduction of co2 and n2 , not to mention 
 hydrogen evolution . for all of these reactions , a 
 central question is the regiochemistry of the protonation . for compounds 
 with metal metal bonds , protonation can in principle occur 
 at a single metal or at the metal the regiochemistry 
 of protonation is relevant to a mechanistic 
 understanding of the [ nife]- and [ fefe]-hydrogenases , which also feature 
 ni  fe and fe  fe bonding interactions . by 
 proton transfer , these complexes equilibrate protons and dihydrogen , 
 i.e. , 2h + 2e  h2 in accordance with the local redox environment and activities of 
 h and h2 . functional models for the active 
 sites of these enzymes present opportunities for translating biological 
 insights into practical catalytic systems . in addition to being fast and operating at low overpotentials , the 
 hydrogenases are attractive targets for functional modeling because 
 they utilize earth abundant metals . for 
 the above reasons , much effort has been directed , both computationally and experimentally , at understanding the details of proton transfer to and from synthetic 
 nife and fefe complexes . while many complexes engage separately 
 in acid base and 
 redox reactions , few exhibit both kinds of reactivity , providing a 
 possible explanation for the paucity of functional hydrogenase ( h2ase ) models . with respect to the [ nife]-h2ases 
 ( figure 1 , left ) , the premier example of such 
 a model is ( oc)3fe(pdt)ni(dppe ) ( 1 ) . tricarbonyl 1 and analogous phosphine - substituted 
 derivatives undergo : ( i ) protonation to afford rare examples of nickel  iron 
 hydrides and ( ii ) 1e oxidation to give fe(i)ni(ii ) 
 derivatives ( figure 1 , right ) . several examples 
 of charge - neutral fe(i)ni(i ) species , as well as their related fe(ii)ni(ii ) 
 hydrides and fe(i)ni(ii ) cations , have been characterized . active site of [ nife]-h2ase , a bidirectional 
 catalyst , 
 participates in redox and acid  base chemistry ( left ) . the [ nife]-h2ase active 
 site features ni bound to four 
 cys ligands in a distorted tetrahedral ( sf4-like ) arrangement . two of the thiolates link to an fe center whose coordination sphere 
 includes the unusual co and cn cofactors , the latter 
 h - bonding to neighboring ser and arg residues . in the active states of the enzyme , the feni core appears to be 
 relatively conformationally rigid , with the feni 
 distance and coordination geometry being rather insensitive both to 
 redox state and the presence / absence of h substrate . the rigidity of the active site is undoubtedly 
 a factor in the high rates and low overpotentials at which the interconversion 
 of h and h2 is mediated . in contrast to the rigid active site , small molecule mimics 
 are 
 conformationally dynamic . for example , 1 undergoes a 
 tetrahedral  square - planar twist at ni upon oxidation or protonation 
 ( figure 1 ) . while the structures of 1 , , and [ 1h ] are established , those of any intermediates are not . such transformations may , for 
 example , involve  pre - twisting  of the ni coordination 
 in 1 ( scheme 1 , top ) . distortion of the ground - state structure of 1 could 
 well affect oxidation states by inducing 2e mixed - valency . furthermore , the degree of fe  ni bonding 
 and , most importantly , reactivity would also be perturbed . while ligand - centered 
 isomerism is a common theme in the chemistry of multimetallic compounds , 
 drastic differences in coordination geometry and metal metal 
 bonding between two ( or more ) interconverting species are extremely 
 rare . they are , however , not without precedent ; for example , jones 
 and co - workers reported x - ray structures for two complexes of formula 
 ( oc)2rh(-pr2)2rh(co)2 ( scheme 1 , bottom ) . rh 
 bonding ( 2.761  ) , in which one rh center is square - planar and 
 the other tetrahedral . a second ( orange ) isomer was also observed , 
 in which non - interacting rh centers ( 3.717  ) adopt square - planar 
 geometries . the present study involves parallel experimental 
 and density functional 
 theory ( dft ) investigations to obtain a complete structural , energetic , 
 and mechanistic picture of complexes derived from feni complex 1 . this work was extended to fepd and fept species such that 
 a homologous series can be interrogated for information regarding 
 conformational motions and metal  metal bonding . the major routes to low - valent feni 
 complexes of type ( oc)3fe(pdt)ni(diphosphine ) involve treatment 
 of ( pdt)ni(diphosphine ) 
 with either ( i ) fe2(co)9 or ( ii ) fei2(co)4/cocp2 . as applied to the pd and pt derivatives , 
 the second route proved more attractive , giving the diamagnetic heterobimetallics 
 ( oc)3fe(pdt)pd(dppe ) ( 2 ) and ( oc)3fe(pdt)pt(dppe ) ( 3 ) ( scheme 2 ) . isolated as dark - green needles , 2 is sensitive to 
 o2 ( decomposing to co - free paramagnetic species ) but is 
 thermally stable in the solid state under n2 . the energies 
 of the two co bands ( a + e modes ) compare well with 
 those observed for 1 ( table 1 ) . the fepd species 2 is 
 similar to 1 in 
 that its room temperature p{h } nmr spectrum 
 features a broad singlet resonance (  51.3 ) for the dppe ligand . 
 at lower temperature , the resonance for 1 decoalesces 
 into two peaks ( well - resolved at 68 c ) , consistent with 
 a dynamic process that interconverts the p sites . the 
 previously reported coalescence temperature ( 30 c ) corresponds to a free energy barrier of 9.5 
 kcal / mol for the feni species 1 . this process presumably 
 occurs via an intermediate or transition state with square - planar 
 ni . no decoalescence was observed for 2 even at 85 
 c , behavior suggestive of either : ( i ) a tetrahedral pd ground 
 state with a particularly low rotational barrier or ( ii ) a square - planar 
 pd ground state . the second possibility is ruled out based on the 
 similarity of the ir data for 1 and 2 . the 
 structure of 2 in the solid state was confirmed crystallographically 
 ( figure 2 ) . ortep of 2 with ellipsoids 
 drawn at the 50% probability 
 level and h atoms omitted for clarity . selected distances (  ) 
 ( mean dft values in parentheses ) : pd1fe1 , 2.561 ( 2.56 ) ; pd1p1 , 
 2.249 ; pd1p2 , 2.271 ( 2.26 ) ; pd1s1 , 2.573 ; pd1s2 , 
 2.402 ( 2.49 ) ; fe1s1 , 2.299 ; fe1s2 , 2.275 ( 2.30 ) ; fe1c30 , 
 1.795 ; fe1c31 , 1.793 ; fe1c32 , 1.798 ( 1.77 ) . selected 
 angles ( deg ) ( mean dft values in parentheses ) : p1pd1s1 , 
 135.2 ; p1pd1s2 , 118.83 ; p2pd1s1 , 115.7 ; 
 p2pd1s2 , 122.5 ( 135.1 ) ; s1fe1c30 , 
 162.2 ; s2fe1c32 , 163.1 ( 161.1 ) . given the covalent radii of pd ( 1.39  ) and low - spin 
 fe ( 1.32 
  ) , the fe pd distance of 
 2.561  suggests the presence of bonding between the metal centers 
 in 2 . metal  metal interactions 
 aside , the pd and fe centers in 2 exist in roughly tetrahedral 
 and square - pyramidal (  = 0.01 indicates a low degree of trigonality ) coordination geometries , respectively . the complex 
 is isostructural to its fe(i)ni(i ) congener , and it is expected that 
 an fe(i)pd(i ) description is appropriate , particularly considering 
 the tetrahedral pd coordination , as such a geometry is unlikely for 
 pd(ii ) . the structures of several complexes reported here were 
 calculated 
 using dft , and the optimized structures were in excellent agreement 
 with available crystal structures ( selected metrics are presented 
 in tables s3 and s4 ) . in the case of 2 , calculations accurately predicted the geometry of pd , as 
 well as its proximity to fe . pd bond to arise 
 from overlap of an fe - centered d(z ) orbital with a pd - centered orbital with d character ( figure s56b ) . nbo analysis 
 of feni congener 1 also revealed metal  metal bonding 
 ( figure s56a ) consistent with previous 
 dft analyses of this system . nbo analysis 
 of other fe  m complexes discussed herein did not indicate mixing 
 of metal d orbitals ; hence , no fe the cyclic 
 voltammogram of 2 features reversible and quasi - reversible 
 oxidations at e1/2 = 0.87 and 
 + 0.26 v , respectively 
 ( vs ferrocene / ferrocenium , fc ; figure 3 ) . the reversible wave , assigned to the   couple , is 300 mv more negative than the wave for , a result reflective 
 of the more reducing nature of the heavier group 10 metal . cyclic voltammograms 
 of 2 ( dotted trace ) and [ 2]bf4 ( solid trace ) acquired in ch2cl2 with 
 100 mm nbu4pf6 . potentials ( v vs fc ) were swept at 100 mv s. dft was used to calculate the 
 potentials associated with the two 
 couples for each [ l(co)2fe(pdt)m(dppe ) ] ( l = co , pcy3 ; m = ni , pd , pt ) system . the calculated 
 potentials are in excellent agreement with the experimental values 
 for the neutral / monocation couples , while the values for the monocation / dication 
 couples are in qualitative but not quantitative agreement . the latter 
 discrepancies are mainly a result of these more anodic couples being 
 only partially reversible , the associated potentials being less thermodynamically 
 reliable . calculated and experimental values 
 for [ 1]bf4 agree by construction and 
 were used as references for the corresponding   waves . calculated and experimental 
 values 
 for [ 2]bf4 agree by construction and 
 were used as references for the corresponding , , and [ 3 ] waves . this value was not determined as 
 difficulties arising from local minima led to inconclusive results . this wave is assigned to a 
 fept / fept couple 
 ( vide infra ) . chemical oxidation of 2 with fcbf4 afforded 
 [ ( oc)3fe(pdt)pd(dppe)]bf4 ( bf4 ) , isolated as an olive - brown powder . positive ion esi - ms 
 analysis ( m / z 749.8 ) was consistent 
 with the formula , and the lability of [ ( oc)3fe(pdt)pd(dppe ) ] was evidenced by observation of co dissociation products 
 ( m / z 721.8 , 693.8 ) . the co bands for   ( 2058 , 1986 cm ) are only modestly shifted from those of 2 , implicating oxidation at the pd center . epr measurements also indicated 
 an fe(i)pd(ii ) description for , the assignment 
 being based on the magnitude of the g shifts and 
 the absence of p hyperfine coupling ( figure 4 , top ; for simulated epr spectra and parameters 
 see figure s23 and table s1 , respectively ) . the data are similar to those for fe(i)ni(ii ) complexes , for which 
 theory indicated an fe - centered d(z ) doublet ground - state singly occupied molecular orbital 
 ( somo ) . these computational results , 
 as well the implied absence of spin  orbit coupling expected 
 for a d(z ) ground state , 
 are consistent with a resonance in the vicinity of ge . owing to the inversion of the pdt ring , the [ ( oc)3fe(pdt)m(dppe ) ] radicals 
 exist as two conformers , one giving rise to an axial pattern and the 
 other to a rhombic pattern . x - band epr spectra ( ch2cl2/phme , 110 k ) of 
 bf4 and [ 2]bf4 . dft analysis of spin densities 
 was used to determine the localization 
 of the unpaired electron in   as well as 
 all other mixed - valence bimetallics ( table 3 ) . in each case , the spin is entirely fe - centered , consistent with 
 the fe(i)m(ii ) assignments from epr analyses . the spin density distribution 
 is unaffected by inversion of the pdt ring , as 
 observed for related feni complexes . cation   undergoes substitution with 
 pcy3 to afford [ cy3p(oc)2fe(pdt)pd(dppe)]bf4 ( [ 2]bf4 ) as a yellow - brown 
 solid . similar to its tricarbonyl precursor , [ 2 ] is also described as an fe(i)pd(ii ) compound , the rhombic 
 signals split by interaction of the somo with pcy3 ( a(p ) = 54 , 61 , 57 mhz ) . the 
 data are distinct from those of the green fe(i)ni(ii ) analog [ 1 ] , which show p hyperfine 
 to be absent due to structural distortions at fe . as with 2 , the cyclic voltammogram of [ 2]bf4 features two waves , the events at e1/2 = 1.14 and 0.20 v being assigned to 
 [ 2 ] and [ 2 ] couples , respectively . the waves are cathodically 
 shifted relative to the tricarbonyl parent , with a smaller shift in 
 the first wave ( e1/2 = 0.27 
 v ) than in the second ( e1/2 = 0.46 
 v ) . the fact that ligand substitution at fe perturbs 
 the anodic wave to a greater degree points to this redox event being 
 fe - centered . these data are consistent with the sequence : fe(i)pd(i ) 
  fe(i)pd(ii )  fe(ii)pd(ii ) . the reversibility of the 
 fe(i)pd(ii)/fe(ii)pd(ii ) ( [ 2 ] ) 
 couple is greater than that of , the 
 ligands in the former case being better suited to stabilize the ferrous 
 state . the protonation of 2 with hbf4 affords the hydride salt [ 2h]bf4 , the formation of which is signaled by the appearance 
 of high energy co bands ( 2081 , 2020 cm ) suggestive of an fe(ii)hpd(ii ) product . a high - field h nmr signal (  4.3 ) and a single downfield p nmr resonance (  62.8 ) are consistent with [ 2h ] adopting a cs - symmetric 
 structure in solution . when the acidification of 2 was 
 performed in the presence of cd3od , esi - ms and h nmr spectroscopy confirmed formation of the deuteride isotopologue 
 [ 2d ] . the evidence for the solution structure 
 of [ 2h ] is complemented by crystallographic 
 analysis ( figure 5 ) . ortep of one of two independent 
 complexes in [ 2h]bf4thf0.5et2o with ellipsoids drawn at 
 the 50% probability level . the solvate molecules , bf4 anion and h atoms ( except the h ligand ) are omitted for clarity . mean distances in the complexes 
 (  ) ( mean dft values in parentheses ) : pd2fe2 , 2.882 ( 2.92 ) ; 
 pd2h2 , 2.173 ( 2.19 ) ; pd2p3 , 2.253 ; pd2p4 , 
 2.270 ( 2.30 ) ; pd2s3 , 2.367 ; pd2s4 , 2.363 ( 2.39 ) ; fe2s3 , 
 2.330 ; fe2s4 , 2.338 ( 2.34 ) ; fe2h2 , 1.483 ( 1.54 ) . mean 
 angles in the complexes ( deg ) ( mean dft values in parentheses ) : pd2h2fe2 , 
 102.4 ( 101.2 ) ; p3pd2s4 , 171.5 ; p4pd2s3 , 
 177.7 ( 177.6 ) ; s3fe2c35 , 165.4 ; s4fe2c33 , 
 167.1 ( 165.8 ) . by single crystal x - ray 
 diffraction , two structurally similar fepd 
 hydrides were found in the asymmetric unit of [ 2h]bf4 , in each case with the fe  pd distance ( 2.845 , 2.918 
  ) being greater than the sum of the covalent radii ( 2.71  ) . protonation at fe is accompanied by rotation 
 of the pd(dppe ) center , affording square - planar pd linked by the pdt group to octahedral fe . indeed , the arrangement of 
 metal centers in [ 2h ] is analogous to that 
 in [ ( oc)3feh(pdt)ni(dppe ) ] ( [ 1h ] ) . the h ligand in [ 2h ] was located in the difference map and allowed to refine ; 
 its final location was significantly closer to the fe center . considering 
 the covalent radius of h ( 0.31  ) , the h ligand in [ 2h ] is formally bonded to fe ( 1.469  ) , whereas its interaction 
 with pd is weak ( 2.168  ) . dft calculations also predict unsymmetrical 
 binding of the hydride ligand , which , despite being poised between 
 the metals , can be viewed as a terminal ligand . the first indication 
 of the unusual structure of ( co)3fe(pdt)pt(dppe ) ( 3 ) , which is red - brown , is that its uv  vis spectrum 
 features two absorptions ( 480 , 360 nm ) , whereas single bands were 
 observed for the green complexes 1 ( 398 nm ) and 2 ( 376 nm ) . the ir spectrum of 3 is also distinctive , 
 with the co bands ( 1962 , 1883 , 1870 cm ) observed at energies 70 cm lower than 
 those for fe(i)ni(i ) and fe(i)pd(i ) analogs . the p{h } nmr spectrum for 3 consists of a singlet ( and pt - coupled doublet ) and is temperature - invariant down to 
 85 c . the chemical shift (  44.1 ) and jppt ( 3277 hz ) are close to those of 
 the ( pdt)pt(dppe ) precursor (  44.5 , jppt = 2717 hz ) . while nmr data are in 
 line with pt coordination being either ( i ) highly fluxional tetrahedral 
 or ( ii ) square - planar , the latter is most consistent with low - energy 
 co bands . square planarity at pt is corroborated 
 by dft geometry optimization ( figure 6 ) . energy - minimized 
 structure of 3 obtained with dft . 
 selected distances (  ) : p2 , 2.28 . selected angles ( deg ) : c1fe  s2 , 
 136.8 ; c2fe s1 , 169.4 ; s1pt  p2 , 176.1 ; 
 s2pt  p1 , 177.7 . indeed , rather than being isostructurual to 1 and 2 , 3 is predicted to feature a square - planar 
 pt site , with the angle between the ptp2 and pts2 planes being 4. the calculated fe pt distance ( 2.87 
  ) exceeds the sum of the covalent radii ( 2.68  ) , suggesting an absence of fe  pt bonding 
 and a purely pentacoordinate fe . this site , in contrast to the square - pyramidal 
 fe(i ) in 2 , is distorted trigonal bipyramidal (  
 = 0.54 ) . as mentioned previously , nbo analysis did not reveal d orbital mixing , which was observed in 1 and 2 . overall , the coordination geometries are consistent with 
 an fe(0)pt(ii ) description for 3 . quasi - reversible 
 oxidations for ( co)3fe(pdt)pt(dppe ) 
 were observed at 0.93 and + 0.33 v ( figure 
 s29 ) , potentials similar to those for the fepd analog 2 . the proposed sequence of 1e events : 
 fe(0)pt(ii )  fe(i)pt(ii )  fe(ii)pt(ii ) . this sequence 
 is supported by the ir spectrum for [ ( oc)3fe(pdt)pt(dppe)]bf4 ( bf4 , prepared from 3 and fcbf4 ) , in which the co bands ( 2058 , 
 1987 cm ) are at virtually identical energies to 
 the bona fide fe(i)m(ii ) species   and . while the epr spectrum 
 of   indicates some unpaired spin resides 
 on pt ( a(pt )  97 mhz , a(p )  11 mhz ) , the g - shifts suggest an fe(i)pt(ii ) description to be appropriate , as 
 mirrored in the spin density calculations ( table 3 ) . the paramagnetic salt 
 bf4 undergoes ligand substitution with 
 pcy3 to afford dicarbonyl [ cy3p(oc)2fe(pdt)pt(dppe)]bf4 ( [ 3]bf4 ) , the characterization for which mirrors that for [ 2]bf4 . as expected , reaction of low - valent 3 with hbf4 furnished the hydride salt [ ( oc)3feh(pdt)pt(dppe)]bf4 ( [ 3h]bf4 ) , for which a h nmr resonance at  3.57 
 is particularly diagnostic . 
 the small coupling observed for the satellites ( jhpt = 173 hz ) indicates that the hpt 
 interactions are rather weak . for comparison , square - planar platinum 
 hydrides exhibit jhpt from 
 650 to 1700 hz , with jhpt being at the low end of this range 
 for five - coordinate complexes of the type [ pt(pr3)4h ] . thus , [ 3h ] is similar to its fepd analog in that the 
 h ligand can be considered terminally fe - bound , 
 as also supported by the p nmr resonances for 3 being insensitive to protonation at fe ( for [ 3h ]  46.4 , jppt = 3220 hz ) . in contrast , the energies of the co bands for the fept conjugate acid and base species are very different  those 
 for [ 3h ] ( 2081 , 2022 cm ) are greatly shifted relative to those for 3 ( the average 
 shift , taking into account the a + e patterns was co = 137 cm ) . a similar shift ( 139 cm ) was observed by angelici and co - workers upon protonation 
 of the fe(0 ) species fe(dppe)(co)3 to give the ferrous 
 hydride [ feh(dppe)(co)3 ] . taken together , these data are consistent with fe(0)pt(ii ) , 
 fe(i)pt(ii ) , and fe(ii)hpt(ii ) assignments for 3 , bf4 , and [ 3h]bf4 , respectively . while only a handful of fept complexes bearing h and co ligands exist , the compounds ( co)3feh(-pr2)pt(pr3)2 are particularly pertinent . these complexes interconvert between isomers 
 with terminal ( ( h)fept0 ) and bridging ( fe0(-h)pt ) hydride ligands . when these fept 
 species feature the substituents r = ph , r = oph , a non - fluxional 
 terminal hydride results ( jhpt = 28 hz ) , whereas when r = cy , r = et the hydride is evenly 
 shared ( jhpt = 520 hz ) . one 
 would expect values double the latter ( 1000 hz ) for purely 
 terminal pt  h moieties . ( 28 ) good agreement was found 
 between experimental and calculated shifts in weighted average co 
 vibrational frequency ( co ) induced by protonation 
 and oxidation of bimetallics 13 ( scheme 3 ) . protonation of the feni and fepd complexes is 
 predicted to shift co by 50 cm . in contrast , the shift induced by protonation of the fept analog 
 is much greater , with co 110 cm . these co values correlate 
 with the formal changes in fe oxidation state : from + i to + ii upon 
 protonation of 1 and 2 as well as from 0 
 to + ii upon protonation of 3 . trends in co for the couples , , and   are similar to 
 those seen for protonation , the effects again being greater for the 
 pt case . the co values for these oxidations 
 are consistent with their occurring remote from the fe(co)3 center in   and   but directly at 
 this center in . as discussed above , the 
 fluxionality of 1 and 2 , evidenced by the 
 broad singlet in their p{h } nmr spectra , 
 implicate the presence of a transient or intermediate species featuring 
 square - planar ni / pd centers ( scheme 1 , top ) . this conformational motion is likely to perturb both the oxidation 
 states and intermetallic bonding of the fe and m centers . two optimized 
 dft structures characterized as minima were calculated for each bimetallic 13 : one in which the ( pdt)m(dppe ) fragment 
 is tetrahedral , and one in which it is square - planar . selected bond 
 lengths and angles are provided in table s5 . in each case , the fe center is within bonding distance of m when 
 the latter adopts a tetrahedral geometry , while the fe m elongation is subtle 
 in the feni case ( 0.13  ) but more pronounced for fepd and fept 
 ( 0.30  ) . the relative stability of the two isomers was 
 computed , and the reaction free energies ( g ) and free energy barriers ( g ) associated with tetrahedral to square - planar isomerization are 
 presented in table 4 . the potential energy surface was 
 found to be extremely flat along the isomerization pathway . as a result , the single imaginary frequency for the transition state ( ts ) was < 20 
 cm for the two barriers given in this table . each 
 ts was verified to lead to the relevant tetrahedral and square - planar 
 geometries by following the intrinsic reaction coordinate ( irc ) for 
 4 or 5 steps in both directions and subsequently optimizing the geometries . 
 as the imaginary frequencies may be below the numerical accuracy of 
 the methodology , and the complete irc was not obtained , the free energy 
 barriers should be viewed with caution . moreover , multiple tss were 
 found for 1 , suggesting a ruffled potential energy surface 
 connecting the two isomers , and only the highest free energy barrier 
 is reported . the ts was 
 not determined for 3 as the isomerization was found to 
 be significantly exergonic . holm 
 relation using the coalescence temperature of 243 k ( detailed in si ) , but the dft free energies were calculated 
 at 298 k for consistency with other experiments . note that the calculated 
 and experimental free energy barriers are not exactly equivalent . for the feni and fepd congeners , the isomerization is close to 
 thermoneutral ( g = 0.67 and 
 + 0.99 kcal / mol , respectively ) . given the error associated with the 
 calculations , it is not possible to determine a thermodynamic preference 
 for the coordination geometry at ni / pd . the reaction free energies 
 calculated using various other density functionals and basis sets 
 with geometry optimization in ch2cl2 solvent 
 are qualitatively consistent with the results presented here , although 
 certain levels of theory do not predict a stable square - planar intermediate 
 for geometry optimizations in the gas phase ( see tables s6s8 ) . the calculated free energy barriers 
 are relatively low , consistent with the fluxionality of 1 and 2 observed in their room temperature nmr spectra . because the potential energy surfaces for 1 and 2 were found to be extremely flat along the isomerization 
 pathway , the calculated free energy barriers may not be quantitatively 
 accurate and can be analyzed only qualitatively . the calculated free 
 energy barrier of 6.66 kcal / mol for 1 is qualitatively 
 consistent with , although somewhat lower than , the free energy barrier 
 of 9.5 kcal / mol estimated from the coalescence temperature in the 
 nmr measurement . moreover , the lower calculated free energy barrier 
 of 3.29 kcal / mol for 2 is consistent with the lack of 
 decoalescence observed in the nmr experiments for 2 . the observation of a particularly flat potential energy surface along 
 the isomerization pathway has implications for the enzymatic system , 
 where the geometry at the ni site is between tetrahedral and square - planar . the similarity in free energies and the relatively low free energy 
 barriers suggest that the two isomers of 1 and 2 may interconvert . in addition , the free energy change associated 
 with oxidation of 1 and 2 is similar for 
 the two different isomers , with the calculated reduction potentials 
 of the square - planar and tetrahedral isomers differing by only 0.03 
 v , which is within the estimated error of the methodology . it is thus 
 a distinct possibility that the ni / pd centers in 1 and 2  preorganize  into square - planar geometries 
 prior to 1e loss . in the case of fept complex 3 , the square - planar isomer is overwhelmingly favored , consistent 
 with the high stability characteristic of square - planar pt(ii ) centers . calculations indicate that co energies for the tetrahedral 
 isomer of 3 , which was found to be a local minimum , are 
 within 10 cm of those for 1 and 2 . square - planar isomers of 1 and 2 , while not observed experimentally , have calculated co bands 5565 cm lower in energy 
 than the respective tetrahedral conformers . this co shift is consistent with the change in conformation inducing a change 
 in oxidation state . given the dynamics in the neutral species 1 ( as well 
 as 2 ) , we set about investigating hydride [ 1h ] to yield further insight into the mechanism of protonation . 
 at room temperature , [ ( co)3feh(pdt)ni(dppe ) ] displays a single p nmr peak owing to its cs - symmetry . in view of the difficulty with determining 
 whether or not ni(dppe ) twisting is occurring here , we interrogated 
 an analogous hydride of lower symmetry , which entailed preparation 
 of the new bimetallic ( co)3fe(pdt)ni(s , s - chiraphos ) ( s , s - chiraphos = 2s,3s - bis(diphenylphosphino)butane , 
 scheme 4 ) . analogous to 1 , this 
 species gave rise to a single p nmr resonance at room 
 temperature , with decoalescence observed on cooling to 90 
 c , at which temperature ni(s , s - chiraphos ) twisting is slowed ( figure s49 ) . while ( co)3fe(pdt)ni(s , s - chiraphos ) twists rapidly at room temperature , its conjugate 
 acid 
 does not ; the nonequivalentp nuclei 
 in the chiral hydride [ ( co)3feh(pdt)ni(s , s - chiraphos ) ] ( scheme 4 , center ) give rise to two broad signals ( resolved into two p - coupled doublets at 70 c ) . if twisting and 
 equivalencing of the p sites were to be facile , it would 
 be necessary for a tetrahedral hydride ( scheme 4 , right ) to be energetically accessible , which appears not to be 
 the case . in general , it is proposed that twisting of neutral species 
 is associated with  on / off  switching of fe it is now evident 
 that the twisting of hydride species is slower , perhaps reflecting 
 the rigidifying influence of the hm interactions 
 ( despite their weakness ) . these observations are supported by 
 computational work : the twisting 
 of hydride [ 1h ] was investigated , and two 
 optimized dft structures characterized as minima were calculated for 
 the isomers of [ 1h ] . the relative free energies 
 of the structures , which feature either square - planar or tetrahedral 
 ( pdt)ni(dppe ) fragments ( figure 7 ) , were also 
 calculated . energy - minimized structures obtained with dft for [ 1h ] with the ( pdt)ni(dppe ) moiety either square - planar 
 ( red ) or distorted tetrahedral ( blue ) . the optimized geometries are 
 superimposed and presented in two views . other ( higher energy ) isomers / tautomers 
 proposed can be found in figure s57 . twisting of the square - planar 
 ( pdt)ni(dppe ) site in [ 1h ] to adopt a tetrahedral 
 geometry was calculated to 
 be significantly endergonic ( g = + 11.47 
 kcal / mol ) . the tetrahedral conformer is predicted by dft to be far 
 ( 10  ) more acidic than the square - planar 
 state ( the experimental pka(mecn ) for 
 [ 1h ] is 10.7 ) . considering the conjugate bases for the two isomers of [ 1h ] , these results suggest that 1 has greatly 
 enhanced basicity when in the square - planar form , consistent with 
 its description as fe(0)ni(ii ) ( scheme 1 ) . thus , it is reasonable that this isomer is responsible for the rich 
 acid  base and redox chemistry exhibited by 1 and 
 related complexes . numerous studies have 
 examined the protonation and 1e oxidation of complexes 
 of the type ( co)3fe(pdt)ni(diphosphine ) 
 and substituted derivatives thereof . this paper presents evidence 
 that the protonations , and possibly the oxidations , proceed via a 
 latent 2e mixed - valence intermediate ( scheme 5 ) . despite previous computational 
 analyses , this key square - planar 
 species had not been 
 identified as an energetically accessible isomer of 1 . the first clue for this unsuspected structure was provided by the p nmr properties of 1 , which implicate a transient 
 or intermediate square - planar species that interconverts the p sites . the new and compelling evidence for the intermediate 
 comes from ( i ) dft calculations that suggested thermodynamically accessible 
 rotation at the non - fe center and ( ii ) spectroscopic and electrochemical 
 properties of the fept complex , the protonation and redox behavior 
 of which mirrors that of fe(0 ) diphosphines . the 2e mixed valency of 1 contrast 
 with that for ( cymene)ru(pdt)ni(dppe ) , which features a rigidly tetrahedral 
 ni(0 ) center . the rotation - induced 
 2e mixed - valence behavior 
 also has implications for understanding active site mimics of the 
 [ fefe]-h2ases . prior to protonation , model complexes almost 
 invariably feature fe2(sr)2l6 cores 
 wherein terminal coordination sites are unavailable 
 for h binding . protonation of these fe(i)fe(i ) complexes 
 has been proposed to proceed via a  rotated intermediate  
 ( scheme 6 ) . fe 
 bonding is weakened and a terminal site on one fe center is vacant . by analogy to the results in this paper , such 
 rotated diiron complexes should perhaps be described as fe(ii)fe(0 ) , 
 thus underlining the role of latent , but highly reactive intermediates . overall , the work reported here shows that the geometry 
 at ni decisively 
 influences the reactivity of a neighboring fe site . for small molecule 
 [ nife]-h2ase mimics , it has been demonstrated that perturbation 
 of the ni site greatly alters the acid  base properties of these 
 complexes . with respect to the hydrogenases themselves , the results 
 suggest that the distorted nature of the ni(cys)4 site 
 is likely an important factor in tuning basicity and redox potentials , 
 thereby enabling their remarkable activity . unless otherwise stated , chemicals were purchased 
 from commercial 
 sources and used as received . chromatography was performed using sio2 ( 4063 m , 230400 mesh ) as the stationary 
 phase . the complexes 1 , fei2(co)4 , cl2ni(s , s - chiraphos ) , ( pdt)pd(dppe ) , 
 and ( pdt)pt(dppe ) were prepared according 
 to the literature methods . all reactions and purifications were conducted 
 in an mbraun glovebox equipped with a solvent purification system ; 
 the concentrations of o2 and h2o in the n2 atmosphere were typically no higher than 2 and 0.2 ppm , respectively . glassware used in the preparation of deuteride [ 2d]bf4 was washed with d2o and dried prior to use . a waters micromass quattro ii spectrometer was used to acquire esi - ms 
 data for analytes in dilute ch2cl2 or cd2cl2 solution . unless otherwise stated , 
 nmr data were acquired at room temperature , with samples under an 
 atmosphere of n2 . h and p{h } nmr spectra were recorded on a varian vxr500 or u500 spectrometer 
 at 500 and 202 mhz , respectively . h nmr spectra were recorded 
 on a varian ui 500nb spectrometer at 77 mhz . chemical shifts ( /ppm ) 
 are referenced to chdcl2/ch2cl2 ( 5.32 
 ppm for h ) and external 85% h3po4 ( 0 ppm for p ) . cyclic voltammetry was carried out in 
 a single compartment glass cell using a ch instruments chi630d electrochemical 
 analyzer . the working , counter , and pseudoreference electrodes were 
 glassy c , pt , and ag , respectively . the analyte ( 1 mm ) and nbu4pf6 ( 100 mm ) were dissolved in ch2cl2 , and potentials ( reported here relative to internal fc / fc ) were swept at 0.1 v s. crystallographic 
 data were collected using a siemens smart diffractometer equipped 
 with a mo k source (  = 0.71073  ) and 
 an apex ii detector . epr spectra of complexes ( 1 mm in ch2cl2/phme , 1:1 ) were recorded on a varian e - line 
 12 century series x - band cw spectrometer . at 
 28 c , a mixture of fei2(co)4 ( 29.5 
 mg , 70 mol ) and ( pdt)pd(dppe ) ( 42.8 mg , 70 mol ) were 
 dissolved in ch2cl2 ( 1 ml ) with stirring . after 
 1 min , the solution was treated with cocp2 ( 26.5 mg , 140 
 mol ) in ch2cl2 ( 1 ml ) and allowed to 
 warm to room temperature , before it was concentrated to 0.5 
 ml and chromatographed ( 10 cm sio2 , ch2cl2 eluent ) . the fourth band , deep green in color , was 
 concentrated to 2 ml , layered with pentane ( 20 ml ) , 
 and allowed to stand overnight at 28 c . the solids that 
 formed were isolated by filtration , washed with pentane ( 2  
 1 ml ) , and dried briefly to afford the title compound as dark - green 
 plates ( 9.0 mg , 12 mol , 17% ) . h nmr ( cd2cl2 ) :  7.74 ( m , 8h , o - ph ) , 7.43 
 ( m , 8h , m - ph ) , 7.42 ( m , 4h , p - ph ) , 
 2.59 ( ddd , jhh = 13.5 hz , jhh = 5.8 hz , jhh = 3.3 hz , 2h , equatorial ch2ch2ch2 ) , 2.35 
 ( m , 2h , pch2ch2p ) , 2.22 ( m , 2h , pch2ch2p ) , 1.94 ( m , 2h , axial ch2ch2ch2 ) , 1.29 ( m , 2h , ch2ch2ch2 ) . p{h } nmr ( cd2cl2 ) :  51.3 . esi - ms : m / z 749.8 [ m ] , 721.8 [ m  co ] , 693.8 [ m  2co ] ( ionization induced by addition of fc[b(c6h3 - 3,5-(cf3)2)4 ] ) . calcd for 
 c32h30o3s2p2nipd0.2ch2cl2 : c , 50.36 ; h , 3.99 ; n , 
 0.00 . green plates of 2 formed upon slow diffusion of pentane layered onto a concentrated 
 ch2cl2 solution of the title compound at 28 
 c . one crystal ( 0.377  0.232  0.058 mm ) was subjected to x - ray diffraction at 168 k. its space group was 
 determined to be triclinic p-1 with cell parameters : a 8.900  , b 12.883  , c 14.778  ,  104.64 ,  106.73 , 
  90.98. integration of 6798 reflections and solution 
 by direct methods using shelxtl v6.12 afforded a model with r1 = 0.0285 and wr2 = 0.0684 . a stirred solution of 2 ( 11.3 
 mg , 15 mol ) in ch2cl2 ( 1 ml ) was treated 
 with fcbf4 ( 4.1 mg , 15 mol ) in ch2cl2 ( 1 ml ) . after 1 min , pentane ( 20 ml ) was added , and the mixture 
 allowed to stand overnight at 28 c . the solids were 
 isolated by filtration , washed with pentane ( 2  1 ml ) , and dried 
 briefly to afford the title compound as an olive - brown powder ( 3.5 
 mg , 4.2 mol , 28% ) . esi - ms : m / z 749.8 [ m  bf4 ] , 721.8 [ m  co  bf4 ] , 693.8 [ m  2co  bf4 ] . calcd for c32h30feo3p2pds2bf40.5ch2cl2 : c , 44.35 ; h , 3.55 ; n , 0.00 . found : c , 44.26 ; h , 3.19 ; n , 0.54 . a solution of 2 ( 22.5 mg , 
 30 mol ) in ch2cl2 ( 1 ml ) was treated 
 with hbf4 ( 54% et2o solution , 14.6 mg , 90 mol ) 
 in ch2cl2 ( 1 ml ) . the solution was evaporated 
 to dryness , and the oily residue triturated with et2o ( 3 
 ml ) . the resulting solid was isolated by filtration , washed with et2o ( 2  1 ml ) , and dried briefly to afford the title compound 
 as an orange powder ( 21.1 mg , 25 mol , 84% ) . h nmr 
 ( cd2cl2 ) :  7.70 ( m , 8h , o - ph ) , 7.64 ( t , jhh = 6.7 
 hz , 4h , p - ph ) , 7.57 ( m , 8h , m - ph ) , 
 2.87 ( m , 4h , pch2ch2p ) , 2.63 ( m , 1h , equatorial ch2ch2ch2 ) , 2.30 ( m , 4h , ch2ch2ch2 ) , 1.29 ( m , 1h , 
 axial ch2ch2ch2 ) , 
 4.3 ( s , 1h , feh ) . orange blocks of [ 2h]bf4thf0.5et2o formed upon slow diffusion 
 of et2o vapor into a thf solution of the title compound 
 at 28 c . mm ) was subjected to x - ray diffraction at 178 k. its space 
 group was determined to be monoclinic p21/n with cell parameters : a 11.466 
  , b 20.692  , c 34.720 
  ,  90.00 ,  97.41 ,  90.00. 
 integration of 9965 reflections and solution by direct methods using 
 shelxtl v6.12 afforded a model with r1 = 0.0369 and wr2 = 0.0754 . a solution of 2 ( 22.5 mg , 
 30 mol ) in ch2cl2 ( 1 ml ) and cd3od ( 0.1 ml ) was treated with hbf4 ( 54% et2o 
 solution , 14.6 mg , 90 mol ) in cd3od ( 0.3 ml ) . the 
 solution was evaporated to dryness , and the oily residue triturated 
 with et2o ( 3 ml ) . the resulting solid was isolated by filtration , 
 washed with et2o ( 2  1 ml ) , and dried briefly to 
 afford the title compound as an orange powder ( 19.6 mg , 23 mol , 
 78% ) . h nmr ( ch2cl2 ) :  4.3 
 ( m ) . p{h } nmr ( ch2cl2 ) :  64.3 . calcd for c32h30dfeo3p2pds2bf40.25ch2cl2 : c , 44.99 ; h , 3.69 ; n , 0.00 . found : c , 45.03 ; h , 3.82 ; n , 0.00 . compound 2 ( 15.0 mg , 20 mol ) and fcbf4 ( 5.4 mg , 20 
 mol ) were dissolved in ch2cl2 ( 1 ml ) 
 with rapid stirring . after 1 min the solution was added dropwise to 
 pcy3 ( 28.0 mg , 100 mol ) in ch2cl2 ( 0.5 ml ) . the solution was stirred for a further 0.5 min , 
 and pentane ( 28 c , 15 ml ) was added and the mixture 
 allowed to stand overnight at 28 c . the solids were 
 isolated by filtration , washed with pentane ( 28 c , 2 
  2 ml ) , and dried briefly to afford the title compound as a 
 yellow - brown powder ( 5.9 mg , 5.4 mol , 27% ) . esi - ms : m / z 1002.4 [ m  bf4 ] , 974.3 [ m  co  bf4 ] . calcd for c49h63feo2p3pds2bf40.25ch2cl2 : c , 53.23 ; h , 5.76 ; n , 0.00 . 
 found : c , 53.16 ; h , 5.82 ; n , 0.20 . a 
 mixture of solid fei2(co)4 ( 25.3 mg , 60 mol ) 
 and ( pdt)pt(dppe ) ( 42.0 mg , 60 mol ) was cooled to 28 after 1 min , the solution was treated with cocp2 ( 22.7 mg , 120 mol ) in ch2cl2 ( 1 ml ) 
 and allowed to warm to room temperature before it was concentrated 
 to 0.5 ml and chromatographed ( 10 cm sio2 , ch2cl2 eluent ) . the olive - brown band ( following 
 the rapidly eluting orange band containing fe2(pdt)(co)6 ) was concentrated to 2 ml , layered with pentane ( 20 
 ml ) , and allowed to stand overnight at 28 c . the supernatant 
 was removed , the dark residue triturated with pentane ( 2  5 
 ml ) , and dried briefly to afford the title compound as a brown powder 
 ( 17.0 mg , 20.2 mol , 34% ) . h nmr ( cd2cl2 ) :  7.86 ( m , 8h , o - ph ) , 7.48 
 ( m , 8h , m - ph ) , 7.46 ( m , 4h , p - ph ) , 
 2.57 ( m , 2h , ch2ch2ch2 ) , 2.22 ( m , 4h , pch2ch2p ) , 1.58 ( m , 2h , ch2ch2ch2 ) , 1.29 
 ( m , 2h , ch2ch2ch2 ) . p{h } nmr ( cd2cl2 ) :  44.1 ( s , d , jppt = 3280 hz ) . esi - ms : m / z 838.9 
 [ m ] ( ionization induced by addition of fc[b(c6h3 - 3,5-(cf3)2)4 ] ) . calcd for c32h30feo3p2pts2 : c , 45.78 ; h , 3.60 ; n , 0.00 . this compound was prepared 
 analogously 
 to bf4 , instead using 3 as 
 the precursor . this compound 
 was prepared analogously 
 to [ 2h]bf4 , instead using 3 as 
 the precursor . h nmr ( cd2cl2 ) :  7.70 ( m , 8h , o - ph ) , 
 7.647.54 ( m , 12h , m , p - ph ) , 
 2.88 ( m , 2h , pch2ch2p ) , 2.7 ( m , 2h , pch2ch2p ) , 2.59 ( m , 1h , equatorial ch2ch2ch2 ) , 2.43 ( m , 4h , ch2ch2ch2 ) , 1.29 ( m , 1h , 
 axial ch2ch2ch2 ) , 
 3.57 ( s , d , jhpt = 
 173 hz , 1h , feh ) . p{h } nmr 
 ( cd2cl2 ) :  46.4 ( s , d , jppt = 3220 hz ) . calcd for c32h31feo3p2pts2bf40.5ch2cl2 : c , 40.25 ; h , 3.33 ; n , 0.00 . found : c , 40.10 ; h , 2.93 ; 
 n , 0.04 . this compound 
 was prepared analogously to [ 2]bf4 , instead using 3 as the precursor . calcd for c49h63feo2p3pts2bf40.75ch2cl2 : c , 48.09 ; h , 
 5.23 ; n , 0.00 . found : c , 48.06 ; h , 5.42 ; n , 0.52 . this compound was 
 prepared analogously to ( pdt)ni(dppe ) , instead 
 using cl2ni(s , s - chiraphos ) 
 as the precursor . h nmr ( cd2cl2 ) :  
 8.15 ( m , 4h , p - ph ) , 7.637.47 ( m , 16h , o , m - ph ) , 2.19 ( m , 4h , ch2ch2ch2 ) , 2.03 
 ( m , 2h , ch ) , 1.81 ( qu , jhh = 6.3 hz , 4h , ch2ch2ch2 ) , 0.96 ( dd , jph = 
 10.6 hz , jhh = 6.2 hz , 6h , 
 ch3 ) . calcd for c31h34s2p2ni : c , 62.96 ; h , 5.79 ; n , 0.00 . 
 found : c , 62.22 ; h , 5.69 ; n , 0.41 . the complexes ( pdt)ni(s , s - chiraphos ) ( 118.3 mg , 200 mol ) 
 and fe2(co)9 ( 72.8 mg , 200 mol ) were 
 dissolved in ch2cl2 ( 3 ml ) with stirring . after 
 18 h , the deep 
 green solution was concentrated to 0.5 ml and chromatographed 
 ( 10 cm sio2 , ch2cl2 eluent ) . an olive - green band was collected , treated with mecn ( 5 ml ) , 
 concentrated to 1 ml and cooled to 28 c . the 
 solids were isolated by filtration , washed with mecn ( 28 c , 
 2 ml ) and pentane ( 28 c , 2  2 ml ) , and dissolved 
 in ch2cl2 ( 1 ml ) and mecn ( 5 ml ) . the crystals that formed were isolated by filtration , washed with 
 mecn ( 28 c , 2 ml ) and pentane ( 28 c , 2 
  2 ml ) , and dried to afford the title complex as dark green 
 crystals ( 46.9 mg , 64.1 mol , 32% ) . h nmr ( cd2cl2 ) :  8.11 ( m , 4h , ph ) , 7.62 ( m , 4h , ph ) , 
 7.527.39 ( m , 12h , ph ) , 2.57 ( ddd , jhh = 13.4 hz , jhh = 7.1 hz , jhh = 7.1 hz , 
 1h , equatorial ch2ch2ch2 ) , 2.17 ( ddd , jhh = 
 13.2 hz , jhh = 7.3 hz , jhh = 7.1 hz , 1h , equatorial ch2ch2ch2 ) , 1.98 ( m , 2h , 
 ch ) , 1.91 ( m , 1h , equatorial ch2ch2ch2 ) , 1.82 ( m , 2h , axial ch2ch2ch2 ) , 1.31 ( m , 1h , 
 axial ch2ch2ch2 ) , 
 0.87 ( m , 6h , ch3 ) . p{h } nmr ( cd2cl2 ) :  63.8 . p{h } nmr ( cd2cl2 , 90 c ) :  
 75.0 , 52.6 . esi - ms : m / z 729.3 [ m ] ( ionization induced by addition 
 of fc[b(c6h3 - 3,5-(cf3)2)4 ] ) . calcd for c34h34o3s2p2nife : c , 55.84 ; h , 4.69 ; n , 0.00 . this compound was prepared analogously 
 to [ 2h]bf4 , instead using ( co)3fe(pdt)ni(s , s - chiraphos ) as the 
 precursor . h nmr ( cd2cl2 ) :  7.89 ( dd , jhh = 10.8 hz , jhh = 8.0 hz , 4h , p - ph ) , 7.807.57 ( m , 16h , o , m - ph ) , 2.60 ( m , 2h , ch ) , 2.37 ( ddd , jhh = 14.4 hz , jhh = 2.9 hz , jhh = 2.9 hz , 1h , equatorial ch2ch2ch2 ) , 2.29 ( dm , jhh = 13.8 hz , 1h , equatorial ch2ch2ch2 ) , 2.15 ( dm , jhh = 13.7 hz , 
 1h , equatorial ch2ch2ch2 ) , 1.91 ( m , 1h , axial ch2ch2ch2 ) , 1.82 ( ddd , jhh = 13.7 hz , jhh = 13.7 hz , jhh = 2.0 hz , 1h , axial ch2ch2ch2 ) , 1.30 ( m , 1h , axial ch2ch2ch2 ) , 0.87 ( m , 6h , ch3)3.57 ( t , jhp = 4.8 hz , 1h , feh ) . p{h } nmr ( cd2cl2 ) :  71.7 , 67.9 . p{h } nmr ( cd2cl2 , 70 c ) : 
  72.6 ( d , jpp = 67 
 hz ) , 67.2 ( d , jpp = 67 hz ) . calcd for c34h35o3s2p2fenibf40.5ch2cl2 : c , 48.10 ; h , 4.21 ; n , 0.00 . found : c , 48.12 ; h , 4.11 ; n , 
 0.40 . dft calculations were 
 performed using 
 the b3p86 density functional with the sdd pseudopotential and basis 
 set for the fe , ni , pd , and pt atoms , 
 the 6 - 31 g * * basis set for -h ligands , 
 and the 6 - 31 g * basis set for all other atoms . the starting geometries for 1 , [ 1h],2 , and [ 2h ] were obtained from crystal 
 structures . the starting coordinates for atoms in 3 and 
 [ 3h ] were obtained from the crystal structure 
 of [ 2h ] , where the metal center and hydride 
 were manually altered prior to optimization . solvation effects were 
 included using the conductor - like polarizable continuum model with bondi atomic radii and included 
 the non - electrostatic contributions of dispersion , repulsion , and cavitation energies . for the results presented in the main 
 paper , the geometry optimizations were performed in the gas phase . geometry optimizations were also performed in solution and were found 
 to be consistent with the gas phase optimizations ; these results are 
 provided in the supporting information . 
 in all cases , the minimum - energy structures the co values were calculated 
 with dft and were scaled by the standard factor of 0.9850 . the transition states ( tss ) for the tetrahedral 
 to square - planar isomerization were identified using the synchronous 
 transit - guided quasi - newton method , and the resulting 
 structures were confirmed to have only a single imaginary frequency . each ts was verified to lead to the relevant tetrahedral and square - planar 
 geometries by following the irc using the local quadratic approximation for 4 or 5 steps in both directions and subsequently 
 optimizing the geometries . because the potential energy surface was 
 found to be very flat , the single imaginary frequencies were small , 
 and the complete irc connecting the two isomers through the ts could 
 not be obtained . thermochemical data were calculated at t = 298.15 
 k. the reaction free energies ( g ) and 
 free energy barriers ( g ) associated with the tetrahedral to square - planar isomerization 
 in solution included zero - point energy , entropic contributions , and 
 solvation effects . the relative reduction potentials and pka s were calculated from the corresponding 
 reaction free energies using methodology described elsewhere . chemical bonding analysis was performed using 
 nbo . all calculations were performed 
 using the gaussian 09 electronic structure program . structures and energies of the systems studied herein are 
 provided in the supporting information .
theory 
 and experiment indicate that the protonation of reduced 
 nife dithiolates proceeds via a previously undetected isomer with 
 enhanced basicity . 
 in particular , it is proposed that protonation 
 of ( oc)3fe(pdt)ni(dppe ) ( 1 ; pdt2 = s(ch2)3s ; dppe = ph2p(ch2)2pph2 ) occurs at the fe site of the two - electron mixed - valence fe(0)ni(ii ) 
 species , not the fe(i)-ni(i ) bond for the homovalence isomer of 1 . 
 the new pathway , which may have implications for protonation 
 of other complexes and clusters , was uncovered through studies on 
 the homologous series l(oc)2fe(pdt)m(dppe ) , where m = ni , 
 pd ( 2 ) , and pt ( 3 ) and l = co , pcy3 . 
 similar to 1 , complexes 2 and 3 undergo both protonation and 1e oxidation to 
 afford well - characterized hydrides ( [ 2h]+ and 
 [ 3h]+ ) and mixed - valence derivatives ( [ 2]+ and [ 3]+ ) , respectively . 
 whereas the pd site is tetrahedral in 2 , the pt site 
 is square - planar in 3 , indicating that this complex is 
 best described as fe(0)pt(ii ) . in view of the results on 2 and 3 , the potential energy surface of 1 was reinvestigated with density functional theory . 
 these calculations 
 revealed the existence of an energetically accessible and more basic 
 fe(0)ni(ii ) isomer with a square - planar ni site .