Oxidative Addition of Dihydrogen to Divanadium in Solid Ne: Multiple‐Bonded Triplet HVVH and Singlet V2(μ‐H)2

Abstract Dinuclear compounds of early transition metals with a high metal–metal bond order are of fundamental interest due to their intriguing bonding situation and of practical interest because of their potential involvement in catalytic processes. In this work, two isomers of V2H2 have been generated in solid Ne by the reaction between V2 and H2 and detected by infrared spectroscopy: the linear HVVH molecule (3Σg − ground state), which is the product of the spin‐allowed reaction between V2 (3Σg − ground state) and H2, and a lower‐energy, folded V2(μ‐H)2 isomer (1A1 ground state) with two bridging hydrogen atoms. Both isomers are characterized by metal–metal bonding with a high bond order; the orbital occupations point to quadruple bonding. Irradiation with ultraviolet light induces the transformation of linear HVVH to folded V2(μ‐H)2, whereas irradiation with visible light initiates the reverse reaction.


Introduction
Many dinuclear complexes of early transition metals have been synthesized in which there are M 2 units that show metalmetal multiple bonding. [1] Due to their electronic properties, such complexes show aunique chemical reactivity that makes them suitable for involvement in catalysis. [2] Among these complexes,there are paddle-wheel complexes like V 2 (DPhF) 4 (DPhF = N,N-diphenylformamidinate), showing short V À V distances of less than 200 pm with at riple bond between the metal atoms. [3,4] To stabilize complexes with astrong bonding between the metal atoms,itisadvisable to reduce the number of ligands as far as possible.H owever,d ue to the increased reactivity of the low-coordinated metals in such complexes,it is necessary to use bulky ligands that shield the metal core. Thep rime example is the dinuclear chromium complex RCrCrR (R = C 6 H 3 -2,6(C 6 H 3 -2,6-i Pr 2 ) 2 )w ith am inimum number of ligands in a trans-bent structure,which even shows quintuple bonding between the two chromium centers with aCrÀCr distance of only 183.5 pm. [5,6] Theelectronic structure of HCrCrH and some other small model systems for this complex have been analyzed theoretically to confirm the presence of fivefold bonding and to understand the origin of the trans-bent alignment. [7,8] However,i ng eneral, it is not clear to what extend the bulky ligands determine the structure.T herefore,itisinparticular desirable to characterize,i fp ossible,e xperimentally,s ystems like the prototypical HMMH molecules with terminally bonded hydrogen atoms and multiple metal-metal bonding.T he matrix-isolation technique is av ersatile means to stabilize and investigate such molecules by isolating them in noble-gas matrices.
An obvious way to generate HMMH molecules in inertgas matrices is the reaction of matrix-isolated bare metal dimers with H 2 in noble-gas matrices.Alarger number of bare transition-metal dimers has been generated in noble-gas matrices, [9] and some studies showed that metal dimers exhibit as pecial reactivity.A st om atrix isolation, an outstanding example is the reactivity of the titanium dimer. Ti 2 reacts with dinitrogen by formation of cyclic Ti 2 (m-N) 2 ,breaking even the strong triple bond, whereas Ti atoms do not show ac orresponding reaction. [10] Another example for the special reactivity of dimers is the platinum dimer cation Pt 2 + ,w hich was shown to be able to activate ammonia in the gas phase (Pt 2 + + NH 3 !Pt 2 NH + + H 2 ), whereas Pt + ,Pt 3 + ,and Pt 4 + were not. [11] However,w ork concerning the reaction of neutral transition-metal dimers with dihydrogen is scarce.T here are many studies in which the reaction of evaporated transition metals Mw ith H 2 has been investigated in matrices. [12] Some of them also report products with the composition M 2 H 2 .The late-transition-metal compounds Rh 2 H 2 [13] and Pd 2 H 2 [14] have been reported to feature rhombic structures (bridging hydrogens), while the compounds HZnZnH, HCdCdH, [15] and HHgHgH [16] show linear structures.Furthermore,absorptions have been tentatively assigned to Mn 2 H 2 , [17] HTiTiH, [18] HZrZrH, [19] and HCrCrH, [20] and the dihydrogen complexes PdPd(H 2 ) [14] and PtPt(H 2 ) [21] have also been observed.
In this work, we report on the generation and characterization of matrix-isolated V 2 H 2 molecules.Sofar, the reaction of V 2 with H 2 has not been reported, only Vatoms have been reacted with dihydrogen. Thermally evaporated Vatoms in Kr and Ar matrices have been found to react with H 2 only upon irradiation (320 nm < l < 380 nm) by formation of VH 2 , whereas Vatoms in their electronic ground state do not react with H 2 . [22] Va toms generated by laser ablation, however, react with H 2 to give VH 2 in Ne and Ar matrices also without specific irradiation. [23] Also,f or some transition-metal-cluster cations,a mong them the dimer cations,t he reactivity with respect to D 2 has been studied, [24] and it was observed that V 2 + reacts in the gas phase with D 2 to yield V 2 D + . [25] Interestingly,v ery recently, the reaction of V 2 + with CO 2 to yield V 2 O 2 + has been investigated, and the surprising observation has been made that the intermediate V 2 O + reacts considerably faster than V 2 + . [26] In this work, it is shown by IR spectroscopy that neutral V 2 reacts with H 2 without as ignificant reaction barrier to linear HVVH under preservation of the spin multiplicity and to the V 2 (m-H) 2 isomer with anonplanar,folded structure (see Figure 1). Thel atter is generated, in particular,u pon irradiation with light of aw avelength in the ultraviolet range between 250 and 385 nm.
To assist in the assignment of the observed bands and to obtain information on the electronic structure of the products, quantum-chemical calculations are performed. Information on the energy and nature of the electronic ground states and low-lying excited states is obtained by multireference configuration-interaction (MRCI) calculations based on completeactive-space self-consistent-field orbitals.V ibrational frequencies are calculated by density-functional calculations.

IR Spectra
TheI Rs pectra recorded after co-depositing vanadium atoms and am ixture of Ne and dihydrogen show broader absorptions at around 1580, 1571, 1569, and 1564 cm À1 ,a nd furthermore absorptions in the region between 331 and 346 cm À1 that belong to the same compound (see Figure 2). Upon annealing to 10 K, four sharper bands appear at 1570.7 (strong), 1569.8 (medium), 1569.4 (medium), and 1568.6 cm À1 (weak), as well as ab and in the low-frequencyr egime at 343.4 cm À1 with ashoulder at 342.5 cm À1 at the expense of the broader bands.U pon irradiation with visible light, the broader bands re-appear with as lightly changed intensity pattern;there is more intensity at around 1580 cm À1 with two maxima at 1581.4 and 1580.2 cm À1 ,a nd in the low-frequency region, there are two pronounced absorptions at 342.4 and 340.3 cm À1 .U pon renewed annealing to 10 K, the sharper bands that appeared in the first annealing again recover intensity at the expense of the broader bands.U pon irradiation with ultraviolet (UV) light, all those bands are extinguished, and upon further annealing to 10 K, the sharp bands only very weakly re-appear.
Another set of absorptions is observed essentially only after annealing (to 10 K), but neither directly after deposition nor directly after irradiation with visible light. These absorptions are found at 1425.1, 1405.3, 917.2, and 565.4 cm À1 (see Figure 3). Theb ands clearly appear upon annealing after deposition, they appear less strongly upon annealing after irradiation with visible light, and they appear most strongly upon annealing after irradiation with UV light. At least after the irradiation with UV light, there are weaker absorptions red-shifted with respect to the clear bands observed after annealing.   . IR spectra at around 1420, 920, and 560 cm À1 of matrices obtained by deposition of Vand H 2 in Ne, a) after deposition, b) after annealing to 10 K, c) after irradiation with visible light, d) after further annealing to 10 K, e) after irradiation with UV light, f) after another annealing to 10 K.
Fora ll those bands,c orresponding counterparts are observed using D 2 or HD.F or the first set of absorptions, there are D 2 counterparts of the four sharper signals that appeared after annealing to 10 Kat1132.6, 1131.9, 1131.8, and 1131.5(sh) cm À1 ,a nd counterparts in the low-frequency regime at 247.0 and 246.2(sh) cm À1 (see Figure 4). Using HD,one group of counterparts is found at 1579.0, 1678.6(sh), 1577.7, and 1577.0 cm À1 near the absorptions found using H 2 , and another group of counterparts is found at 1138.6, 1138.3(sh), 1137.8, and 1137.4(sh) cm À1 near the absorptions found with D 2 .Inthe low-frequencyregion, the HD counterparts are observed at 276.7 and 275.9(sh) cm À1 .Furthermore, using HD,t here appears another weaker band at 442.8 cm À1 not found in the experiments with H 2 or D 2 .
Forthe second set of absorptions,the D 2 counterparts are found at 1035.7, 1021.4, 678.0, and 399.3 cm À1 (see Figures 5 and 6). Using HD,a bsorptions are obtained at 1414.0 and 1412.5(sh) cm À1 between the two high-frequencyb ands with H 2 ,another absorption is obtained at 1030.0 cm À1 between the two high-frequencybands with D 2 .T he HD counterparts for the lower-frequencya bsorptions are observed at 886.8 and 495.6 cm À1 ,b etween the corresponding H 2 and D 2 signals.I n the present experiments,t he signals previously assigned to VH 2 and VH 2 (H 2 )i ns olid Ne [23] have been observed weakly after irradiation.

Assignments
Thef irst set of bands is assigned to the linear HVVH molecule,t he second set of bands is assigned to the folded V 2 (m-H) 2 molecule with bridging Hatoms.T he assignment is based on the number of bands observed, the positions of the bands,a nd their isotopic shifts,a nd it is confirmed by computational results (density functional calculations with the TPSS functional and the TZVP basis set).
Within the first set of bands,w hen using H 2 ,t here is one band in the V À Hstretching region at 1570.7 cm À1 .(Note:for VH 2 in Ne,the V À Hstretching modes are found at 1492.9 and 1553.9 cm À1 . [23] )W henu sing D 2 ,t he band shifts to 1132.2 cm À1 ,c onfirming the presence of aV ÀHu nit. Using HD,two bands in these regions at 1579.0 and 1138.6 cm À1 are found, one in the V À Ha nd one in the V À Dr egion, blueshifted with respect to their H 2 or D 2 counterparts.T his indicates that the latter belong to pairs of bands,t he higherlying member of which is not observed for symmetry reasons, pointing to asymmetrical arrangement of two VÀHgroups in accordance with linear HVVH. Forl inear HVVH, at otal of two modes out of five (s u + , p u )s hould be observable in infrared spectra, and indeed, another mode is observed at 343.4 cm À1 ,t he ungerade combination of the bending motions.W henu sing HD,d ue to symmetry reduction, yet another mode is observed at 442.8 cm À1 ,the formerly gerade combination (p g )o ft he bending vibrations.T he densityfunctional results nicely confirm the assignment (see below).
Within the second set of bands,w hen using H 2 ,t here are two signals at 1425.1 and 1405.3 cm À1 in the region of the stretching vibrations.They have lower wavenumbers than the signals assigned to HVVH, consistent with Hi nabridging configuration. Thei sotopic shifts to 1035.7 and 1021.4 cm À1 when using D 2 confirm that these bands are related to the motion of Hatoms.The observation of two bands in the V À H stretching region already with the homonuclear reactants points to an on-symmetric,l ikely non-planar, structure.F or folded C 2v -symmetric V 2 (m-H) 2 ,f ive out of six modes (3 a 1 , b 1 ,b 2 )should, in principle,beobservable in infrared spectra. Tw oa dditional modes are observed at 917.2 and 565.4 cm À1 , the shift of the Hatoms along the direction of the VÀVaxis at 917.2 cm À1 ,the folding motion at 565.4 cm À1 .Thus,out of the five in principle observable modes,f our are found. The density-functional calculations confirm the assignment to folded C 2v V 2 (m-H) 2 and indicate that the other mode which, in principle,c ould be observed, is very weak, am ode dominated by the VÀVstretching motion.   . IR spectra at around 570, 490, and 400 cm À1 of matrices obtained by deposition of Vand H 2 ,D 2 ,orHDi nNeafter irradiation with UV light and annealing to 10 K.

Calculated Structures and Energies
Theq uantum-chemical calculations,b oth MRCI and density-functional calculations,y ield as the lowest-lying isomer of V 2 H 2 afolded V 2 (m-H) 2 isomer with a C 2v -symmetric structure,and, at an energy higher by about 0.5 eV,alinear HVVH isomer with D 1h symmetry (Figure 7).
According to the MRCI calculations,t he electronic ground state of the folded C 2v isomer is a 1 A 1 state.T he V À Vd istance amounts to 184.4 pm and the V À Hd istance to 179.6 pm. Thetorsional angle between the two HV 2 planes is 120.68 8.T he density-functional calculations yield ab rokensymmetry state that corresponds to the 1 A 1 term. Thev alues for the VÀVand VÀHd istances of 182.9 and 180.1 pm, and the torsional angle between the HV 2 planes of 113.38 8 obtained by the density-functional calculations are close to the MRCI values and confirm that the density-functional calculations give,qualitatively,the correct picture.The relative energies of low-lying excited states (by MRCI) at the structure of the C 2v 1 A 1 ground term are shown in Figure 7. From arelative energy of 0.79 eV above the ground state,t here are different triplet and singlet terms with small energy separations,t he lowest ones are a 3 B 2 and a 1 B 2 term at 0.79 and 0.89 eV.The lowestlying term of the linear HVVH isomer is,a ccording to the MRCI calculations,a 3 S g À term at an energy of 0.48 eV relative to the 1 A 1 state.T he VÀVd istance amounts to 170.7 pm and the VÀHdistance to 170.5 pm. Thesame term is also obtained by the density-functional calculations at ar elative energy of 0.56 eV,i ng ood agreement with the MRCI result. Thev alues for the V À Vand V À Hd istances of 171.1 and 171.6 pm obtained by the density-functional calculations are again close to the MRCI values and confirm the qualitatively correct picture of the density-functional calculations also in the case of the linear isomer. Ther elative energies of the lower-lying terms at the structure of the 3 S g À term are also shown in Figure 7. At 0.15 and 0.17 eV with respect to the 3 S g À term, there are a 1 G g and a 1 S g + term, respectively.F urther singlet and triplet terms follow at energies higher than 1.21 eV.

Calculated Vibrational Frequencies
Thev ibrational frequencies obtained from the densityfunctional calculations support the assignment of the observed bands to linear HVVH and folded V 2 (m-H) 2 .A ll vibrational modes for which the calculations indicate transitions with substantial intensity have been observed with wavenumbers not too far from the calculated values.F or the linear isomer, the wavenumbers of the allowed transitions are calculated to be 1613 and 322 cm À1 (see Table 1), whereas the observed transitions have values of 1570 and 343 cm À1 .T he differences amount to 43 and 21 cm À1 .The calculated isotopic shifts of 461 and 92 cm À1 show an even better agreement with the measured shifts of 438 and 97 cm À1 .For the folded isomer, the calculated wavenumbers of 1522, 1502, 984, and 571 cm À1 (see Table 1) deviate from the measured values of 1425, 1405, 917 and 565 cm À1 by 97, 97, 66, and 5cm À1 ,respectively.Thus, the observed splitting between the symmetric and antisymmetric vibrations at 1425 and 1405 cm À1 is almost quantitatively reproduced by the calculations.A gain, the calculated isotopic shifts of 438, 431, 275, and 162 cm À1 still show abetter agreement with the observed shifts of 389, 384, 239, and 166 cm À1 .S uch deviations of some 10 cm À1 are in the typical range for frequencies obtained by density-functional calculations and have to be expected.
To obtain an estimate for the reaction energy of the formation of V 2 (m-H) 2 and the fragmentation of HVVH, additional density-functional calculations were performed on V 2 ( 3 S g À ), VH( 5 D), and H 2 .A ccording to the calculations,t he formation of V 2 (m-H) 2 via V 2 + H 2 !V 2 (m-H) 2 is exotherm by 1.18 eV.Hence,t he formation of the linear HVVH isomer is still exotherm by 0.62 eV.The 5 D ground state for VH is in line with previous calculations. [27,28,29,30] Thec alculated fragmentation energy of HVVH by HVVH( 3 S g À )!2VH( 5 D)amounts to 2.77 eV.T his value is similar to the experimental value of 2.75 eV for the dissociation energy of V 2 [31] and higher than the value of 1.53 eV for Cr 2 , [32] and supports the presence of multiple bonding in HVVH.

Discussion
Forb oth HVVH and V 2 (m-H) 2 ,c lear,s harp signals are observed only after annealing (not after deposition or irradiation), pointing to the population of af ew (welldefined) matrix sites.F or HVVH, the broader signals found after deposition or irradiation are clearly apparent (Figure 2). Such signals,h owever, are not obvious for V 2 (m-H) 2 .N evertheless,u pon close inspection of the spectra after deposition or irradiation, also for V 2 (m-H) 2 ,broad absorption features somewhat red-shifted to sharper signals can be perceived (Figure 3). Additionally,i nt he spectra after deposition or irradiation, no signals are detected that would indicate the formation of yet another different product. Therefore,i ti sa ssumed that V 2 (m-H) 2 is also present in the matrices after annealing or irradiation, but that the signals are too broad to be clearly seen. Notice that the signals of V 2 (m-H) 2 are,i ng eneral, weaker than those of HVVH: Thestrongest signal of V 2 (m-H) 2 has only about one quarter of the intensity of the strongest signal of HVVH. Thus,the detection of broad signals is expected to be more difficult for V 2 (m-H) 2 .
Theexperiments show that H 2 reacts with V 2 by formation of both al inear HVVH isomer and af olded C 2v V 2 (m-H) 2 isomer.Thus,itis likely that the reaction proceeds without or with only asmall barrier. Thefolded isomer is the more stable one,n evertheless,t he linear isomer is formed in substantial amount. This observation is consistent with the ground-state properties of folded V 2 (m-H) 2 and linear HVVH, as revealed by the calculations.HVVH has a 3 S g À ground state,the same as the diatomic V 2 .T hus,t he formation of HVVH from V 2 and H 2 is aspin-allowed process,whereas the formation of the folded V 2 (m-H) 2 isomer,which has a 1 A 1 ground state,isspinforbidden although energetically favorable.I rradiation with UV light leads to the destruction of HVVH and almost exclusive formation of the more stable V 2 (m-H) 2 .O bviously, by irradiation with UV light, higher-lying excited states are populated that allow an easy isomerization to the folded V 2 (m-H) 2 ,for example,bychange of the spin state due to spinorbit coupling and subsequent isomerization or vice versa.
It is observed that upon irradiation with visible light, the signals of V 2 (m-H) 2 decrease,w hereas those of HVVH increase,a nd upon irradiation with UV light, the signals of HVVH almost entirely vanish, whereas those of V 2 (m-H) 2 markedly increase.T he experiments indicate the conversion of linear HVVH to non-planar V 2 (m-H) 2 by irradiation with UV light and the conversion of V 2 (m-H) 2 to HVVH by irradiation with visible light (Scheme 1).
Thec hemistry of V 2 and H 2 is ar eminiscence of the reactivity of the gallium dimer with dihydrogen. Thegallium dimer Ga 2 ,despite spin restrictions,spontaneously reacts with dihydrogen by formation of cyclic Ga 2 (m-H) 2 ,w hereas ground-state Ga atoms do not react with H 2 .(GaH 2 is formed only upon photoexcitation.) [33,34,35] Furthermore,G a 2 (m-H) 2 features an interesting photochemistry.I rradiation (l = 546 nm) of Ga 2 (m-H) 2 leads to the formation of HGaGaH and H 2 GaGa, and upon irradiation of these products using

Research Articles
other wavelengths (l = 365 nm or l > 700 nm), cyclic Ga 2 (m-H) 2 is reformed. Furthermore,the present results for V 2 H 2 are similar to the findings of at heoretical (density-functional) study on Ti 2 H 2 . [36] There,also afolded isomer with bridging H atoms and al inear HTiTiH isomer were found, the isomer with bridging ligands being the lowest in energy. TheV À Vb ond distance of 170.7 pm for HVVH is surprisingly short, even shorter than that of V 2 ( 3 S g À )o f 180.4 pm, [37] although the leading configuration for V 2 , (6s g + ) 2 (7s g + ) 2 (3p u ) 2 (3p u ) 2 (1d g ) 1 (1d g ) 1 ,even points to aquintuple bond (The occupation numbers of the CASSCF natural orbitals for V 2 yield av alue for the bond order of 4.27). An explanation of this contradictory observation is likely the following:d ue to the formation of the V À Hb onds,t he 4s contribution to the electron density at the Vatoms and in the VÀVb onding region decreases,a nd therefore also the shielding of the Vnuclear charge,and the V 2 core has partial V 2 2+ character leading to ac ontraction of the VÀVb onding orbitals.Although for both V 2 (m-H) 2 and HVVH, the results point to quadruple bonding,the V À Vbond distance in V 2 (m-H) 2 is longer by 13.7 pm than in HVVH. Thev alues for the VÀVb ond order are in qualitative agreement with this observation:t he values point to quadruple bonding for both isomers,b ut the value for HVVH it is still somewhat larger than for V 2 (m-H) 2 .H owever,t he bonding situation differs in some respects.I nV 2 (m-H) 2 ,t here are bridging Ha toms,a nd hence,t here is associated electron density close to the V À V axis.F urthermore,H VVH has at riplet ground state.T herefore,inits leading configuration, two of the five orbitals that contribute to the bonding are occupied by one electron only, whereas in V 2 (m-H) 2 ,t here are four VÀVb onding orbitals occupied by pairs of electrons.T hus,i nH VVH, for the two unpaired electrons,t here is the possibility to keep al arger distance.I ti sl ikely that both factors influence the final bonding situation.
TheV ÀVb ond distances of 184.4 pm and 170.7 pm in V 2 (m-H) 2 and HVVH, respectively,a re still shorter than the values in the paddlewheel complexes V 2 (DPhF) 4 with aformal triple bond (197.8/197.9 pm) [3] and K[V 2 (DPhF) 4 ]w ith af ormal bond order of 3.5 (192.9 pm), [4] which have V 2 4+ and V 2 3+ cores,a nd far shorter than the value of 246.0 pm in ad ivanadium(IV) complex with aV ÀVs ingle bond, [38] supporting the presence of aquadruple bond in V 2 H 2 .HVVH and V 2 (m-H) 2 are further examples of bimetallic metal hydride compounds with ahigh metal-metal bond order,like HCrCrH, HMoMoH, and HWWH. [39] However,t he latter ones with a trans-bent alignment have been characterized only theoretically to date and are still awaiting experimental confirmation.

Conclusion
Multiple bonds between two transition metals are intensely studied due to their intriguing electronic structure and potential applications.T he bond order is particularly high if the number of ligands is decreased to an absolute minimum. However,t he synthesis of RMMR compounds (M = transition metal) by conventional techniques in solution is adifficult task and, by all means,requires bulky ligands R. In this work, we demonstrate the use of the matrix-isolation technique to experimentally characterize simple HMMH compounds with multiple MM bonds.H ence,t wo isomers of unprecedented V 2 H 2 ,f olded V 2 (m-H) 2 and linear HVVH, are generated in solid Ne and identified by their IR spectra. UV irradiation induces the transformation of linear HVVH to folded V 2 (m-H) 2 .I rradiation with visible light leads to the transformation of the folded V 2 (m-H) 2 isomer to linear HVVH. Quantumchemical calculations (MRCI) show that the folded V 2 (m-H) 2 is the lower-lying isomer with a 1 A 1 ground state.T he linear HVVH with a 3 S g À electronic ground state is calculated to be at an energy of 0.48 eV with respect to the folded isomer. Both isomers are characterized by multiple V À Vb onding interactions and the orbital occupations point to quadruple bonding.T he calculated vibrational frequencies (densityfunctional calculations) support the assignment of the observed transitions.A lthough the folded isomer is the lowerlying one,w ithout irradiation, the linear isomer is formed in as ubstantial amount. This observation is consistent with the electronic ground states of the isomers.Since the V 2 molecule has a 3 S g À ground state,the formation of the folded V 2 (m-H) 2 isomer ( 1 A 1 )b yr eaction of H 2 and V 2 is as pin-forbidden process,w hereas the formation of linear HVVH is spinallowed. Thus,i ti sr easonable that as ubstantial amount is trapped as HVVH. In ongoing work, we extend our work on other multiple-bonded HMMH compounds.T he experimental characterization of these small molecules is important for the detailed understanding of the bond properties and, thereby,also the reactivity of these systems.