Observation and Characterization of the Hg-O Diatomic Molecule: A Matrix-Isolation and Quantum-Chemical Investigation.

Mercuric oxide is a well-known and stable solid, but the diatomic molecule Hg-O is very fragile and does not survive detection in the gas phase. However, laser ablation of Hg atoms from a dental amalgam alloy target into argon or neon containing about 0.3 % of 16 O2 or of 18 O2 during their condensation into a cryogenic matrix at 4 K allows the formation of O atoms which react on annealing to make ozone and new IR absorptions in solid argon at 521.2 cm-1 for Hg-16 O or at 496.4 cm-1 for Hg-18 O with the oxygen isotopic frequency ratio 521.2/496.4=1.0499. Solid neon gives a 529.0 cm-1 absorption with a small 7.8 cm-1 blue shift. CCSD(T) calculations found 594 cm-1 for Hg16 O and 562 cm-1 for Hg18 O (frequency ratio=1.0569). Such calculations usually produce harmonic frequencies that are slightly higher than the anharmonic (observed) values, which supports their relationship. These observed frequencies have the isotopic shift predicted for Hg-O and are within the range of recent high-level frequency calculations for the Hg-O molecule. Spectra for the related mercury superoxide and ozonide species are also considered for the first time.

(Noble Gas)n -NC+ Molecular Ions in Noble Gas Matrices: Matrix Infrared Spectra and Electronic Structure Calculations.

An investigation of pulsed-laser-ablated Zn, Cd and Hg metal atom reactions with HCN under excess argon during co-deposition with laser-ablated Hg atoms from a dental amalgam target also provided Hg emissions capable of photoionization of the CN photo-dissociation product. A new band at 1933.4 cm-1 in the region of the CN and CN+ gas-phase fundamental absorptions that appeared upon annealing the matrix to 20 K after sample deposition, and disappeared upon UV photolysis is assigned to (Ar)n CN+ , our key finding. It is not possible to determine the n coefficient exactly, but structure calculations suggest that one, two, three or four argon atoms can solvate the CN+ cation in an argon matrix with C-N absorptions calculated (B3LYP) to be between 2317.2 and 2319.8 cm-1 . Similar bands were observed in solid krypton at 1920.5, in solid xenon at 1935.4 and in solid neon at 1947.8 cm-1 . H13 CN reagent gave an 1892.3 absorption with shift instead, and a 12/13 isotopic frequency ratio-nearly the same as found for 13 CN+ itself in the gas phase and in the argon matrix. The CN+ molecular ion serves as a useful infrared probe to examine Ng clusters. The following ion reactions are believed to occur here: the first step upon sample deposition is assisted by a focused pulsed YAG laser, and the second step occurs on sample annealing: (Ar)2 + +CN→Ar+CN+ →(Ar)n CN+ .

M←NCCH3, M-η2-(NC)-CH3, and CN-M-CH3 Prepared by Reactions of Ce, Sm, Eu, and Lu Atoms with Acetonitrile: Matrix Infrared Spectra and Theoretical Calculations.

The reactions of laser-ablated Ce, Sm, Eu, and Lu atoms with acetonitrile were studied by matrix infrared spectra in a neon matrix, and M←NCCH3, M-η2-(NC)-CH3, and CN-M-CH3 were identified with isotopic substitution and quantum chemical calculations. The major product is the insertion complex (CN-M-CH3), while the end-on and side-on complexes (M←NCCH3 and M-η2-(NC)-CH3) are also trapped in the matrix. The CCN antisymmetric stretching mode for Ce-η2-(NC)-CH3 was observed at 1536.9 cm-1, which is much lower than the same modes observed for other lanthanides. NBO analysis reveals that Ce exhibits a remarkable 4f-orbital contribution in bonding to N and to C, reconfirming an active 4f-orbital contribution of cerium in bonding in the side-on complex, while the 4f contributions of Sm and Eu to the M-N and M-C bonds are much lower and the 4f orbital of Lu is not involved in bonding.

Cyanides, Isocyanides, and Hydrides of Zn, Cd and Hg from Metal Atom and HCN Reactions: Matrix Infrared Spectra and Electronic Structure Calculations.

Zinc and cadmium atoms from laser ablation of the metals and mercury atoms ablated from a dental amalgam target react with HCN in excess argon during deposition at 5 K to form the MCN and MNC molecules and CN radicals. UV irradiation decreases the higher energy ZnNC isomer in favor of the lower energy ZnCN product. Cadmium and mercury atoms produce analogous MCN primary molecules. Laser ablation of metals also produces plume radiation which initiates H-atom detachment from HCN. The freed H atom can add to CN radical to produce the HNC isomer. The argon matrix also traps the higher energy but more intensely absorbing isocyanide molecules. Further reactions with H atoms generate HMCN and HMNC hydrides, which can be observed by virtue of their C-N stretches and intense M-H stretches. Computational modeling of IR spectra and relative energies guides the identification of reaction products by providing generally reliable frequency differences within the Zn, Cd and Hg family of products, and estimating isotopic shifts using to 13 C and 15 N isotopic substitution for comparison with experimental data.

Matrix Infrared Spectroscopic Studies of B-NCCN, B-η2-(NC)-CN, NCBCN, CNBCN, CNBNC, and High-Order Products Produced in Reactions of Boron Atoms with Cyanogen.

The products in reactions of laser-ablated boron atoms with cyanogen in excess argon have been identified via investigation of the matrix spectra and their variation on photolysis, annealing, and isotopic substitutions. DFT calculations have been performed for the plausible products and reaction paths, providing helpful guides. B-NCCN and B-η2-(NC)-CN were observed in the original deposition spectra, but they disappear on photolysis with λ > 220 nm while more stable NCBCN, CNBCN, and CNBNC were produced. Besides these primary products, high-order products [(NC)2B-NCCN, (CN)(NC)B-NCCN, (CN)2B-NCCN, and (NC)2B-B(CN)2] were also observed, which increased in the later stage of annealing. Our calculations show that initially produced B-NCCN is interconvertible to B-η2-(NC)-CN and the more stable boron cyanide and isocyanide, consistent with the observed results. The formation of high-order products demonstrates that boron highly prefers the trivalent state in reactions with cyanogen, similar to aluminum.

Matrix Infrared Spectroscopic and Theoretical Investigations of M···NCCN, M···CNCN, M···C(N)CN, NCMCN, CNMNC, CNMCN, and [M···NCCN]+ Produced in the Reactions of Group 11 Metal Atoms with Cyanogen.

Reactions of group 11 metals with cyanogen, N≡C-C≡N, in excess argon and neon have been carried out, and the products were identified via examination of the matrix spectra and their variation upon photolysis, annealing, and isotopic substitutions. Density functional theory calculations provided helpful information for the plausible products and reaction paths. While M···NCCN and M···CNCN were observed in all three metal systems, the cyanide and isocyanide products (NCMCN, NCMNC, and CNMNC) were identified only in the Cu reactions, and M···C(N)CN was identified in the Cu and Au spectra. Intrinsic reaction coordinate calculation results along with the observed spectral variation upon photolysis and annealing suggest that Cu···C(N)CN was the pathway to cyanide and isocyanide. The product absorptions with exceptionally high C-N stretching frequencies in the Au system have been tentatively assigned to a cation [Au···NCCN+]. The group 11 metal cyanides and isocyanides that require two chemical bonds to the central metal are energetically favorable only in the lightest metal system.

Cyanides and Isocyanides of Zinc, Cadmium and Mercury: Matrix Infrared Spectra and Electronic Structure Calculations for the Linear MNC, NCMCN, CNMNC, NCMMCN, and CNMMNC Molecules.

Cadmium atoms from laser ablation react with cyanogen, NC=CN, in excess argon during co-deposition at 4 K, and even more on UV irradiation of the cold samples. Final annealing to 35 K increases bands at 2187.3 and 2089.2 cm-1 at the expense of weaker bands at 2194.6 and 2092.2 cm-1 through addition of another cadmium atom. Reaction products were identified by comparison with B3LYP and CCSD(T) computed frequencies and energies, through frequency differences between Zn and Cd products, and by cyanogen isotopic substitution. The CN radical, ZnNC, and CdNC were observed on sample deposition. Hg arc ultraviolet (UV) irradiation activates the insertion of Cd and Zn to form the NCCdCN, CNCdNC, NCZnCN and CNZnNC molecules. Next annealing increased the dimetal products NCCdCdCN, CNCdCdNC, NCZnZnCN, and CNZnZnNC at the expense of their single metal analogs. Laser ablated mercury amalgam also produced NCHgCN, NCHg-HgCN, CNHgNC and CNHg-HgNC. The Group12 metals form both cyanide and isocyanide products, and the argon matrix also traps the higher energy but much more intensely absorbing isocyanides. In the isocyanide case bond polarity results in very intense infrared absorptions. Group 12 metals produce shorter M-M bonds in the dimetal cyanides NCM-MCN and isocyanides CNM-MNC than in the M-M itself, and their computed M-M bond lengths compare favorably with those measured for dimetal complexes stabilized by large ring containing molecular ligands.

Matrix Infrared Spectroscopic and Theoretical Investigations of X2CX···MX and CX3-MX Provided in Reactions of Ag and Au with Tetrahalomethanes.

Reactions of laser-ablated silver atoms with tetrahalomethanes have been carried out in excess argon, and the products were identified via examination of the matrix spectra and their variation on photolysis and annealing. While production of the insertion products (CX3-AgX) was evident in all Ag reactions, different sets of product absorptions were also observed in the higher frequency region (1260-720 cm-1). They increased on photolysis but decreased on annealing, opposite to the absorptions of the insertion products. They have been assigned to weakly bound complexes of CX3 and AgX (X2CX···AgX), which were generated in halogen abstraction by the metal atom from CX4. These Cl-mediated Ag complexes are only slightly higher in energy than the insertion products due to the d10s1 electron configuration of the group 11 metal. NBO analyses reveal that the CX3 radical is coordinated to an ionic species (Ag+Cl-) via electron-rich X. The product absorptions marked "m" in the previous Au + CX4 study also most probably originated from the weakly bound product, which is more stable than the methylidene (X2C-AuX2).

Formation of Short Zn-Zn Bonds Stabilized by Simple Cyanide and Isocyanide Ligands.

Cyanogen diluted in argon was reacted with laser ablated Zn atoms to produce the NCZnCN and NCZnZnCN cyanides and higher energy isocyanides ZnNC, CNZnNC, and CNZnZnNC, which were isolated in excess argon at 4 K. These reaction products, identified from the matrix infrared spectra of their -CN and -NC chromophore ligand stretching modes, were confirmed by 13 C and 15 N isotopic substitution and comparison with frequencies calculated by the B3LYP and CCSD(T) methods using the all electron aug-cc-pVTZ basis sets. The cyanide and isocyanide products were increased markedly by mercury arc UV photolysis, which covers the zinc atomic absorption. The above electronic structure calculations that produce appropriate ligand frequencies for these dizinc products also provide their Zn-Zn bond lengths: CCSD(T) calculations find a short 2.367 ŠZn-Zn bond in the NCZnZnCN cyanide, a shorter 2.347 ŠZn-Zn bond in the 37.4 kJ mol-1 higher energy isocyanide CNZnZnNC, and a longer 4.024 Šbond in the dizinc van der Waals dimer. Thus, the diatomic cyanide (-CN) and isocyanide (-NC) ligands are as capable of stabilizing the Zn-Zn bond as many much larger ligands based on their measured and our calculated Zn-Zn bond lengths. This is the first example of dizinc complexes stabilized by different ligand isomers. Additional weaker bands in this region can be assigned to the analogous trizinc molecules NCZnZnZnCN and CNZnZnZnNC.

Infrared Spectra of CH3CN→M, M-η2-(NC)-CH3, CH3-MNC Prepared by Reactions of Laser-Ablated Fe, Ru, and Pt Atoms with Acetonitrile in Excess Argon.

Reactions of laser-ablated Fe, Ru, and Pt atoms with acetonitrile have been carried out in excess argon, and the products identified in the matrix spectra. CH3CN→Fe and Fe-η2-(NC)-CH3 observed in the original deposition spectra converted to CH3-FeNC on uv irradiation. CH3CN→Ru, the only product detected in the Ru system, dissociated on uv irradiation, but was partly reproduced on subsequent visible irradiation and annealing. Similar behavior was found for CH3CN→Pt. The major products (CH3-FeNC, CH3CN→Ru, and CH3CN→Pt) are the most stable constituents in the previously proposed reaction path for Group 4, 5, 6, and 7 metal atoms and acetonitrile, parallel with the previous results. The Group 8 metal π-coordination products are weakly bound complexes due to limited back-donation to the π*-orbitals of CH3CN. Calculations show that the Fe insertion product has a much less bent structure than the Ru analogue, in line with its higher s-character from the first row transition-metal to the C-Fe bond, and the group 8 metal methylidenes are not agostically distorted. The Pt to N bond in CH3CN→Pt is the strongest of all the metals we have investigated owing in large part to its higher electron affinity, which prevents nitrogen lone pair density from entering the pi* orbitals of the C-N group.

Matrix Infrared Spectroscopic and Theoretical Studies for Products Provided in Reactions of Sn with Ethane and Halomethanes.

Tin insertion products (oxidation state 2+) were observed in reactions of laser-ablated Sn atoms with ethane, and halomethanes in excess argon, parallel to the Pb reactions. The CSnX bond angles of the observed Sn complexes are close to right angles, and natural bond orbital calculations show that Sn also utilizes mostly its p-orbitals to make chemical bonds. Bridged Sn complexes [CX2(X)-SnX] were also provided in reactions of tetrahalomethanes via photo-isomerization of the insertion products, showing that the p-orbitals of Sn are more accessible than those of Pb. These products were identified from the matrix infrared spectra on the basis of isotopic shifts and density functional theory frequencies. Considering the previously reported high-oxidation-state products of the lighter group 14 elements and the Pb products with primarily oxidation state 2+ because of the relativistic effects, the observed Sn complexes show a trend that the high-oxidation-state complexes are less favored with increasing atomic mass in group 14, which is opposite to that observed in transition-metal columns.

Mercury Cyanides and Isocyanides: NCHgCN and CNHgNC as well as NCHgHgCN and CNHgHgNC: Simple Molecules with Short, Strong Hg-Hg Bonds.

Mercury atoms, laser-ablated from an amalgam dental filling target, react with cyanogen in excess argon during condensation at 4 K to form two major products in the 2200 cyanide M-C-N stretching region of the IR spectrum, which were assigned to NCHgCN and NCHgHgCN from their antisymmetric C-N stretching mode absorptions at 2213.8 and 2180.1 cm-1 . Two broader bands in the isocyanide region at 2098.2 and 2089.6 cm-1 were assigned to CNHgNC and CNHgHgNC. The N-bonded isomers were computed to be 603/33 and 823/69 times more intense IR absorbers than the C-bonded isomers at the CCSD level of theory. The dissociation energy for the NCHg-HgCN molecule into two HgCN molecules was calculated to be 296 kJ mol-1 and that for CNHg-HgNC into two HgNC molecules is 304 kJ mol-1 . These simple molecules with two cyanide or two isocyanide ligands have two of the shortest and strongest known Hg-Hg single bonds as the two electronegative CN ligands withdraw antibonding electron density from the bonding region.

Matrix Infrared Spectra and Electronic Structure Calculations of Linear Alkaline Earth Metal Di-isocyanides CNMNC, Ionic (NC)M(NC) Bowties, and Ionic (MNC)2 Rings.

Laser-ablated group 2 metal atoms exhibit different reactivities with (CN)2 in excess argon and neon during condensation at 4 K. UV irradiation (220-290 nm) is required to activate Be to produce the linear CNBeNC di-isocyanide molecule with a strong antisymmetric C-N stretching band at 2104.3 cm-1 and a C-N-Be-N-C stretching mode at 1265.7 cm-1. The di-isocyanide appears at lower frequency and exhibits more nitrogen and less carbon isotopic shift than the cyanide counterpart, which is confirmed by B3LYP isotopic frequency calculations. Two weak bands were observed for the cyanide NCBeCN, and three absorptions were found for the mixed ligand CNBeCN molecule, which would be difficult to synthesize and put into a bottle. Mg reacts with (CN)2 to form the CNMgNC counterpart at 2085.8 cm-1 on annealing to 25 K. Absorptions for the Ca(NC)2, Sr(NC)2, and Ba(NC)2 molecules at 2060.8, 2048.1, and 2045.9 cm-1 increase on sample annealing with these more reactive heavier alkaline earth metal atoms, which have calculated twisted bowtie side-bound (CN) structures of C2 symmetry with shorter computed M-N than M-C distances. NBO calculations for the latter molecules reveal natural charges of +1.54, 1.64, 1.71 e on the metal centers and - 0.77, 0.82, and 0.855 e on the corresponding CN subunits, with a doubly occupied sp-sp σ and two p-p π bonds on each (CN), which supports an ionic model for bonding in these molecules. A weaker band at 2056.6 cm-1 behaves nearly the same as the Ca bowtie band in the spectrum on 25 K annealing and photolysis, and it is assigned to the 6 kJ/mol higher-energy linear CNCaNC isomer. Additional similar sharp absorptions for a new Ca, Sr, and Ba reaction product at 2036.3, 2040.6, and 2036.0 cm-1 increase on annealing at the expense of adjacent broader bands: B3LYP and NBO calculations validate their assignment to ionic hexagonal C2 h (MNC)2 rings from the reaction of two M atoms with (CN)2.

Matrix Infrared Spectra, Photochemistry and Density Functional Calculations of Cl--HCCl2, ClHCl-, Cl-ClCCl, and Cl--HCHCl Produced from CHCl3 and CH2Cl2 Exposed to Irradiation from Laser Ablation.

Strong absorptions for Cl--HCCl2 with D and 13C isotopes were observed in the spectra of CHCl3 codeposited with laser-ablated metal atoms, cations, electrons, and vacuum ultraviolet radiation, which shows that the precursor is an effective electron scavenger. The IR spectra, isotopic shifts, and DFT calculations identified the major product as Cl--HCCl2, which is characterized by a strong, broad C-H stretching mode interacting with the overtone of the H-C-Cl bending fundamental. These absorptions decreased on subsequent annealing and photolysis treatments while the ClHCl- absorptions increased, suggesting that dissociation of the chloroform anion generates the stable symmetrical hydrogen dichloride anion as does the reaction of HCl and Cl-. A new set of strong, broad absorptions in the deposition spectra that diminished on the early annealing and photolysis are assigned to the Cl-ClCCl radical isomer. Dominant spectral features in the C-H stretching region for the experiments with CH2Cl2 are assigned to the symmetric C-H and the antisymmetric Cl-H-C-H stretching bands of the methylene chloride anion Cl--HCHCl. The stronger, broader, lower frequency bands are due to the hydrogen-bonded hydrogen stretching, and the weaker, sharper, higher frequency absorptions are due to the terminal C-H bond stretching. Similar experiments with CHBr3 produced absorptions for the analogous Br--HCBr2 and BrHBr- anions.

Matrix Infrared Spectroscopic and Theoretical Studies for the Products of Lead Atom Reactions with Ethane and Halomethanes.

The insertion products of laser-ablated Pb atom reactions with ethane and mono-, di-, tri-, and tetrahalomethanes in excess argon were prepared and identified from their matrix infrared spectra on the basis of DFT computed frequencies and observed isotopic shifts. Unlike the lighter elements in group 14, the heaviest member lead exists primarily in the oxidation state 2+ using 6p orbitals in reaction products due to relativistic contraction of the 6s orbital. The C-Pb-X (X = H, F, Cl) bond is close to a right angle, indicating that Pb contributes mostly p-character to the C-Pb and Pb-X bonds. The lead reaction product with ethane is CH3CH2-Pb-H. The lower energy product in the CH2FCl reaction is CH2F-PbCl, which is photoisomerized to CH2Cl-PbF. A lead methylidene (CCl2-PbCl2) was identified only in reactions with CCl4. The relatively small energy difference between the insertion and methylidene products in the tetrachloride system allows photochemical conversion from the insertion product to the unusual 3+ Pb oxidation-state methylidene. In this case Pb uses three p orbitals in bonding and C is sp2 hybridized leaving spin paired but not bonding by symmetry single electrons in the C 2p orbital perpendicular to the CCl2 plane and in the Pb 6s orbital. More often lead uses 6p orbitals in bonding due to the high electronic promotion energy for 6s electrons.

Reactions of Laser-Ablated Aluminum Atoms with Cyanogen: Matrix Infrared Spectra and Electronic Structure Calculations for Aluminum Isocyanides Al(NC)1,2,3 and Their Novel Dimers.

Laser-ablated Al atoms react with (CN)2 in excess argon during condensation at 4 K to produce AlNC, Al(NC)2, and Al(NC)3, which were computed (B3LYP) to be 27, 16, and 28 kJ/mol lower in energy, respectively, than their cyanide counterparts. Irradiation at 220-580 nm increased absorptions for the above molecules and the very stable Al(NC)4- anion. Annealing to 30, 35, and 40 K allowed for diffusion and reaction of trapped species and produced new bands for the Al(NC)1,2,3 dimers including a rhombic ring core (C)(AlN)2(C) with C's attached to the N's, a (NC)2Al(II)-Al(II)(NC)2 dimer with a computed Al-Al length of 2.557 Å, and the dibridged Al2(NC)6 molecule with a calculated D2 h structure and rhombic ring core like Al2H6. In contrast, the Al(NC)4- anion was destroyed on annealing presumably due to neutralization by Al+. B3LYP calculations also show that aluminum chlorides form the analogous molecules and dimers. In our search for possible new products, we calculated Al(NC)4 and found it to be a stable molecule, but it was not detected here.

Tungsten Hydride Phosphorus- and Arsenic-Bearing Molecules with Double and Triple W-P and W-As Bonds.

Laser ablation of tungsten metal provides W atoms which react with phosphine and arsine during condensation in excess argon and neon, leading to major new infrared (IR) absorptions. Annealing, UV irradiation, and deuterium substitution experiments coupled with electronic structure calculations at the density functional theory level led to the assignment of the observed IR absorptions to the E≡WH3 and HE═WH2 molecules for E = P and As. The potential energy surfaces for hydrogen transfer from PH3 to the W were calculated at the coupled-cluster CCSD(T)/complete basis set level. Additional weak bands in the phosphide and arsenide W-H stretching region are assigned to the molecules with loss of H from W, E≡WH2. The electronic structure calculations show that the E≡WH3 molecules have a W-E triple bond, the HE═WH2 molecules have a W-E double bond, and the H2E-WH molecules have a W-E single bond. The formation of multiple E-W bonds leads to increasing stability for the isomers.

Matrix Infrared Spectra of Insertion and Metallacyclopropane Complexes [CH3CH2-MH and (CH2)2-MH2] Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Ethane.

CH3CH2-MH and (CH2)2-MH2 were identified in the matrix IR spectra from reactions of laser-ablated group 3 metal atoms with ethane, and they were characterized via theoretical investigations. The observed products are the most stable in the proposed reaction path. Because of the small number of valence electrons, the group 3 metal high oxidation-state complexes are less stable. The C-C insertion product [(CH3)2M], which was predicted to be more stable than the observed ones, was not observed probably because of the high energy barrier and a likely slower rate for insertion into one C-C bond than one of six C-H bonds. The C-C bond of the metallacyclopropanes is the shortest among the early transition-metal analogues, and its stretching frequencies are the highest, revealing the weakest interaction between the metal dihydride and ethylidene groups. The undetected ethylidene is not agostic, parallel to the previously examined methylidene.

Observation and Characterization of CH3CH2-MH, (CH2)2-MH2, CH2═CH-MH3, and CH3-C≡MH3- Produced by Reactions of Group 5 Metal Atoms with Ethane.

The primary products in reactions of laser-ablated group 5 metal atoms with ethane were identified in argon matrix IR spectra and characterized via density functional theory computations. The second- and third-row transition metals Nb and Ta produced insertion, metallacyclopropane, vinyl trihydrido, and anionic ethylidyne complexes (CH3CH2-MH, (CH2)2-MH2, CH2═CH-MH3, and CH3C≡MH3-), while the first-row transition metal V yielded only the insertion and metallacyclopropane products. The energetically higher ethylidenes and neutral ethylidynes (CH3CH═MH2 and CH3C[Formula: see text]MH3) were not detected. The unique anionic ethylidynes are the most stable anionic species in the Nb and Ta systems. Evidently back-donation from the metal center to the C-C π* orbital is stronger than that in the group 6 metal analogue but weaker than that in the corresponding group 4 metal complex. The C-M bond for the Nb and Ta ethylidyne anions is a true triple bond.

Thorium and Uranium Hydride Phosphorus and Arsenic Bearing Molecules with Single and Double Actinide-Pnictogen and Bridged Agostic Hydrogen Bonds.

Thorium atoms from laser ablation react with phosphine during condensation in excess argon to produce two new infrared absorptions at 1467.2 and 1436.6 cm-1 near weak bands for ThH and ThH2, which increase on annealing to 25 and 30 K, indicating spontaneous reactions. Analogous experiments with uranium produced two similar bands at 1473.4 and 1456.7 cm-1 above UH at 1423.8 cm-1 and another absorption at 1388.2 cm-1. Electronic structure calculations at the coupled cluster CCSD(T) for Th and density functional theory calculations for U as well as their proximity to other actinide hydride absorptions support assignments of these bands to the simplest molecules HP═ThH2, HP═UH2, and PH2-UH. Arsine gave the analogous products HAs═ThH2, HAs═UH2, and AsH2-UH. The HE═AnH2 molecules (E = P, As; An = Th, U) have strong agostic An-H(E) interactions with H-E-An angles in the range of 60-64°. The calculated agostic bond distances are 9% to 12% longer than terminal single An-H bonds, which suggests that these strong agostic bonds can be considered as bridge bonds since similar relationships are found for the dibridged M2H6 molecules (M = Al, Ga, In). The NBO analysis and the molecular orbitals show the presence of a σ and a π bond for HE═AnH2 molecules that are heavily polarized with most of the density on the P or As.