Infrared Spectra of M‑η²‑C₂H₂, HM–CCH, and HM–CCH^<b>–</b> Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Acetylene

The major HM–CCH and M-η²-C₂H₂ products are observed in the matrix infrared spectra from reactions of laser-ablated group 3 metal atoms with acetylene, while the vinylidene product is not detected. These results reveal that coordination of group 3 metal atoms to the acetylene π-bond and H-migration from C to M readily occur during codeposition and photolysis afterward. The product absorption assigned to the La–vinylidene complex in a previous study is reassigned to one of the absorptions of La-η²-C₂H₂···C₂H₂. Two strong Sc–H stretching absorptions are assigned to the free HSc–CCH^– anion, in accord with a previous study, but a lower frequency counterpart is reassigned to HSc–CCH^– coordinated to acetylene based on the increasing relative intensity of the low-frequency component at higher acetylene concentration. The group 3 metals evidently form weaker π-complexes than the group 4 metals. The addition of an electron to HM–CCH elongates the M–H and M–C bonds in the anionic species due to the lower ionic contributions to the bonding

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≐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

Infrared Spectra of CH₃–MX and CH₂X–MH Prepared in Reactions of Laser-Ablated Gold, Platinum, Palladium, and Nickel Atoms with CH₃Cl and CH₃Br

Au, Pt, Pd, and Ni insertion complexes have been produced in reactions with methyl chloride and bromide. The primary products are identified in the matrix IR spectra on the basis of deuterium shifts in the vibrational frequencies and correlation with DFT calculated frequencies. However, fragments of the insertion complexes and the higher oxidation state derivatives, which have been observed in previous studies for reactions with methane and fluoromethane, were not detected. The Pt C–H insertion complex (CH₂Cl–PtH) is also produced, parallel to the fluorinated analogue. The observed Pt and Pd products have highly bent CMX­(H) moieties, which are traced to the unusually high d orbital contributions from the metal atom to the C–M and M–X­(H) bonds

Infrared Spectra of CX₃–AuCl and CX₂–AuCl₂ Generated in Reactions of Laser-Ablated Gold Atoms with Chlorofluoromethanes and Carbon Tetrachloride

Laser-ablated gold atoms have been reacted with CF₃Cl, ¹³CF₃Cl, CF₂Cl₂, CFCl₃, CCl₄, and ¹³CCl₄ in excess argon during condensation, and the CX₃–AuCl (X = F, Cl) insertion products are generated. In the reactions of CFCl₃ and CCl₄ the methylidene complexes (CXCl–AuCl₂) are also produced, and CFCl–AuCl₂ is interconvertible with the insertion product CFCl₂–AuCl upon photolysis. The tetrachloroauric insertion and methylidene complexes are energetically comparable and also structurally close to each other. These products reveal that C–Cl insertion by Au readily occurs, consistent with our previous Au investigations, and subsequent X migration also follows in case the methylidene is energetically reachable. The C–Au bonds of the insertion and methylidene complexes are comparable, unlike those of the previously studied transition-metal systems, and NBO results also show that the methylidene C–Au bond (B3LYP computed 2.04 Å) is a effectively a single bond. Evidently the gold atom with filled 5d and 4f orbitals cannot accommodate a π system for a methylidene-like complex without larger stabilizing/conjugating ligands on carbon

Infrared Spectra of the Complexes Os←NCCH₃, Re←NCCH₃, CH₃–ReNC, CH₂Re(H)NC, and CHRe(H)₂NC and their Mn Counterparts Prepared by Reactions of Laser-Ablated Os, Re, and Mn Atoms with Acetonitrile in Excess Argon

Acetonitrile forms primarily N-coordination complexes (M←NCCH₃) with Os and Re metal atoms, but these metal atoms produce exclusively methylidyne complexes (HCMH₂X) in similar previous reactions with small alkanes and halomethanes. The Os complex increases on visible photolysis and dissociates partially on UV irradiation without the generation of other new products, whereas the Re complex converts to other products (CH₃–ReNC, CH₂Re­(H)­NC, and CHRe­(H)₂NC) on photolysis. The primary formation of the N-coordination complex originates from its stability relative to that of the nitrile π-complex in these systems. The agostic interaction in the methylidene complex is apparently insignificant, and this rare observation of a methylidyne product with Re reflects that H migration in the isocyanide system is less favorable than those in the hydride and halide analogues. Experiments with Mn gave weaker counterpart product absorptions

Infrared Spectra of Manganese Insertion, Vinyl, and Cyclic Complexes Prepared in Reactions of Laser-Ablated Mn Atoms with Methane, Ethane, Ethyl Chloride, and 1,2-Dichloroethane

Manganese insertion, vinyl, and cyclic complexes are prepared in direct reactions of excited Mn atoms with CH₄, C₂H₆, C₂H₅Cl, and CH₂ClCH₂Cl, all with sextet ground states. The only organometallic product observed in the reaction with methane is CH₃–MnH. The analogous insertion product C₂H₅–MnH is observed with ethane, but hydrogen elimination is accompanied by generation of the vinyl product (CH₂CH–MnH). The unusual stabilities of metallacyclic over carbene products in the haloethane systems are in line with the previously observed group 4 metallacyclopropanes. NBO analyses reveal that the distinctively low metal d-orbital contribution to the C–M and M–H bonds is responsible for the linear backbones of CH₃–MnH and the group 12 metal analogues, which are similar to those of the Grignard reagents. Systematic NBO calculations for the first-row transition-metal CH₃–MH complexes show that a low metal d-contribution to the C–M and M–H bonds gives a linear molecular backbone and that increasing d-character in these bonds decreases the C–M–H angle. The stabilities of the half-filled and filled d-orbitals evidently make the group 7 and 12 metals similar to the group 2 metals. The tendency of increasing preference for higher oxidation state complexes with heavier members of the group is most dramatic for the group 7 metals Mn and Re

Infrared Spectra and Density Functional Calculations for Singlet CH₂SiX₂ and Triplet HC–SiX₃ and XC–SiX₃ Intermediates in Reactions of Laser-Ablated Silicon Atoms with Di‑, Tri‑, and Tetrahalomethanes

Reactions of laser-ablated silicon atoms with di-, tri-, and tetrahalomethanes in excess argon were investigated, and the products were identified from the matrix infrared spectra, isotopic shifts, and density functional theory energy, bond length, and frequency calculations. Dihalomethanes produce planar singlet silenes (CH₂SiX₂), and tri- and tetrahalomethanes form triplet halosilyl carbenes (HC–SiX₃ and XC–SiX₃). The Si-bearing molecules identified are the most stable, lowest-energy product in the reaction systems. While the C–Si bond in the silene is a true double bond, the C–Si bond in the carbene is a shortened single bond enhanced by hyperconjugation of the two unpaired electrons on C to σ*­(Si–X) orbitals, which contributes stabilization through a small amount of π-bonding and reduction of the HCSi or XCSi angles. The C–Si bond lengths in these carbenes (1.782 Å for HC–SiF₃) are between the single-bond length in the unobserved first insertion intermediate (1.975 Å for CHF₂–SiF) and the double-bond length in the silene (1.704 Å for CHFSiF₂). The silicon s²p² and titanium s²d² electron configurations produce similar primary products, but the methylidyne with Ti has a bond to carbon stronger than that of the halosilyl carbene

Matrix Infrared Spectra and Density Functional Calculations for New <i>iso</i>-Halomethanes: CHCl₂–Cl, CHFCl–Cl, CFCl₂–Cl, CHBr₂–Br, and CBr₃–Br in Solid Argon

Laser ablation of transition metals for reactions with halocarbons to produce new metal bearing molecules also exposed these samples to laser plume radiation and its resulting photochemistry. Investigations with CCl₄ also produced several known neutral and charged intermediate species, including the <i>iso-</i>tetrachloromethane CCl₃–Cl observed in previous work and identified by the Maier group. CHCl₂–Cl, CHFCl–Cl, and CFCl₂–Cl, photoisomers of CHCl₃, CHFCl₂, and CFCl₃, were also identified in matrix IR spectra. The new C–X bonds are shorter than those of the reactants, and the Cl atom that is weakly bonded to the residual Cl atom forms an unusually strong C–Cl bond. NBO analysis reveals substantial CCl double-bond character, and the weak Cl···Cl bond is largely ionic. Therefore, the CHX₂–X species can be represented as HXCX^δ+···X^δ−. Ionic properties are revealed for CCl₃–Cl, which has an average C–Cl bond length near the median for the CCl₃ radical and cation and a natural charge of +0.49 for the CCl₃ subunit. IRC computations reproduce smooth interconversion between the reactants and products, and the transition state is energetically close to the product, which is consistent with its disappearance on visible irradiation

Matrix Infrared Spectra and Quantum Chemical Calculations of Ti, Zr, and Hf Dihydride Phosphinidene and Arsinidene Molecules

Laser ablated Ti, Zr, and Hf atoms react with phosphine during condensation in excess argon or neon at 4 K to form metal hydride insertion phosphides (H₂P-MH) and metal dihydride phosphinidenes (HPMH₂) with metal phosphorus double bonds, which are characterized by their intense metal–hydride stretching frequencies. Both products are formed spontaneously on annealing the solid matrix samples, which suggests that both products are relaxed from the initial higher energy M-PH₃ intermediate complex, which is not observed. B3LYP (DFT) calculations show that these phosphinidenes are strongly agostic with acute H–PM angles in the 60° range, even smaller than those for the analogous methylidenes (carbenes) (CH₂MH₂) and in contrast to the almost linear H-NTi subunit in the imines (H-NTiH₂). Comparison of calculated agostic and terminal bond lengths and covalent bond radii for HPTiH₂ with computed bond lengths for Al₂H₆ finds that these strong agostic Ti–H bonds are 18% longer than single covalent bonds, and the bridged bonds in dialane are 10% longer than the terminal Al–H single bonds, which show that these agostic bonds can also be considered as bridged bonds. The analogous arsinidenes (HAsMH₂) have 4° smaller agostic angles and almost the same metal–hydride stretching frequencies and double bond orders. Calculations with fixed H–P–Ti and H–As–Ti angles (170.0°) and <i>C</i>_<i>s</i> symmetry find that electronic energies increased by 36 and 44 kJ/mol, respectively, which provide estimates for the agostic/bridged bonding energies

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