Reactions of gold atoms and small clusters with CO: Infrared spectroscopic and theoretical characterization of Au(n)CO (n = 1-5) and Au(n)(CO)2 (n = 1, 2) in solid argon

Laser-ablated Au atoms have been co-deposited with CO molecules in solid argon to produce gold carbonyls. In addition to the previously reported Au(CO)n (n = 1, 2) and Au2(CO)2 molecules, small gold cluster monocarbonyls Au(n)CO (n = 2-5) are formed on sample annealing and characterized using infrared spectroscopy on the basis of the results of the isotopic substitution and CO concentration change and comparison with theoretical predictions. Of particular interest is that the mononuclear gold carbonyls, Au(CO)n (n = 1, 2), are favored under the experimental conditions of higher CO concentration and lower laser energy, whereas the yields of the gold cluster carbonyls, Au(n)CO (n = 2-5) and Au2(CO)2, remarkably increase with lower CO concentration and higher laser power. Density functional theory (DFT) calculations have been performed on these molecules and the corresponding small naked gold clusters. The identities of these gold carbonyls Au(n)CO (n = 1-5) and Au(n)(CO)2 (n = 1, 2) are confirmed by the good agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts.     

Theoretical studies on molecular and structures of mono- and binuclear chromium carbazole derivatives for optoelectronics

Studies on the molecular geometries, electronic properties and second-order nonlinearities of a series of mono- and binuclear chromium carbazole complexes: (N-vinylcarbazole)Cr(CO)(3) (M1), (N-vinylcarbazole)Cr(CO)(2)PPh(3) (M2), (CO)(3)Cr(N-vinylcarbazole)Cr(CO)(3) (B1), and (CO)(3)Cr(N-vinylcarbazole)Cr(CO)(2)PPh(3) (B2) were carried out, using the density functional theory (DFT) at the B3LYP//LanL2DZ/6-31G(d) level. The experimental singlet metal-to-ligand charge transfer ((1)MLCT) spectra of these complexes can also be well simulated and discussed by the time-dependent DFT (TDDFT) at the B3LYP//LanL2DZ/6-311+G(d) level associated with the polarizable continuum model (PCM). The computational results show that an unusual characteristic of chromium carbazole structures is explained in terms of interaction between frontier molecular orbitals of the metal and its ligands. The highest occupied molecular orbitals (HOMOs) of these complexes are composed of a set of distorted degenerated Cr 3d orbitals, whereas the lowest unoccupied molecular orbitals (LUMOs) are predominantly the N-vinylcarbazole ligand π* orbitals. The HOMO-LUMO energy gaps decrease in the order NVC > M1 > B1 > M2 > B2. The considerable coupling between the carbazole and (CO)(3) in M1 creates an asymmetric environment about the chromium atom, leading to modest second-order responses. The PPh(3) ligand is acting as a donor which increases the donating strength of the d(π) orbitals in chromium carbazole species, resulting in the large electronic asymmetry in M2. As for the binuclear chromium carbazole chromophores, a wide-range (1)MLCT band and large oscillator strength are found, allowing for the electronic interactions between two metal centers which can be modified by altering the ligand bound to the metals to induce peculiar asymmetry. Essentially, Cr(CO)(3) acceptor and Cr(CO)(2)PPh(3) donor units in B2 make significant contribution to the charge-transfer process or NLO responses via conventional push-pull chromophoric architecture.     

X-ray absorption spectroscopic studies of chromium nitroso complexes. Crystal and molecular structure of (Ph4P)3[Cr(NO)(NCS)5].2.4(CH3)2CO

X-ray absorption spectroscopy (XAS) provides a direct means of solving the controversy on Cr oxidation states in nitroso complexes. The first XAS studies of four known Cr-NO complexes, [Cr(NO)(OH(2))(5)](2+), [Cr(NO)(acac)(2)(OH(2))], [Cr(NO)(CN)(5)](3)(-), and [Cr(NO)(NCS)(5)](3)(-), have been performed, in comparison with the related Cr(III) complexes, [Cr(OH(2))(6)](3+), [Cr(acac)(3)], [Cr(CN)(6)](3)(-), and [Cr(NCS)(6)](3)(-). The X-ray absorption near-edge structure (XANES) spectra of the Cr-NO complexes are distinguished from those of the corresponding Cr(III) complexes by increased intensities of pre-edge absorbancies due to the 1s --> 3d transition, as well as with slight shifts (by 0.2-1.0 eV) of the edge positions to lower energies, with no major changes in the edge shape. These features, together with the available structural data on Cr-NO complexes, show that the effective Cr oxidation states in such complexes are close to Cr(III), due to the pi-back-bonding within the Cr-NO moiety. Multiple-scattering fitting of X-ray absorption fine structure (XAFS) spectra of [Cr(NO)(acac)(2)(OH(2))] supported the assignment of this complex as a trans-isomer (Keller, A.; Jezovska-Trzebiatowska, B. Polyhedron 1985, 4, 1847-1852). The first crystal structure of a Cr nitroso-isothiocyanato complex, (Ph(4)P)(3)[Cr(NO)(NCS)(5)].2.4(CH(3))(2)CO, has been determined.     

Density functional and Ab initio study of Cr(CO)n (n = 1-6) complexes

Cr(CO)n (n = 1-6) systems were studied for all possible spin states using density functional and high-level ab initio methods to provide a more complete theoretical understanding of the structure of species that may form during ligand dissociation of Cr(CO)6. We carried out geometry optimizations for each system and obtained vibrational frequencies, sequential bond dissociation energies (BDE), and total CO binding energies. We also compared the performance of various DFT functionals. Generally, the ground states of Cr(CO)6, Cr(CO)5, and Cr(CO)4, whose spin multiplicity is a singlet, are in good agreement with both previous theoretical results and currently available experimental data. Calculations on Cr(CO)3, Cr(CO)2, and CrCO provide new findings that the ground state of Cr(CO)3 might be a quintet with C2v symmetry instead of a singlet with C3v symmetry, and the ground state of Cr(CO)2 is not a linear quintet, as suggested by previous DFT calculations, but rather a linear septet. We also found that nonet states of Cr(CO)2 and CrCO display partial C-O bond breakage.     

Spectroscopic detection and theoretical confirmation of the role of Cr2(CO)5(C5R5)2 and .Cr(CO)2(ketene)(C5R5) as intermediates in carbonylation of N=N=CHSiMe3 to O=C=CHSiMe3 by .Cr(CO)3(C5R5) (R = H, CH3)

Conversion of N=N=CHSiMe3 to O=C=CHSiMe3 by the radical complexes .Cr(CO)3C5R5 (R = H, CH3) derived from dissociation of [Cr(CO)3(C5R5)]2 have been investigated under CO, Ar, and N2 atmospheres. Under an Ar or N2 atmosphere the reaction is stoichiometric and produces the Cr[triple bond]Cr triply bonded complex [Cr(CO)2(C5R5)]2. Under a CO atmosphere regeneration of [Cr(CO)3(C5R5)]2 (R = H, CH3) occurs competitively and conversion of diazo to ketene occurs catalytically as well as stoichiometrically. Two key intermediates in the reaction, .Cr(CO)2(ketene)(C5R5) and Cr2(CO)5(C5R5)2 have been detected spectroscopically. The complex .Cr(13CO)2(O=13C=CHSiMe3)(C5Me5) has been studied by electron spin resonance spectroscopy in toluene solution: g(iso) = 2.007; A(53Cr) = 125 MHz; A(13CO) = 22.5 MHz; A(O=13C=CHSiMe3) = 12.0 MHz. The complex Cr2(CO)5(C5H5)2, generated in situ, does not show a signal in its 1H NMR and reacts relatively slowly with CO. It is proposed to be a ground-state triplet in keeping with predictions based on high level density functional theory (DFT) studies. Computed vibrational frequencies are also in good agreement with experimental data. The rates of CO loss from 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)2(C5H5)]2 and CO addition to 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)3(C5H5)]2 have been measured by kinetics and show DeltaH approximately equal 23 kcal mol(-1) for both processes. Enthalpies of reduction by Na/Hg under CO atmosphere of [Cr(CO)n(C5H5)]2 (n = 2,3) have been measured by solution calorimetry and provide data for estimation of the Cr[triple bond]Cr bond strength in [Cr(CO)2(C5H5)]2 as 72 kcal mol(-1). The complex [Cr(CO)2(C5H5)]2 does not readily undergo 13CO exchange at room temperature or 50 degrees C implying that 3Cr2(CO)5(C5H5)2 is not readily accessed from the thermodynamically stable complex [Cr(CO)2(C5H5)]2. A detailed mechanism for metalloradical based conversion of diazo and CO to ketene and N2 is proposed on the basis of a combination of experimental and theoretical data.     

A coupled cluster study of the electronic spectroscopy and photochemistry of Cr(CO)6

The transition energies to the low-lying singlet and triplet excited states of Cr(CO)(6) are computed by equation-of-motion coupled cluster singles and doubles (EOM-CCSD) and similarity transformed equation-of-motion coupled cluster singles and doubles (STEOM-CCSD) methods with all-electrons basis sets. Both experimental and optimized geometries are used for the calculations. Calculations with various basis sets, among them one of the largest calculations performed at the EOM-CCSD level, based on atomic natural orbitals with 627 functions, were used to evaluate the basis set influence on computed transition energies. The presence of a shoulder at 3.9 eV in the experimental absorption spectrum, assigned to the (1)A(1g)-->(1)T(2u) transition, which was not reproduced by recent density functional theory (DFT) or multi-state complete active space perturbation theory (MS-CASPT2) is supported by the present STEOM-CCSD calculations with a theoretical value of 3.92 eV. In addition to this weak (1)A(1g)--> a (1)T(2u) absorption, we observe two strong absorptions corresponding to (1)A(1g)--> a (1)T(1u) at 4.37 eV (vs. an experimental value of 4.46 eV) and (1)A(1g)--> b (1)T(1u) at 5.20 eV (vs. an experimental value of 5.53 eV). Both are characterized as metal-to-ligand charge-transfer (MLCT) allowed transitions. The first metal-centered (MC) absorption at 4.37 eV in our best calculation is degenerate with the lowest MLCT absorbing state. The one-dimensional potential energy curves associated to the low-lying singlet MLCT and MC states as a function of the chromium axial carbonyl bond distance q(a) = [Cr-CO(axial)] show that an avoided crossing exists between the a (1)T(1g) (MC) and a (1)T(1u) (MLCT) states near 1.92 A, which is very close to the equilibrium Cr-CO distance. Moreover, the MC state seems to be dissociative for the CO loss. These two important features could explain the ultra-fast dissociation of CO (100 fs) observed in recent low intensity laser probed gas phase experiments.     

Infrared spectroscopic and density functional theory studies on the CO dissociation by scandium and yttrium dimers

Reactions of laser-ablated scandium and yttrium atoms with dilute carbon monoxide molecules in solid argon have been investigated using matrix-isolation infrared spectroscopy. On the basis of the results of the isotopic substitution, the change of laser power and CO concentration and the comparison with density functional theory (DFT) calculations, the absorption at 1193.4 cm(-1) is assigned to the C-O stretching vibration of the Sc(2)[eta(2)(mu(2)-C,O)] molecule, which has a single bridging CO that is tilted to the side. This CO-activated molecule undergoes ultraviolet-visible photoinduced rearrangement to the CO-dissociated molecule, c-Sc(2)(mu-C)(mu-O). The cyclic c-Sc(2)(mu-C)(mu-O) molecule has a bridging carbon on one side of the Sc(2) unit and a bridging oxygen on the other. The analogous Y(2)[eta(2)(mu(2)-C,O)] molecule has not been observed, but the CO-dissociated c-Y(2)(mu-C)(muO) molecule has been observed in the Y + CO experiments. DFT calculations of the geometry structures, vibrational frequencies, and IR intensities strongly support the assignments. The CO activation mechanism has also been discussed. Our experimental and theoretical results schematically depict an activation process to CO dissociation.     

Molecular and electronic structure of chromium(V) nitrido complexes with azide and isothiocyanate ligands

The first use of [Cr(N)Cl4]2- as a starting material in chromium(v) nitrido chemistry is demonstrated in simple, high yield, metathesis reactions with the pseudohalogens SCN- and N3- yielding five-coordinate, labile complexes: [Cr(N)(NCS)4]2- and [Cr(N)(N3)4]2-, which have been crystallized and characterized by single-crystal X-ray diffraction. Reaction of [Cr(N)(NCS)4]2- with 1,10-phenanthroline furnishes six-coordinate [Cr(N)(NCS)3(phen)]- wherein phenanthroline coordinates to the position trans to the nitrido ligand. The trans influence of the nitrido ligand leads to a bond length difference of 0.223 A between the axial and equatorial ligators from the phenanthroline ligand. The absorption band with lowest energy in these pseudo-linear complexes is assigned as the electric dipole forbidden transition d(xy) --> d(x-y) based on intensities and its variation with the nature of the equatorial ligators. This absorption provides the spectrochemical series for the equatorial ligands, which is found to be numerically almost identical to that determined for chromium(III). DFT calculations reproduce the observed structures and corroborate the ligand field picture of the electronic structure of these complexes.     

Time-resolved vibrational spectroscopy of [FeFe]-hydrogenase model compounds

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe-Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) and Fe(2)(μ-S(2)C(2)H(4))(CO)(6) is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) were studied with density functional theory (DFT) calculations. For Fe(2)(μ-S(2)C(3)H(6))(CO)(6) excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe-Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) with a semibridging CO is reminiscent of the geometry of the Fe(2)S(2) subcluster of the active site observed in Fe-Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis.     

Why is Re-Re bond formation/cleavage in [Re(bpy)(CO)3]2 different from that in [Re(CO)5]2? Experimental and theoretical studies on the dimers and fragments

The Re(NN)(CO)3(THF) (NN=bpy=2,2'-bipyridine or dmb=4,4'-dimethyl-2,2'-bipyridine) radical, produced by homolysis of [Re(NN)(CO)3]2 in THF solution by visible irradiation, dimerizes with rate constants kd=20 +/- 3 and 11 +/- 4 M(-1) s(-1) for NN=dmb and bpy, respectively. The dimerization processes are strikingly slow compared to those of typical metal radicals including Re(CO)5 (kd approximately 10(9) M(-1) s(-1)). In order to explain such slow reactions, we have performed B3LYP hybrid DFT and fully ab initio RHF and MP2 calculations on several conformations of [Re(bpy)(CO)3]2 (cis, trans, skewed cis, skewed trans) and [Re(CO)5]2 (staggered) and on their constituent monomer radicals and anions. The calculations show that the most stable geometry of [Re(bpy)(CO)3]2 is skewed cis, and the experimental infrared spectrum and photochemical properties of the [Re(bpy)(CO)3]2 dimer are best described by the calculated properties of the skewed cis conformer in which there is no low-lying unoccupied orbital that is predominantly sigma(MM) in character. The Re(bpy)(CO)3(THF) ligand radical is more stable than the 5-coordinate "17-electron" metal radical, Re(bpy)(CO)3, suggesting that the extremely slow dimerization rate most likely arises from the solvent blocking the binding site (i.e., the estimated fraction of the five-coordinate monomer is 1.6 x 10(-2)). Theoretical results are consistent with our experimental results that the dimerization process proceeds via the Re centered radical, which is involved in a pre-equilibrium favoring the ligand-centered radical. Furthermore, time-dependent DFT calculations on [Re(bpy)(CO)3]2 and [Re(bpy)(CO)3]- identify the origin of UV-vis absorption in THF.     

Description of excited states in [Re(Imidazole)(CO)3(Phen)]+ including solvent and spin‐orbit coupling effects: Density functional theory versus multiconfigurational wavefunction approach

The low‐lying electronic excited states of [Re(imidazole)(CO)₃(phen)]⁺ (phen = 1,10‐phenanthroline) ranging between 420 nm and 330 nm have been calculated by means of relativistic spin‐orbit time‐dependent density functional theory (TD‐DFT) and wavefunction approaches (state‐average‐CASSCF/CASPT2). A direct comparison between the theoretical absorption spectra obtained with different methods including SOC and solvent corrections for water points to the difficulties at describing on the same footing the bands generated by metal‐to‐ligand charge transfer (MLCT), intraligand (IL) transition, and ligand‐to‐Ligand‐ charge transfer (LLCT). While TD‐DFT and three‐roots‐state‐average CASSCF (10,10) reproduce rather well the lowest broad MLCT band observed in the experimental spectrum between 420 nm and 330 nm, more flexible wavefunctions enlarged either by the number of roots or by the number of active orbitals and electrons destabilize the MLCT states by introducing IL and LLCT character in the lowest part of the absorption spectrum. © 2016 Wiley Periodicals, Inc.     

Photodissociation of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)L and Fe(CO)(4)L: a theoretical study

The photochemistry of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)PH(3) and ax-Fe(CO)(4)PH(3) is studied with time-dependent DFT theory to explore the propensity of the excited molecules to expel their ligands. The influence of the PH(3) ligand on the properties of these complexes is compared with the photodissociation behavior of the binary carbonyl complexes Cr(CO)(6) and Fe(CO)(5). The lowest excited states of Cr(CO)(5)PH(3) are metal-to-ligand charge transfer (MLCT) states, of which the first three are repulsive for PH(3) but modestly bonding for the axial and equatorial CO ligands. The repulsive nature is due to mixing of the initial MLCT state with a ligand field (LF) state. A barrier is encountered along the dissociation coordinate if the avoided crossing between these states occurs beyond the equilibrium distance. This is the case for expulsion of CO but not for the PH(3) group as the avoided state crossing occurs within the equilibrium Cr-P distance. The lowest excited state of ax-Fe(CO)(4)PH(3) is a LF state that is repulsive for both PH(3) and the axial CO. Excited-state quantum dynamics calculations for this state show a branching ratio of 99 to 1 for expulsion of the axial phosphine ligand over an axial CO ligand. The nature of the phosphorus ligand in these Cr and Fe complexes is only of modest importance. Complexes containing the three-membered phosphirane or unsaturated phosphirene rings have dissociation curves for their lowest excited states that are similar to those having a PH(3) ligand. Analysis of their ground-state Cr-P bond properties in conjunction with frontier orbital arguments indicate these small heterocyclic groups to differ from the PH(3) group mainly by their enhanced sigma-donating ability. All calculations indicate that the excited Cr(CO)(5)L and Fe(CO)(4)L molecules (L = PH(3), PC(2)H(5), and PC(2)H(3)) prefer dissociation of their phosphorus substituent over that of an CO ligand. This suggests that the photochemical approach may be a viable complement to the ligand exchange and redox methods that are currently employed to demetalate transition metal complexed organophosphorus compounds.     

Multimetal Fischer carbene complexes of Group VI transition metals: synthesis, structure and substituent effect investigation

Fischer carbene complexes of tungsten with substituents containing up to two additional different transition metals, with all the metals in electronic contact with the carbene carbon atom, were synthesised and studied both in solution and in the solid state. For the complexes of the type [W(CO)(5){C(OR')R}], the substituents chosen were heteroaromatic 2-benzo[b]thienyl (2-BT), or 2-BT π-bonded to a chromium tricarbonyl fragment ([Cr(CO)(3)(2-η(6)-BT)]) or ferrocenyl (Fc) as the R-substituent, while the OR'-substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1b (R' = Et, R = 2-BT), 2b (R' = Et, R = [Cr(CO)(3)(2-η(6)-BT)]), 3b (R' = TiCp(2)Cl, R = 21-BT), 4b (R' = TiCp(2)Cl, R = [Cr(CO)(3)(2-η(6)-BT)]), 5b (R' = Et, R = Fc) and 6b (R' = TiCp(2)Cl, R = Fc). The structural features and their relevance to bonding in the multimetal carbene compounds of both these tungsten and the analogous chromium complexes were investigated as they represent indicators of possible reactivity sites in multimetal carbene assemblies. The possibility of using DFT calculations to quantify the effect of metal-containing substituents on the carbene ligands was tested and correlated with experimental parameters by employing methods such as vibrational spectroscopy, molecular orbital analysis, and cyclic voltammetry.     

Photochemistry of (η6-anisole)Cr(CO)3 and (η6-thioanisole)Cr(CO)3: evidence for a photoinduced haptotropic shift of the thioanisole ligand, a picosecond time-resolved infrared spectroscopy and density functional theory investigation

The photochemistry of (η(6)-anisole)Cr(CO)(3) and (η(6)-thioanisole)Cr(CO)(3) was investigated by picosecond time-resolved infrared spectroscopy in n-heptane solution at 298 K. Two independent excited states are populated following 400 nm excitation of each of these complexes. An excited state with some metal-to-CO charge-transfer character is responsible for the CO-loss process, which is slow compared to CO-loss from Cr(CO)(6). Observed first order rate constants of 1.8 × 10(10) s(-1) and 2.5 × 10(10) s(-1) were obtained for the anisole and thioanisole complexes, respectively. The second excited state has metal-to-arene charge transfer character and results in a haptotropic shift of the thioanisole ligand. DFT calculations characterized the excited states involved and the nature of the haptotropic shift intermediate observed for the thioanisole species.     

Vibrational and electronic absorption spectroscopy of dibenzo[b,def]chrysene and its ions

The vibrational and electronic absorption spectra of dibenzo[b,def]chrysene (DBC) and its ions in argon matrixes have been recorded. Assignment of the observed infrared (IR) bands has been made by comparison with the density functional theory (DFT) computations of harmonic vibrational frequencies (with 6-31G(d,p) or 6-311+G(d,p) basis sets). Extensive time-dependent (TD) DFT calculations of vertical excitation energies have aided in the assignment of the experimental electronic absorption transitions. In general, the theoretical predictions are in good agreement with the observed ultraviolet and visible bands. By correlating IR and UV-visible band intensities (after UV photolysis), it has been shown that both DBC cations and anions are formed. The IR band intensity distributions of the DBC ions differ markedly from neutral DBC. A synthetic spectrum composed of neutral, cationic, and anionic DBC contributions compares reasonably well with the interstellar features of the "unidentified infrared" (UIR) bands from the reflection nebula NGC 7023. Finally, it is shown that the electronic absorption bands of the DBC ions lie in close proximity to several of the diffuse interstellar visible absorption bands (DIBs).     

The involvement of metal-to-CO charge transfer and ligand-field excited states in the spectroscopy and photochemistry of mixed-ligand metal carbonyls. A theoretical and spectroscopic study of [W(CO)(4)(1,2-ethylenediamine)] and [W(CO)(4)(N,N'-bis-alkyl-1,4-diazabutadiene)

A new interpretation of the electronic spectroscopy, photochemistry, and photophysics of group 6 metal cis-tetracarbonyls [M(CO)(4)L(2)] is proposed, that is based on an interplay between M --> L and M --> CO MLCT excited states. TD-DFT and resonance Raman spectroscopy show that the lowest allowed electronic transition of [W(CO)(4)(en)] (en = 1,2-ethylenediamine) has a W(CO(eq))(2) --> CO(ax) charge-transfer character, whereby the electron density is transferred from the equatorial W(CO(eq))(2) moiety to pi orbitals of the axial CO ligands, with a net decrease of electron density on the W atom. The lowest, emissive excited state of [W(CO)(4)(en)] was identified as a spin-triplet W(CO(eq))(2) --> CO(ax) CT excited state both computationally and by picosecond time-resolved IR spectroscopy. This state undergoes 1.5 ps vibrational relaxation/solvation and decays to the ground state with a approximately 160 ps lifetime. The nu(CO) wavenumbers and IR intensity pattern calculated by DFT for the triplet W(CO(eq))(2) --> CO(ax) CT excited state match well the experimental time-resolved spectrum. For [W(CO)(4)(R-DAB)] (R-DAB = N,N'-bis-alkyl-1,4-diazabutadiene), the W(CO(eq))(2) --> CO(ax) CT transition follows in energy the W --> DAB MLCT transition, and the emissive W(CO(eq))(2) --> CO(ax) CT triplet state occurs just above the manifold of triplet W --> DAB MLCT states. No LF electronic transitions were calculated to occur in a relevant energetic range for either complex. Molecular orbitals of both complexes are highly delocalized. The 5d(W) character is distributed over many molecular orbitals, while neither of them contains a predominant metal-ligand sigma 5d(W) component, contrary to predictions of the traditional ligand-field approach. The important spectroscopic, photochemical, and photophysical roles of M(CO(eq))(2) --> CO(ax) CT excited states and the limited validity of ligand field arguments can be generalized to other mixed-ligand carbonyl complexes.     

Spectral and thermal studies of some chromium and molybdenum complexes with ONO donor Schiff bases

Reactions of M(CO)(6), where M=Cr and Mo with Schiff bases prepared by the condensation of ethanolamine with either acetylacetone or benzoylacetone were investigated. The reactions of Cr(CO)(6) in benzene resulted in the formation of the tricarbonyl drivatives [Cr(CO)(3)(HL)], HL=acaceaH or baceaH. The HL proved to act as a tridentate ligand. The corresponding reactions with Mo(CO)(6) in dioxane gave the oxo complexes [Mo(2)O(6)(HL)(2)] with HL was a bidentate. All prepared complexes were investigated using elemental analysis, IR, mass spectrometry, UV-vis absorption spectra and magnetic measurement. Thermal behaviors of the complexes were studied using by thermogravimetry (TG). Schemes for the thermal decomposition were proposed along with their mass fragmentation patterns.     

Haptotropic rearrangement in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl

The para-aminobiphenyl compound [(η(6)-C(6)H(5))(C(6)H(4)-4-NH(2))]Cr(CO)(3) (1) has an arene-phenyl dihedral angle of 38.01(6)°, as determined by single-crystal X-ray crystallography, and 34.7(11)°, as determined by DFT calculations. It undergoes haptotropic rearrangement at 140 °C in solution to form [(η(6)-C(6)H(4)-4-NH(2))(C(6)H(5))]Cr(CO)(3) (2), even though previous reports have suggested that such rearrangements should not be observed in compounds with arene-phenyl dihedral angles greater than 22°. NMR analysis gave a rate constant of k = 5.0 × 10(-5) s(-1) for the rearrangement of 1 to 2. The ortho-substituted analog [(η(6)-C(6)H(5))(C(6)H(4)-2-NH(2))]Cr(CO)(3) (3) has an arene-phenyl dihedral angle of 67.70(7)°, as determined by single-crystal X-ray crystallography, and 51.9(10)°, as determined by DFT calculations. Surprisingly, even though it displays a more extreme canting of arene rings, 3 rearranges to [(η(6)-C(6)H(4)-2-NH(2))(C(6)H(5))]Cr(CO)(3) (4) at 140 °C in solution with a rate constant of k = 2.6 × 10(-4) s(-1). This approximately five-fold rate enhancement likely results from the ortho-amino group providing intramolecular stabilization for intermediates formed during the rearrangement.     

Oxidation of carbon monoxide on group 11 metal atoms: matrix-isolation infrared spectroscopic and density functional theory study

Laser-ablated Cu, Ag, and Au atoms react with CO and O2 mixture in solid argon to produce carbonyl metal oxides, (O2)Cu(CO)(n) (n = 1, 2), (eta(1)-OO)MCO (M = Ag, Au), OCAuO2CO, and OAuCO, as well as group 11 metal carbonyls and oxides. These carbonyl metal oxides are characterized using infrared spectroscopy on the basis of the results of the isotopic substitution and the CO concentration change. Density functional theory (DFT) calculations have been performed on these molecules. The identifications of these carbonyl metal oxides are confirmed by the good agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts. Carbon dioxide is eliminated from these carbonyl metal oxides upon UV irradiation, providing the evidence for the oxidation of carbon monoxide on group 11 metal atoms. The present experiments also reveal that the reactivity of copper toward CO is prior to O2, and the reactivity of silver toward O2 is prior to CO, whereas the reactivity of gold toward CO is comparable to O2.     

Reactions of laser-ablated zinc and cadmium atoms with CO: infrared spectra of the Zn(CO)x (x = 1-3), CdCO-, and Cd(CO)2 molecules in solid neon

Reactions of laser-ablated zinc and cadmium atoms with carbon monoxide molecules in solid neon have been investigated using matrix-isolation infrared spectroscopy. Based on the isotopic substitution, absorptions at 1852.2, 1901.9, 1945.9, and 1995.2 cm(-1) are assigned to the C-O stretching vibrations of the ZnCO, Zn(CO)(2), and Zn(CO)(3) molecules. Absorptions at 1735.8, 1961.3, and 2035.7 cm(-1) are assigned to the C-O stretching vibrations of the CdCO(-) and Cd(CO)(2) molecules. In contrast with the previous argon experiments, more species and more valuable information about the reaction of zinc and cadmium atoms with CO have been obtained in solid neon. Density functional theory calculations have been performed on these zinc and cadmium carbonyls. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts substantiates the identification of these carbonyls from the matrix infrared spectrum. The present experiments also reveal that zinc is more reactive with CO than cadmium.