Infrared spectroscopic investigation of poly(2-methoxyethyl vinyl ether) during thermosensitive phase separation in water

Hydration changes of poly(2-methoxyethyl vinyl ether) (PMOVE) synthesized via living cationic polymerization have been investigated during a temperature-responsive phase separation in water by using infrared spectroscopy. An aqueous PMOVE solution has lower critical solution temperatures (LCSTs) of 66 degrees C in H2O and 65 degrees C in D2O at approximately 15 wt %. During phase separation, the C-H stretching (nu(C-H)) bands of PMOVE shift downward (red shift). In particular, the IR band assigned to the antisymmetric stretching vibration of the terminal methyl groups exhibits a remarkably large red shift by 16 cm-1. The band also exhibits a red shift with increasing polymer concentration at T < Tp. Density functional theory (DFT) calculations of the models of hydrated PMOVE indicate that the shift is due mainly to the breaking of hydrogen bonds (H-bonds) between the oxygen of the methoxy groups and water and partially to the breaking of the CH...O H-bond to them.     

Cooperativity between OH...O and CH...O hydrogen bonds involving dimethyl sulfoxide-H2O-H2O complex

The cooperativity between the O-H...O and C-H...O hydrogen bonds has been studied by quantum chemical calculations at the MP2/6-311++G(d,p) level in gaseous phase and at the B3LYP/6-311++G(d,p) level in solution. The interaction energies of the O-H...O and C-H...O H-bonds are increased by 53 and 58%, respectively, demonstrating that there is a large cooperativity. Analysis of hydrogen-bonding lengths, OH bond lengths, and OH stretching frequencies also supports such a conclusion. By NBO analysis, it is found that orbital interaction plays a great role in enhancing their cooperativity. The strength increase of the C-H...O H-bond is larger than that of the O-H...O H-bond due to the cooperativity. The solvent has a weakening effect on the cooperativity.     

LCST and UCST behavior of poly(N-isopropylacrylamide) in DMSO/water mixed solvents studied by IR and micro-Raman spectroscopy

Temperature-responsive phase separations of poly(N-isopropylacrylamide) (PNiPAm)/dimethylsulfoxide (DMSO)/water mixtures have been investigated by infrared and confocal micro-Raman spectroscopy. The ternary mixtures exhibited lower critical solution temperature (LCST) and upper critical solution temperature (UCST) phenomena at low and high DMSO concentrations, respectively. The amide I band of PNiPAm consists of two components; the intensity of the 1650 cm-1 component increased, and that of the 1625 cm-1 component decreased with increasing temperature during both LCST and UCST phase transitions. Gradual red shifts of the C-H stretching and the amide II bands with increasing temperature or increasing DMSO concentration indicate a removal of water molecules from the alkyl and N-H groups. Raman microscopic measurements showed that DMSO is excluded from the polymer-rich phases upon both LCST and UCST phase separation. On the basis of the experimental results and the quantum chemical calculations, a model that explains the solvation change of the polymer during phase transitions was proposed.     

An electrostatic model for the frequency shifts in the carbonmonoxy stretching band of myoglobin: correlation of hydrogen bonding and the stark tuning rate

The effect of internal and applied external electric fields on the vibrational stretching frequency for bound CO (nu(CO)) in myoglobin mutants was studied using density functional theory. Geometry optimization and frequency calculations were carried out for an imidazole-iron-porphine-carbonmonoxy adduct with various small molecule hydrogen-bonding groups. Over 70 vibrational frequency calculations of different model geometries and hydrogen-bonding groups were compared to derive overall trends in the C-O stretching frequency (nu(CO)) in terms of the C-O bond length and Mulliken charge. Simple linear functions were derived to predict the Stark tuning rate using an approach analogous to the vibronic theory of activation.(1) Potential energy calculations show that the strongest interaction occurs for C-H or N-H hydrogen bonding nearly perpendicular to the Fe-C-O bond axis. The calculated frequencies are compared to the structural data available from 18 myoglobin crystal structures, supporting the hypothesis that the vast majority of hydrogen-bonding interactions with CO occur from the side, rather than the end, of the bound CO ligand. The nu(CO) frequency shifts agree well with experimental frequency shifts for multiple bands, known as A states, and site-directed mutations in the distal pocket of myoglobin. The model calculations quantitatively explain electrostatic effects in terms of specific hydrogen-bonding interactions with bound CO in heme proteins.     

Study on hydration of poly(N-vinylcaprolactam) microgels by near-IR and mid-IR spectroscopy

Hydration of poly(N-vinylcaprolactam) microgels was investigated by near-infrared (NIR) and mid-infrared (MIR) spectroscopy. The thermosensitive microgels were prepared by emulsion polymerization, and turbidity, dynamic light scattering, and differential scanning calorimetry measurements were carried out. In MIR spectra, carbonyl bands consist of three components due to double, single, and zero hydrogen-bonding carbonyl groups as verified by density functional theory calculations. The relative intensities changed critically at the volume phase transition temperature upon heating. In NIR spectra, two absorbance peaks around 5,900?cm^?1 were observed, which can be assigned to the first overtone of C–H bands. Both of them undergo red shifts during the phase transition in a similar way to that of fundamental bands in MIR spectra. The result suggests that NIR spectroscopy may be a new general method that can provide new information for research on hydration of thermosensitive microgels.     

Evidence of CO(2) molecule acting as an electron acceptor on a nanoporous metal-organic-framework MIL-53 or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2))

The adsorption mode of CO(2) at low coverage in the nanoporous metal benzenedicarboxylate MIL-53(Cr) or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2)) has been identified using IR spectroscopy; the red shift of the nu(3) band and the splitting of the nu(2) mode of CO(2) in addition to the shifts of the nu(OH) and delta(OH) bands of the MIL-53(Cr) hydroxyl groups provide evidence that CO(2) interacts with the oxygen atoms of framework OH groups as an electron-acceptor via its carbon atom; this is the first example of such an interaction between CO(2) and bridged OH groups in a solid.     

Hydrogen bond stabilization in 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate probed by high pressure: the role of charge-enhanced C-H...O interactions in the room-temperature ionic liquid

The hydrogen bonding structures of room-temperature ionic liquids 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate have been studied by infrared spectroscopy. High-pressure infrared spectral profiles and theoretical calculations allow us to make a vibrational assignment of these compounds. The imidazolium C-H bands of 1,3-dimethylimidazolium methyl sulfate display anomalous non-monotonic pressure-induced frequency shifts. This discontinuity in frequency shift is related to enhanced C-H...O hydrogen bonding. This behavior is in contrast with the trend of blue shifts in frequency for the methyl C-H stretching mode at ca. 2960 cm(-1). Our results indicated that the imidazolium C-H groups are more favorable sites for hydrogen bonding than the methyl C-H groups in the pure 1,3-dimethylimidazolium methyl sulfate. Nevertheless, both methyl C-H and imidazolium C-H groups are favorable sites for C-H...O hydrogen bonding in a dilute 1,3-dimethylimidazolium methyl sulfate/D(2)O mixture. Hydrogen bond-like C-H...F interactions were observed between PF(6)(-) and H atoms on the alkyl side chains and imidazolium ring for 1-butyl-3-methylimidazolium hexafluorophosphate.     

Structural organization in aqueous solutions of 1-Butyl-3-methylimidazolium halides: a high-pressure infrared spectroscopic study on ionic liquids

High-pressure infrared spectroscopy was applied to study the hydrogen-bonding structures of 1-butyl-3-methylimidazolium halides/D2O mixtures. No drastic changes were observed in the concentration dependence of the alkyl C-H band frequency at high concentration of 1-butyl-3-methylimidazolium chloride. Nevertheless, the alkyl C-H exhibits an increase in frequency upon dilution at low concentration. These observations may indicate a clustering of the alkyl groups at high concentration and the formation of a certain water structure around alkyl C-H groups in the water-rich region. The imidazolium C-H band at ca. 3051 cm(-1) displays a monotonic blue-shift in frequency as the sample was diluted at high concentration of 1-butyl-3-methylimidazolium chloride. That is, water can be added to change the structural organization of 1-butyl-3-methylimidazolium chloride in the ionic liquid-rich composition region by introducing water-imidazolium C-H interactions. Analyzing the pressure dependence of the imidazolium C-H stretches yielded anomalous nonmonotonic pressure-induced frequency shifts. This result may reflect the strengthening of C-H-O interactions between imidazolium C-H groups and the water clusters. Density functional theory calculations also revealed that the characteristic bonded C2-H vibration may be shifted via the modification of C2-H-Cl- associations.     

Peptide C(alpha)D(alpha) stretch frequencies in a hydrated conformation are perturbed mainly by C(alpha)-D(alpha)...O hydrogen bonding

We have shown (J. Phys. Chem. A 2004, 108, 10923; 2007, 111, 5300) that the C(alpha)D(alpha) stretch frequency, nu(CD), can discriminate between uniform alpha(R), beta, and polyproline II conformations of isolated peptides. Similar results for such peptides to which explicit waters are hydrogen bonded exhibit shifts in nu(CD) from those of the isolated structures. We demonstrate that the main source of these frequency shifts is the formation of C(alpha)-D(alpha)...O hydrogen bonds to water. Taking into account C-H...O(water) hydrogen bonding, together with the traditional bonding of peptide groups to water, can be expected to increase our understanding of the interaction of proteins with their aqueous environment.     

Evidence of rotational isomerism in 1-butyl-3-methylimidazolium halides: a combined high-pressure infrared and Raman spectroscopic study

High-pressure methods were applied to investigate the rotational isomerism and the hydrogen-bonding structures of 1-butyl-3-methylimidazolium bromide and 1-butyl-3-methylimidazolium chloride, respectively. Conformation changes of the butyl chain were observed above a pressure of 0.3 GPa. Under ambient pressure, Raman spectra indicate that the more thermodynamically stable butyl structure of the cations is the gauche-anti (GA) and all-anti forms for 1-butyl-3-methylimidazolium bromide and 1-butyl-3-methylimidazolium chloride, respectively. Nevertheless, the high-pressure phases arise from the perturbed GA conformer. The imidazolium C-H bands of 1-butyl-3-methylimidazolium chloride display anomalous nonmonotonic pressure-induced frequency shifts. This discontinuity in the frequency shift is related to the modification of the imidazolium C-H---Cl- contacts upon compression. The alkyl C-H---Cl- interactions are suggested to be a compensatory mechanism to provide additional stability. Density-functional-theory-calculated results also support the high-pressure results that the methyl and butyl C-H groups are suitable proton donor sites for the GA conformer.     

pi-Acid/pi-base carbonyloxo, carbonylsulfido, and mixed-valence complexes of tungsten

Carbonyloxotungsten(IV) complexes, TpWOX(CO), are produced in the reactions of dioxygen (for X = Cl, Br, I) or pyridine N-oxide [for X = S(2)P(OPr(i))(2), S(2)PPh(2)] with TpWX(CO)(2) [Tp = hydrotris(3,5-dimethylpyrazol-1-yl)borate]. Analogous carbonylsulfidotungsten(IV) species, TpWSX(CO), result from the reactions of TpWX(kappa(2)-MeCN)(CO) with propylene sulfide. The carbonyloxo complexes exhibit nu(CO) and nu(W=O) IR bands in the 1995-1965 and 957-951 cm(-1) regions, respectively; the nu(CO) and nu(W=S) bands of the carbonylsulfido species appear at 1970-1937 and 512-502 cm(-1), respectively. The complexes possess C(1) symmetry and display carbonyl (13)C NMR resonances at delta 272-287, with J(WC) 160-196 Hz. The crystal structures of TpWO(S(2)PPh(2))(CO) and TpWS(S(2)PPh(2))(CO).0.5CHCl(3) reveal distorted octahedral tungsten centers coordinated by a fac tridentate Tp ligand and mutually cis, monodentate chalcogenido [d(W=O) = 1.698(4) A; d(W=S) = 2.135(4) Angstroms], carbonyl, and dithiophosphinato ligands. In refluxing toluene, TpWOI(CO) converts into purple, mixed-valence TpW(III)I(CO)(mu-O)W(V)OITp. The dinuclear complex contains a nearly linear [173.1(6) degrees] mu-oxo bridge connecting disparate distorted octahedral tungsten centers. The metrical parameters and spectroscopic properties are consistent with the presence of a W(III)/W(V) mixed-valence species, possessing a filled, delocalized three-center (W-O-W) pi bond and a localized (on W(III)), filled d(pi) orbital that back-bonds to the carbonyl ligand.     

cis-Dioxomolybdenum(VI) and oxo(phosphine oxide)molybdenum(IV) complexes: steric and electronic fine-tuning of cis-[MoOS]2+ precursors

The complexes cis-Tp(iPr)Mo(VI)O2(OAr) (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate, -OAr = phenolate or naphtholate derivative) are formed upon metathesis of Tp(iPr)MoO2Cl and HOAr/NEt3 in dichloromethane. The orange, diamagnetic dioxo-Mo(VI) complexes exhibit strong nu(MoO2) IR bands at ca. 930 and 905 cm(-1) and NMR spectra indicative of C(s) symmetry. They undergo electrochemically reversible, one-electron reductions at potentials in the range -0.714 to -0.855 V vs SCE (in MeCN) and react with PEt3 to produce Tp(iPr)Mo(IV)O(OAr)(OPEt3). The green, diamagnetic oxo-Mo(IV) complexes display a single nu(MoO) IR band at ca. 950 cm(-1) and exhibit NMR spectra indicative of C1 symmetry. The crystal structures of eight dioxo-Mo(VI) complexes have been determined to assess the degree of frontal (O3-donor face) steric congestion at the Mo center, to identify complexes amenable to conversion into monomeric oxosulfido-Mo(VI) derivatives. The complexes display distorted octahedral geometries, with a cis arrangement of terminal oxo ligands, with d(Mo=O)av = 1.694 A and angle(MoO2)av = 103.4 degrees. Maximal frontal steric congestion is observed in the 2-phenolate derivatives, and these are identified as precursors for strictly monomeric(solid and solution state) oxosulfido-Mo(VI) counterparts.     

Solvent effects on infrared spectra of methyl 4-hydroxybenzoate in pure organic solvents and in ethanol/CCl4 binary solvents

Infrared spectroscopy studies of methyl 4-hydroxybenzoate (MHB) in 17 different organic solvents and in ethanol/CCl4 binary solvent were undertaken to investigate the solvent-solute interactions. The frequencies of carbonyl stretching vibration nu(C=O) of MHB in single solvents were correlated with the solvent acceptor number (AN) and the linear solvation energy relationships (LSER). The assignments of the two bands of nu(C=O) of MHB in alcohols and the single one of that in non-alcoholic solvents were discussed. The shifts of nu(C=O) of MHB in ethanol/CCl4 binary solvents showed that several kinds of solute-solvent hydrogen bonding interactions coexisted in the mixture solvents, with a change in the mole fraction of ethanol in the binary solvents.     

Structural and magnetic studies on cyano-bridged rectangular Fe2M2 (M = Cu, Ni) clusters

Using the tricyano precursor, (Bu4N)[(Tp)Fe(CN)3] (Tp = Tris(pyrazolyl) hydroborate) (1), four new tetranuclear clusters, [(Tp)Fe(CN)3Cu(Tp)]2.2H2O (2), [(Tp)Fe(CN)3Cu(bpca)]2.4H2O (3) (bpca = bis(2-pyridylcarbonyl)amidate anion), [(Tp)Fe(CN)3Ni(tren)]2(ClO4)2.2H2O (4) (tren = tris(2-amino)ethylamine), and [(Tp)Fe(CN)3Ni(bipy)2]2[(Tp)Fe(CN)3]2.6H2O (5) (bipy = 2,2'-bipyridine), have been synthesized and structurally characterized. The four clusters possess similar square structures, where FeIII and MII (M = CuII or NiII) ions alternate at the rectangle corners. There exist intermolecular - stacking interactions through pyrazolyl groups of Tp- ligands in complexes 2 and 4, which lead to 1D chain structures. Complex 5 shows a 3D network structure through the coexistence of - stacking effects and hydrogen-bonding interactions. Magnetic studies show intramolecular ferromagnetic interactions in all four clusters. The exchange parameters are +11.91 and +1.38 cm(-1) for clusters 2 and 3, respectively, while uniaxial molecular anisotropy can be detected in complex 3 due to the distorted core in its molecular structure. Complex 4 has a ground state of S = 3 and shows SMM behavior with an effective energy barrier of U = 18.9 cm(-1). Unusual spin-glass-like dynamic relaxations are observed for complex 5.     

A novel 3D supramolecular coordination polymer based on tetranuclear complex of lead(ii) with terephthalic acid and 8-hydroxyquinolin, [Pb4(8-Quin)4(Tp)2(DMF)2] n

A novel two-dimensional coordination polymer, [Pb₄(8-Quin)₄(Tp)₂(DMF)₂] _n (Tp = terephthalic dianion, 8-Quin = 8-Hydroxyquinolate, and DMF = N,N-dimethylformamide) has been synthesized and characterized by elemental analysis, FT-IR, and single crystal X-ray diffraction. The single crystal X-ray analysis shows that the complex is a two-dimensional (2D) polymer and the dimensionality is further increased to 3D as a result of weak C-H???? and C-H??O interactions.     

Unusual infrared spectrum of ethane adsorbed by gallium oxide

The present study reports an unusual diffuse reflection Fourier transform (DRIFT) spectrum of ethane adsorbed by gallium oxide. One of the stretching C-H bands in this spectrum with a maximum at 2753 cm(-1) is more than by 100 cm(-1) shifted toward lower frequencies in comparison with gaseous ethane. In addition, the relative intensity of this band is unusually high. This indicates a very strong polarizability of the corresponding chemical bonds resulting from perturbation of ethane by the low coordinated Ga(3+) cations. The assignment of this band to the very strongly perturbed initially fully symmetric nu(1) C-H stretching vibration is confirmed by a DFT modeling of ethane adsorption by the simplest Ga(2)O(3) cluster. The obtained results also indicate heterolytic dissociative adsorption and dehydrogenation of ethane by Ga(3+) Lewis sites at elevated temperature. This is evidenced by the appearance of new IR bands from zinc alkyl fragments and acidic protons followed by decomposition of resulting zinc ethyl species. In parallel, the most intense IR band at lower frequency from the most strongly polarized C-H chemical bond decreased in intensity. The obtained results indicate that these vibrations are involved in subsequent heterolytic dissociative adsorption. The obtained results demonstrate that, similar to the shifts of C-H stretching vibrations to the low-frequency, intensities of IR C-H stretching bands can be also used as an index of chemical activation of adsorbed paraffins via their polarization by the low-coordinated cations.     

Xenophilic complexes bearing a Tp(R) Ligand, [Tp(R)M--M'Ln] [Tp(R) = Tp(iPr2), Tp(#) (Tp(Me2,4-Br)); M=Ni, Co, Fe, Mn; M'Ln = Co(CO)4, Co(CO)3(PPh3), RuCp(CO)2]: the two metal centers are held together not by covalent interaction but by electrostatic attraction

A series of dinuclear complexes, [Tp(R)M--M'L(n)] [Tp(iPr(2) )M--Co(CO)(4) (1; M=Ni, Co, Fe, Mn); Tp(#)M--Co(CO)(4) (1'; M=Ni, Co); Tp(#)Ni--RuCp(CO)(2) (3')] (Tp(iPr(2) )=hydrotris(3,5-diisopropylpyrazolyl)borato; Tp(#) (Tp(Me(2),4-Br))=hydrotris(3,5-dimethyl-4-bromopyrazolyl)borato), has been prepared by treatment of the cationic complexes [Tp(iPr(2) )M(NCMe)(3)]PF(6) or the halo complexes [Tp(#)M--X] with the appropriate metalates. Spectroscopic and crystallographic characterization of 1-3' reveals that the tetrahedral, high-spin Tp(R)M fragment and the coordinatively saturated carbonyl-metal fragment (M'L(n)) are connected only by a metal-metal interaction and, thus, the dinuclear complexes belong to a unique class of xenophilic complexes. The metal-metal interaction in the xenophilic complexes is polarized, as revealed by their nu(CO) vibrations and structural features, which fall between those of reference complexes: covalently bonded species [R--M'L(n)] and ionic species [M'L(n)](-). Unrestricted DFT calculations for the model complexes [Tp(H(2) )Ni--Co(CO)(4)], [Tp(H(2) )Ni--Co(CO)(3)(PH(3))], and [Tp(H(2) )Ni--RuCp(CO)(2)] prove that the two metal centers are held together not by covalent interactions, but by electrostatic attractions. In other words, the obtained xenophilic complexes can be regarded as carbonylmetalates, in which the cationic counterpart interacts with the metal center rather than the oxygen atom of the carbonyl ligand. The xenophilic complexes show divergent reactivity dependent on the properties of donor molecules. Hard (N and O donors) and soft donors (P and C donors) attack the Tp(R)M part and the ML(n) moiety, respectively. The selectivity has been interpreted in terms of the hard-soft theory, and the reactions of the high-spin species 1-3' with singlet donor molecules should involve a spin-crossover process.     

IR spectrum of the H(5)O(2)(+) cation in the context of proton disolvates L-H(+)-L

The H(5)O(2)(+) ion has been studied in chlorocarbon, benzene, and weakly coordinating anion environments to bridge the gap between the gas-phase and traditional condensed-phase investigations. Symmetrical cations of the type [H(5)O(2)(+) x 4Solv] are formed via H-bonding with the terminal O-H groups. In the infrared spectrum, the nu(s)OH and nu(as)OH vibrations behave in a manner similar to those of common water molecules: the stronger is the H-bonding interaction with the surroundings, the lower is the frequency shift. A consistent pattern of IR bands from the central O-H(+)-O group is identified, regardless of the strength of the interaction of H(5)O(2)(+) with its environment. Three intense bands develop: a (860-995 cm-1), b (1045-1101 cm(-1)), and c (1672-1700 cm(-1)), as well as two weak bands, d ( approximately 1300 cm(-1)) and e ( approximately 1400-1500 cm(-1)). These fingerprint bands are highly characteristic for vibrations of O-H-O group irrespective of formal charge. They are seen in symmetrical proton disolvates of the type L-H(+)-L, where L is an O-atom donor (alcohol, ether, ketone, phosphate, etc.), and in [A-H-A](-) acid salts (A(-) = oxyanion). The commonality is equivalency of the two O-atoms, a short O...O distance (ca. 2.40 Angstrom), and a flat-bottomed potential well for the bridging proton, that is, a short, strong, low-barrier H-bond. Assignments for bands a-e are suggested in an attempt to resolve inconsistencies between experimental and calculated data.     

Chemical shift correlations in the 13C, 17O and 31P NMR spectra of some Mo(CO)4((PPh2O)2Y(R)R′) (Y(R) = P(O), Si(Me); R′ = alkyl,haloalkyl, aryl) and [Mo(CO)4(PPh2O)2]2Si complexes

The syntheses and ¹³C, ¹⁷O, ²⁹Si and ³¹P NMR spectra of a series of Mo(CO)₄((PPh₂O)₂Y(R)R′) (Y(R) = P(O), Si(Me); R′=alkyl, haloalkyl, aryl) and [Mo(CO)₄(PPh₂O)₂]₂Si complexes are given. The chemical shift ranges of the cis and trans carbonyl ¹³C and ¹⁷O, phenyl C(1) ¹³C and ³¹P resonances are relatively large and, with the exception of the cis carbonyl ¹⁷O chemical shifts, the correlations between the chemical shifts of the various resonances are excellent. These correlations are consistent with the model of metal carbonyl ¹³C and ¹⁷O chemical shifts proposed by Bodner and Todd. In addition they allow the model to be extended to include the diphenylphosphinite ³¹P chemical shifts in these complexes. The excellent correlations may be due to the presence of the chelate ring which limits the rotation around the molybdenum-phosphorus bond and to the fact that all three groups directly bonded to the phosphorus remains constant.     

Oxygen atom transfer in models for molybdenum enzymes: isolation and structural, spectroscopic, and computational studies of intermediates in oxygen atom transfer from molybdenum(VI) to phosphorus(III)

Intermediates in the oxygen atom transfer from Mo(VI) to P(III), [Tp(iPr)MoOX(OPR3)] (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate; X = Cl-, phenolates, thiolates), have been isolated from the reactions of [Tp(iPr)MoO2X] with phosphines (PEt3, PMePh2, PPh3). The green, diamagnetic oxomolybdenum(IV) complexes possess local C(1) symmetry (by NMR spectroscopy) and exhibit IR bands assigned to nu(Mo==O) (approximately 950 cm(-1)) and nu(P==O) (1140-1083 cm(-1)) vibrations. The X-ray crystal structures of [Tp(iPr)MoOX(OPEt3)] (X = OC6H4-2-sBu, SnBu), [Tp(iPr)MoO(OPh)(OPMePh2)], and [Tp(iPr)MoOCl(OPPh3)] have been determined. The monomeric complexes exhibit distorted octahedral geometries, with coordination spheres composed of tridentate fac-Tp(iPr) and mutually cis monodentate terminal oxo, phosphoryl (phosphine oxide), and monoanionic X ligands. The electronic structures and stabilities of the complexes have been probed by computational methods, with the three-dimensional energy surfaces confirming the existence of a low-energy steric pocket that restricts the conformational freedom of the phosphoryl ligand and inhibits complete oxygen atom transfer. The reactivity of the complexes is also briefly described.