Thermodynamic and kinetic studies of H atom transfer from HMo(CO)3(eta(5)-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: direct insertion of benzonitrile into the Mo-H bond of HMo(N[t-Bu]Ar)3 forming (Ph(H)C=N)Mo(N[t-Bu]Ar)3

Synthetic studies are reported that show that the reaction of either H2SnR2 (R = Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp = eta(5)-C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar = 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct (PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)C=N)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of 1-H and 2 in toluene solution has been measured by solution calorimetry (DeltaH = -13.1 +/- 0.7 kcal mol(-1)) and used to estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol(-1). The enthalpy of reaction of 1-H and 2-NCPh in toluene solution was determined calorimetrically as DeltaH = -35.1 +/- 2.1 kcal mol(-1). This value combined with the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (1(2)) gives an estimated value of 90 kcal mol(-1) for the BDE of the ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrile insertion into 2-H would be exothermic by approximately 36 kcal mol(-1), and this reaction was observed experimentally. Stopped flow kinetic studies of the rapid reaction of 1-H with 2-NCPh yielded DeltaH(double dagger) = 11.9 +/- 0.4 kcal mol(-1), DeltaS(double dagger) = -2.7 +/- 1.2 cal K(-1) mol(-1). Corresponding studies with DMo(CO)3(Cp) (1-D) showed a normal kinetic isotope effect with kH/kD approximately 1.6, DeltaH(double dagger) = 13.1 +/- 0.4 kcal mol(-1) and DeltaS(double dagger) = 1.1 +/- 1.6 cal K(-1) mol(-1). Spectroscopic studies of the much slower reaction of 1-H and 2 yielding 2-H and 1/2 1(2) showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 1(2). The presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant for H atom transfer from 1-H has been measured: 0.09 +/- 0.01 M(-1) s(-1) at 1.3 degrees C and 0.26 +/- 0.04 M(-1) s(-1) at 17 degrees C. Study of the rate of reaction of 1-D yielded kH/kD = 1.00 +/- 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N)3Mo...HMo(CO)3(Cp) is rate limiting.     

Trifluoromethyl substituted ring systems: synthesis,and reactivity towards metal carbonyls

Hexafluoro-but-2-yne and actafluoro-but-2-ene both readily add to cyclopentadiene. Similar Diels-Alder reactions occur between hexafluoro-but-2-yne and cycloheptatriene and cyclooctatetraene. 2,3-Bis(trifluoromethyl)bicyclo[2.2.1]hepta-2,5-diene reacts with chromium and molybdenum hexacarbonyls, and with enneacarbonyl di-iron to give metal complexes [M(diene)(CO)₄] (M = Cr, Mo) and [Fe(diene)(CO)₃], respectively. 6,7-Bis-(trifluoromethyl)tricyclo[3.2.2.0^2,4]nona-6,8-diene obtained from hexafluoro-but-2-yne and cycloheptatriene and 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0^2,5]deca-3,7,9-triene formed from hexafluoro-but-2-yne and cyclooctatetraene also react with molybdenum hexacarbonyl to form complexes of molybdenum di- and tetracarbonyl groups, respectively. ¹H, ¹⁹F and ¹³C n.m.r. spectra of the compounds are described.     

Structural Effects of Group VI Metal Tricarbonyl Binding to Benzenoid Rings: Interruption of Conjugation or Enhanced Aromaticity?

The effect of tricarbonyl (group VI metal) complexation on the geometric aromatic character of benzenoid rings is studied as a function of bond‐length alternation (localization) in the parent arene. Good agreement between theory and experiment is established for (η⁶‐benzene) tricarbonylchromium, ‐molybdenum, and ‐tungsten. It is found that, whereas the electrons of benzene become slightly more localized upon tricarbonyl metal complexation, those of ‘cyclohexatriene' mimics, like in‐starphenylene, become more delocalized. A combination of ab initio quantum‐mechanical and high‐accuracy X‐ray methods leads to a linear structure? structure correlation between the free and metal‐bound arene bond‐alternation geometry. In all cases, the average bond length in the arene increases upon complexation. The computational observation that the average bond length increases more in benzene complexes than in in‐starphenylene implies stronger back bonding in the benzene complexes and coincides with the experimental observation that more‐delocalized arenes form thermodynamically favored complexes. The rotational barriers about the tricarbonylmetal‐to‐arene axis were computed for 1‐Cr, 1‐Mo , and 1‐W as well as for 5‐Cr, 5‐Mo , and 5‐W . Barriers for the former group are characteristically low, almost negligible (0.05 kcal/mol for 1‐Cr ; 0.01 kcal/mol for 1‐Mo ; 0.27 kcal/mol for 1‐W ), whereas for the latter group they are substantial (11.2 kcal/mol for 5‐Cr ; 15.2 kcal/mol for 5‐Mo ; 13.6 kcal/mol for 5‐W ). The higher barriers found in 5‐M compounds are consistent with previous findings.     

Heats of reaction of HMo(CO)3C5H5 with CCl4 and CBr4 and of NaMo(CO)3C5H5 with I2 and CH3. Solution thermochemical study of the MoX bond for X = H,Cl, Br,I and CH3

The heats of reaction of HMo(CO)₃C₅H₅ with CX₄ (X = Cl, Br) producing XMo(CO)₃C₅H₅ have been measured by solution calorimetry and are −31.8±0.9 and −34.4±2.0 kcal/mole, respectively. The heats of reaction of NaMo(CO)₃C₅H₅ with I₂ and CH₃I producing IMo(CO)₃C₅H₅ and H₃CMo(CO)₃C₅H₅ are −32.3± 1.3 and −7.7± 0.3 kcal/mole. Oxidation with Br₂CCl₄ yielding Br₃Mo(CO)₂C₅H₅ was measured for the following complexes: (C₅H₅(CO)₃Mo)₂, (−92.0±1.0 kcal/mole), BrMo(CO)₃C₅H₅ (−24.9± 2.0 kcal/mole) and HMo(CO)₃C₅H₅ (−60.7± 2.0 kcal/mole). These and other data are used to calculate the Mo–X bond strength for X = H, Cl, Br, I, and CH₃. These bond strength estimates are compared to those reported for X₂Mo(C₅H₅)₂. Iodination of H₃CMo(CO)₃C₅H₅, reported in the literature to yield CH₃I and IMo(CO)₃C₅H₅, actually produces CH₃C(O)I and I₃Mo(CO)₂C₅H₅.     

On the reactions of molybdenum and tungsten carbonyls with trimethyl- and triethyl-aluminium

Reactions of hexacarbonylmolybdenum, hexacarbonyltungsten and arene complexes of tricarbonylmolybdenum and tricarbonyltungsten with trimethyl- and triethyl-aluminium have been studied. It has been found, based on IR and NMR spectra, that trialkylaluminium does not form complexes with hexacarbonyls of molybdenum and tungsten. Arene (mesitylene, toluene and benzene) complexes of tricarbonylmolybdenum form 1 : 1 complexes with triethylaluminium, and arene complexes of tricarbonyltungsten form complexes with trimethyl- and triethyl-aluminium. Regardless of the molar ratios of reactants (arene)M(CO)₃/AlEt₃, only one of the three CO groups bonded to molybdenum or tungsten forms a complex with AlEt₃. Fast exchange between free and complexed trialkylaluminium and an exchange of trialkylaluminium between all three carbonyl groups have been observed in benzene, toluene and decalin solutions. In the ¹H NMR spectra of the products of the reactions of (mesitylene)Mo(CO)₃ with AIEt₃ and AIMe₃, signals at –9 to –14ppm (characteristic for molybdenum hydrides) were present. It confirmed an alkylation of molybdenum followed by β- or α-hydrogen elimination with the formation of the corresponding molybdenum hydrides, the actual catalyst of aromatic hydrocarbon hydrogenation.     

Benchmarking the Fluxional Processes of Organometallic Piano-Stool Complexes

The correlation consistent Composite Approach for transition metals (ccCA-TM) and density functional theory (DFT) computations have been applied to investigate the fluxional mechanisms of cyclooctatetraene tricarbonyl chromium ((COT)Cr(CO)₃) and 1,3,5,7-tetramethylcyclooctatetraene tricarbonyl chromium, molybdenum, and tungsten ((TMCOT)M(CO)₃ (M = Cr, Mo, and W)) complexes. The geometries of (COT)Cr(CO)₃ were fully characterized with the PBEPBE, PBE0, B3LYP, and B97-1 functionals with various basis set/ECP combinations, while all investigated (TMCOT)M(CO)₃ complexes were fully characterized with the PBEPBE, PBE0, and B3LYP methods. The energetics of the fluxional dynamics of (COT)Cr(CO)₃ were examined using the correlation consistent Composite Approach for transition metals (ccCA-TM) to provide reliable energy benchmarks for corresponding DFT results. The PBE0/BS1 results are in semiquantitative agreement with the ccCA-TM results. Various transition states were identified for the fluxional processes of (COT)Cr(CO)₃. The PBEPBE/BS1 energetics indicate that the 1,2-shift is the lowest energy fluxional process, while the B3LYP/BS1 energetics (where BS1 = H, C, O: 6-31G(d′); M: mod-LANL2DZ(f)-ECP) indicate the 1,3-shift having a lower electronic energy of activation than the 1,2-shift by 2.9 kcal mol⁻¹. Notably, PBE0/BS1 describes the (CO)₃ rotation to be the lowest energy process, followed by the 1,3-shift. Six transition states have been identified in the fluxional processes of each of the (TMCOT)M(CO)₃ complexes (except for (TMCOT)W(CO)₃), two of which are 1,2-shift transition states. The lowest-energy fluxional process of each (TMCOT)M(CO)₃ complex (computed with the PBE0 functional) has a ΔG^‡ of 12.6, 12.8, and 13.2 kcal mol⁻¹ for Cr, Mo, and W complexes, respectively. Good agreement was observed between the experimental and computed ¹H-NMR and ¹³C-NMR chemical shifts for (TMCOT)Cr(CO)₃ and (TMCOT)Mo(CO)₃ at three different temperature regimes, with coalescence of chemically equivalent groups at higher temperatures.     

Organosilyl and organogermyl isocyanide complexes of group vib metal carbonyls

Syntheses of twelve M(CO)₅L complexes (M  Cr, Mo, W; L  CNSiR₂R′, CNGeR₂R′ for R,R′  Me,Ph) were accomplished by carbonyl displacement from M(CO)₆ by L. Several cis-Mo(CO)₄L₂ complexes and one fac complex, Mo(CO)₃(CNGeMe₃)₃, are also reported, prepared by displacement of bicycloheptadiene or cycloheptatriene from Mo(CO)₄(bicycloheptadiene) and Mo(CO)₃(cycloheptatriene). Infrared and ¹³C NMR spectra confirm that the ligands are isocyanides rather than cyanides although the latter is the stable and predominate form of the pure ligands. The mono-substituted compounds are only moderately stable when sealed in vacuo; otherwise stored they decompose rapidly probably by virtue of reaction with oxygen. The phenylsilyl and phenylgermyl isocyanide complexes are harder to store than the methyl analogues. The bis and tris complexes were very difficult to study, being thermally very unstable as well as reactive toward oxygen so that characterization of these species was only marginally successful.     

The hydrogenation of α-methylstyrene by tricarbonyl(cyclopentadienyl)hydride compounds of tungsten and molybdenum; support for a radical mechanism

Rates of reaction of the hydrides of tungsten and molybdenum of the form HM(η5-C₅H₅(CO)₃, with β-methylstyrene have been determined. The rate law is first order in olefin and in hydride. A mechanism involving a rate limiting step of hydrogen atom transfer to the olefin is consistent with the rate law, isotope effect and the absence of CO inhibition. The activation enthalpy for the reactions of HW(η5-C₅H₅)(CO)₃ and HMo(η5-C₅H₅)(CO)₃ are 97.5 ± 4.2 and 89.1 ± 3.3 kJ/mol, respectively. The rate constant for the reaction of styrene and HW(β5-C₅H₅)(CO)₃ is approximately that of β-methylstyrene, while β-methylstyrene was not observed to react under the conditions of the previous determinations. This suggests that attack by the hydride occurs at the β-carbon and this process is inhibited by substituents at that location.     

Highly fluxional tetrahapto-cyclooctatetraene complexes of ruthenium(0)

In complexes of the type Ru(arene)(cot) (cot = cyclooctatetraene) the cyclooctatetraene ring is 1—4-η-bonded as shown by X-ray diffraction study of the hexamethylbenzene derivative, and the barrier to intramolecular exchange of the bound and unbound halves of the eight-membered ring is unusually low (< 6 kcal/mol).     

DFT study on mechanism of carbonyl hydrosilylation catalyzed by high-valent molybdenum (IV) hydrides

Density functional theory (DFT) calculations have been employed to investigate hydrosilylation of carbonyl compounds catalyzed by three high-valent molybdenum (VI) hydrides: Mo(NAr)H(Cp)(PMe₃) (1A), Mo(NAr)H(PMe₃)₃ (1B), and Mo(NAr)H (Tp)(PMe₃) (Tp?=?tris(pyrazolyl) borate) (1C). Three independent mechanisms have been explored. The first mechanism is “carbonyl insertion pathway”, in which the carbonyls insert into Mo?H bond to give a metal alkoxide complex. The second mechanism is the “ionic hydrosilylation pathway”, in which the carbonyls nucleophilically attacks η¹-silane molybdenum adduct. The third mechanism is [2 + 2] addition mechanism which was proposed to be favorable for the high-valent di-oxo molybdenum complex MoO₂Cl₂ catalyzing the hydrosilylation. Our studies have identified the “carbonyl insertion pathway” to be the preferable pathway for three molybdenum hydrides catalyzing hydrosilylation of carbonyls. For Mo(NAr)H (Tp)(PMe₃) (Tp?=?tris(pyrazolyl) borate), the proposed nonhydride mechanism experimentally is calculated to be more than 32.6?kcal/mol higher than the “carbonyl insertion pathway”. Our calculation results have derived meaningful mechanistic insights for the high-valent transition metal complexes catalyzing the reduction reaction.     

The heats of hydrogenation of the metal—metal bonded complexes [M(CO)3C5H5]2 (M = Cr,Mo, W)

Direct measurement of the enthalpy of decomposition of HCr(CO)₃C₅H₅ to [Cr(CO)₃C₅H₅]₂ and H₂ was made by differential scanning calorimetry. The heat of hydrogenation of 1,3-cyclohexadiene by HM(CO)₃C₅H₅ for M = Cr, Mo, and W was measured by solution calorimetry. The enthalpies of iodination of [M(CO)₃C₅H₅]₂ and HM(CO)₃C₅H₅ were measured for M = Mo and W. These data have been used to calculate the heats of hydrogenation for each of the metal—metal bonded dimers, [M(CO)₃C₅H₅]₂ (M = Cr, Mo, and W).C₅H₅(CO)₃M-M(CO)₃C₅H₅(s) + H₂(g) → 2HM(CO)₃C₅H₅(s)Addition of hydrogen has been found to be exothermic for M = Cr, W (?3.3 kcal/mol and ?1.5 kcal/mole, respectively) but endothermic for M = Mo (+6.3 kcal/mol). These results are consistent with the trend of increasing MH bond strengths upon descending Group VI. Addition of H₂ to [Cr(CO)₃C₅H₅]₂ is favored by the unusually weak chromium—chromium bond.     

Synthesis and Crystal Structures of the Molybdenum(II) Complexes with the N,N‐dimethylthiocarbamoyl Containing Ligand: Crystal Structures of β [Mo(CO)2{η2‐S2P(OEt)2}(η2‐SCNMe2)(PPh3)] and [Mo(CO)2(η3‐Tp)(η2‐SCNMe2)(PPh3)]

Reactions of the thiocarbamoyl‐molybdenum complex [Mo(CO)₂(η²‐SCNMe₂)(PPh₃)₂Cl] 1 , and ammonium diethyldithiophosphate, NH₄S₂P(OEt)₂, and potassium tris(pyrazoyl‐1‐yl)borate, KTp, in dichloromethane at room temperature yielded the seven coordinated diethyldithiophosphate thiocarbamoyl‐molybdenum complexe [Mo(CO)₂{η²‐S₂P(OEt)₂}(η²‐SCNMe₂)(PPh₃)] β‐3 , and tris(pyrazoyl‐1‐yl)borate thiocabamoyl‐molybdenum complex [Mo(CO)₂(η³‐Tp)(η²‐SCNMe₂)(PPh₃)] 4 , respectively. The geometry around the metal atom of compounds β‐3 and 4 are capped octahedrons. The α‐ and β‐isomers are defined to the dithio‐ligand and one of the carbonyl ligands in the trans position in former and two carbonyl ligands in the trans position in later. The thiocabamoyl and diethyldithiophosphate or tris(pyrazoyl‐1‐yl)borate ligands coordinate to the molybdenum metal center through the carbon and sulfur and two sulfur atoms, or three nitrogen atoms, respectively. Complexes β‐3 and 4 are characterized by X‐ray diffraction analyses.     

Quantum chemical study of the structures and dynamic behavior of tricarbonyl complexes of Group 6 metals (Cr,Mo, W) with polyaromatic hydrocarbons using the density functional theory

The quantum chemical study of the mechanism was performed for tricarbonyl η⁶-complexes of coronene I-M and kekulene II-M (M = Cr, Mo, W) by the density functional method. The activation barriers of η⁶,η⁶-interring haptotropic rearrangements (IHR), being the migration of the metaltricarbonyl group M(CO)₃ from one six-membered aromatic ring to another, were determined. The processes of η⁶,η⁶-IHR in the metal tricarbonyl complexes with relatively high polycyclic aromatic hydrocarbons (PAH) I and II occur with close energy barriers (ΔG^≠ ≈ 20—25 kcal mol^–1), which are lower than the barriers (ΔG^≠ ~ 30 kcal mol^–1) of similar transformations measured or calculated earlier for the chromium tricarbonyl complexes of naphthalene and its derivatives and other PAH. For the molybdenum tricarbonyl complexes the activation barriers of η⁶,η⁶-IHR decrease additionally by ~ 5 kcal mol^–1 compared to those for the chromium tricarbonyl complexes, whereas for the tungsten tricarbonyl complexes they increase again and become approximately equal to the activation barriers of similar chromium tricarbonyl complexes. All stationary states on the potential energy surface determining the mechanism of η⁶,η⁶-IHR are characterized by a decrease in hapticity compared to the initial and final complexes.     

Synthesis and reactivity of naphthalene sandwich complexes of molybdenum. X-ray structure of [Mo(η6-naphthalene) {P(OMe)3}3] and [Mo(H)(η6-naphthalene){P(OMe)3}3][BF4]

The sandwich complexes bis(η⁶-naphthalene)molybdenum(0) ( 1 ), bis(η⁶-1-methylnaphthalene)molybdenum(0) ( 2 ), and bis(η⁶-1,4-dimethylnaphthalene)molybdenum(0) ( 3 ) are synthesized by cocondensation of Mo-atoms with the naphthalene ligands. Complexes 1–3 are also obtained by reduction of MoCl₅ or MoCl₄. 2THF with highly activated Mg in the presence of the naphthalene ligands. Mg was activated by sublimation of the metal in a simple rotating solution reactor. Complex 2 exists as a mixture of regio- and stereoisomers. Three regioisomers, 3a–c , are formed in reactions of Mo-atoms with 1,4-dimethylnaphthalene, whereas 3a , the isomer with the Mo-atom coordinated to the unsubstituted rings, is formed selectively via the reductive method. The ligands in 1–3 are highly labile. CO displaces both naphthalene rings in 2 and 3 to give [Mo(CO)₆], while PF₃, P(OMe)₃, and PMe₃ displace only one coordinated naphthalene in 1 to yield the [Mo(η⁶-naphthalene)L₃] complexes 4–6 . In toluene, arene exchange is a competitive process in reactions of 1 with PF₃. Complexes 5 (L = P(OMe)₃) and 6 (L = PMe₃) react with HBF₄ to give the cationic metal hydride complexes 8 and 9 . The X-ray crystal structures of [Mo(η⁶-naphthalene) {P(OMe)₃}₃] ( 5 ) and [Mo(H)(η⁶-naphthalene) {P(OMe)₃}₃][BF₄] ( 8 ) are reported.     

On the coordination chemistry of corannulene, the smallest “buckybowl”

A variety of methods, conventional and non-conventional, are used in attempts to prepare the compounds (η⁶-corannulene)M(CO)₃ (M = Cr, Mo, W), all unsuccessful. Conventional methods are also utilized in attempts to prepare the compound [CpFe(η⁶-corannulene)]PF₆, but these result in mixtures of cationic CpFe(arene) complexes containing partially hydrogenated corannulene; similar results have been reported for other polyaromatic hydrocarbons. DFT calculations on the compound (η⁶-corannulene)Cr(CO)₃ suggest that the (η⁶-corannulene)-Cr linkage is only a few kcal/mol weaker than the corresponding bond in (η⁶-benzene)Cr(CO)₃, implying that failures in syntheses arise from kinetic, not thermodynamic problems.     

Tris(3,5‐dimethylpyrazolyl)methane‐Based Heterobimetallic Complexes that Contain Zn and CdTransition‐Metal Bonds: Synthesis,Structures, and Quantum Chemical Calculations

Reactions of the tris(3,5‐dimethylpyrazolyl)methanide amido complexes [M′{C(3,5‐Me₂pz)₃}{N(SiMe₃)₂}] (M′=Mg ( 1 a ), Zn ( 1 b ), Cd ( 1 c ); 3,5‐Me₂pz=3,5‐dimethylpyrazolyl) with two equivalents of the acidic Group 6 cyclopentadienyl (Cp) tricarbonyl hydrides [MCp(CO)₃H] (M=Cr ( 2 a ), Mo ( 2 b )) gave different types of heterobimetallic complex. In each case, two reactions took place, namely the conversion of the tris(3,5‐dimethylpyrazolyl)methanide ligand (Tpmd*) into the ‐methane derivative (Tpm*) and the reaction of the acidic hydride M?H bond with the M′?N(SiMe₃)₂ moiety. The latter produces HN(SiMe₃)₂ as a byproduct. The Group 2 representatives [Mg(Tpm*){MCp(CO)₃}₂(thf)] ( 3 a / b ) form isocarbonyl bridges between the magnesium and chromium/molybdenum centres, whereas direct metal–metal bonds are formed in the case of the ions [Zn(Tpm*){MCp(CO)₃}]⁺ ( 4 a / b ; [MCp(CO)₃]^? as the counteranion) and [Cd(Tpm*){MCp(CO)₃}(thf)]⁺ ( 5 a / b ; [Cd{MCp(CO)₃}₃]^? as the counteranion). Complexes 4 a and 5 a / b are the first complexes that contain Zn?Cr, Cd?Cr, and Cd?Mo bonds (bond lengths 251.6, 269.8, and 278.9 pm, respectively). Quantum chemical calculations on 4 a / b* (and also on 5 a / b* ) provide evidence for an interaction between the metal atoms.     

Ethylene addition to group-6 transition metal oxo complexes - A theoretical study

Quantum chemical calculations using density functional theory (B3LYP) were carried out to elucidate the reaction pathways for ethylene addition to the chromium and molybdenum complexes CrO(CH₃)₂(CH₂) (Cr1) and MoO(CH₃)₂(CH₂) (Mo1). The results are compared with previously published results of the analogous tungsten system WO(CH₃)₂(CH₂) (W1). The comparison of the group-6 elements shows that the molybdenum and tungsten compounds Mo1 and W1 have a similar reactivity while the chromium compound has a more complex reactivity pattern. The kinetically most favorable reaction pathway for ethylene addition to Mo1 is the [2+2]_Mo,C addition across the MoCH₂ double bond which has an activation barrier of only 8.4 kcal/mol. The reaction is slightly exothermic with ΔE_R = −0.6 kcal/mol. The [2+2]_Mo,O addition across the MoO double bond and the [3+2]_C,O addition have much higher barriers and are strongly endothermic. The thermodynamically mostly favored reaction is the [1+2]_Mo addition of ethylene to the metal atom which takes place after prior rearrangement of the Mo(VI) compound Mo1 to the Mo(IV) isomer Mo1g. The reaction is −19.2 kcal/mol exothermic but it has a large barrier of 34.5 kcal/mol. The kinetically and thermodynamically most favorable reaction pathway for ethylene addition to the chromium homologue Cr1 is the multiple-step process with initial rearrangements Cr1 → Cr1c → Cr1g which are followed by a [1+2]_Cr addition yielding an ethylene π complex Cr1g + C₂H₄ → Cr1g-1. The highest barrier comes from the first step Cr1 → Cr1c which has an activation energy of 14.2 kcal/mol. The overall reaction is exothermic by −26.3 kcal/mol.     

The reactivity of complexed carbocycles : XII. Neutral dimetallic complexes with bridging cyclooctatetraene. X-ray structure of μ-[1–4:5–8η-cyclooctatetraene]cyclopentadienylcobalt-tricarbonylmolybdenum (CoMo)

The reaction of CpCoC₈H₈ with (diglyme)Mo(CO)₃, (CH₃CN)₃Cr(CO)₃ and (DMF)₃W(CO)₃ yields dimetallic complexes CpCoC₈H₈M(CO)₃ which contain bridging fluctional cyclooctatetraene. The electron deficiency of the Mo(CO)₃ groups relative to CpCo is believed to be balanced by a π-donor metal—metal bond. This is indicated by an unusually low absorption band in the IR spectrum, and by the X-ray structure of the CoMo dimer, which shows a shortened MoCO bond trans to the metal—metal bond. A further product is the nonfluctional dimer CpCoC₈H₈Mo(CO)₄, in which cyclooctatetraene retains a rigid “tub” conformation. Reaction of C₈H₈Fe(CO)₃ with (diglyme)Mo(CO)₃ also yields a dimeric product (CO)₃FeC₈H₈Mo(CO)₃ with a fluctional bridge ligand.     

Synthesis, electrochemistry, and crystal and molecular structures of some molybdenum(0) arene derivatives with fluorinated and phenyl-substituted arene ligands

Compounds of general formula Mo(η⁶-arene)(CO)₃, arene=diphenyl, 1; 1,3,5-triphenylbenzene, 2; C₆H₅F, 3; C₆H₅CF₃, 4, have been prepared in good yields by reacting fac-Mo(CO)₃(DMF)₃, DMF=N,N-dimethylformamide, with BF₃ · OEt₂ and the appropriate arene. The crystal and molecular structures of 1, 3, and 4, are reported. The dinuclear derivative Mo₂(η⁶:η⁶-C₆H₅-C₆H₅)(CO)₆, 5, was obtained by thermal reaction of Mo(η⁶-toluene)(CO)₃ with Mo(η⁶-diphenyl)(CO)₃. An electrochemical study has been performed on the new complexes, showing that the dimolybdenum complex undergoes a single two-electron reduction at about the same potential as the corresponding dichromium complex, the molybdenum dianion being less stable than the chromium analogue.     

Fluxional behavior of 7-substituted cycloheptatriene iron tricarbonyls

The reaction between C₇H₇Fe(CO)₃^/t- and Me₃MC1, M = Si and Ge, produces 7-substituted cycloheptatriene iron tricarbonyl derivatives. Both compounds, unlike the parent molecule, are fluxional. It is shown that an oscillatory motion of the Fe(CO)₃ group, having the effect of a 1,3-shift with respect to the ring, is responsible for the observed fluxionality.