Extended Huckel Molecular Orbital Calculations Research Articles

Nickel poly-acetylide carbonyl clusters: structural features, bonding and electrochemical behaviour

The reactions of [NEt(4)](2)[Ni(6)(CO)(12)] with miscellaneous carbon halides (e.g. CCl(4), C(4)Cl(6)) in CH(2)Cl(2) have been extensively investigated particularly focusing on the stoichiometric ratio of the reagents and reaction temperature. This allowed the preparation of the previously known acetylide clusters [Ni(16)(C(2))(2)(CO)(23)](4-), [HNi(25)(C(2))(4)(CO)(32)](3-) and [Ni(22)(C(2))(4)(CO)(28)Cl](3-), as well as isolation and full characterisation of the closely related [Ni(17)(C(2))(2)(CO)(24)](4-) and [Ni(25)(C(2))(4)(CO)(32)](4-) tetraanions. From a structural point of view, all these clusters are based on a Ni(16) square orthobicupola which contain interstitial C(2), Ni(η(2)-C(2))(4) or Ni(2)(μ-η(2)-C(2))(4) moieties, displaying rather short C-C bonds. Electrochemical studies reveal that all these species undergo different redox processes, even if their stability is rather limited. This is corroborated by an extensive analysis of the interaction between interstitial C(2) acetylide units and the metal cluster cage by Extended Huckel Molecular Orbital (EHMO) calculations, which indicates that tightly bonded C-C units are less effective than isolated C-atoms in stabilising the cluster cage.

Xenon difluoride plasma fluorination of polymer surfaces

Xenon difluoride (XeF 2) plasma treatment of a series of polymers containing different repeat units gives rise to varying levels of surface fluorination. Alkene and aromatic C–H bonds appear to be more susceptible towards reaction compared to their sp 3 counterparts. The extent of fluorine incorporation can be accounted for in terms of a structure–behaviour relationship derived from extended Huckel molecular orbital calculations. Comparison with CF 4 plasma modification shows that XeF 2 electrical discharges are more effective at fluorinating polymer surfaces.

ChemInform Abstract: Long-Range Interaction Between Nonbonded Hydrides: Attractive in the Case of Transition Metals?

Extended Huckel molecular orbital calculations give a positive Mulliken overlap population between cis non bonded dihydride and trihydrides in transition-metal complexes (i.e., cis-(H) 2 Os(CO) 4 and CpIr(PH 3 )(H) 3 + ). In contrast, isolobal organic systems (CH 4 and D 3h CH 5 + , respectively) show no tendency for such a positive long-range interaction. The results are interpreted in terms of variable electron transfer from the symmetry-adapted polyhydride H n molecular orbitals into those of the metal or organic fragment

Chemical reactivity of second-row transition metal clusters from Hückel-type calculations

Chemical reactivity of mixed transition metal clusters of the second-row is investigated by extended Huckel molecular orbital calculations including a parametrized core repulsion term. Using atomization energies and charge or Fukui indices for bond and site attacks, respectively, it is found that reactivity is enhanced in mixed clusters in comparison with pure ones in agreement with available experimental results.

Super-Exchange Interaction in a Chair-Piperazine Bridged Dicopper(II/II) Complex: Synthesis, Crystal Structure, Magnetic Properties and Molecular Orbital Calculations

Abstract Reaction of the /i-bis(tridentate) ligand H3L' (L' = 1,3-bis[N-(5-cliloro-2-hydroxybenzylidene)- 2-ainiiioetliylene]-2-(5-cliloro-2-hydroxy!phenyl)iniidazoliduie) with eopper(II) chlonde diliydrate gives the chair-piperazine bridged complex [Cu2(μ-L)Cl2]. The halves of the binuclear complex are related by crystallographic inversion symmetry. The intramolecular Cu ・・・Cu separation is 6.954(3) Å. Temperature-dependent magnetic susceptibility measurements of the complex show a weak intramolecular antiferromagnetic eouphng. The super-exchange coupling constant (J) is - 10.5 cm-1. Semi-empirical extended Huckel molecular orbital (EHMO) calculations have been performed in order to gain msight into the molecular orbitals that participate in the super-exchange pathway.

Two-dimensional molecular magnets based on [Pt(mnt)2]− ions: Structures and magnetic properties

Two compounds of composition [FBzPy][Pt(mnt) 2 ] ( 1 ) and [FBzPy][Pt(mnt) 2 ] · 0.25-MeCN ( 2 ) ([FBzPy] + = 1-(4′-fluorobenzyl)pyridinium and mnt 2− = maleonitriledithiolate) were prepared and their crystal structures were determined. The solvent molecule, MeCN, incorporated into lattice leads the anionic stacking pattern to be distinct between 1 and 2 . The regular anionic stacking columns are connected by the anionic dimers to construct into 2D anionic networks in 1 , while the anionic fourfold subunit develop into anionic layers though lateral S⋯S interactions in 2 . As for two compounds, the magnetic susceptibilities in 2–350 K were measured, and the magnetic exchange schemes were built based on the analyses both crystal structures and extended Huckel molecular orbital calculations. The magnetic coupling model of an alternating AFM Heisenberg chain with an isolated AFM coupling dimer was dealt with 1 , while a spin dimer with s = 1/2 with 2 , and the magnetic coupling parameters were further gained via simulating the temperature dependent magnetic susceptibility data of two compounds.

Estimation of Magnetic Interactions and Magnetic Structures in Charge-transfer Salts Based on [Cr(isoq)2(NCS)4]-(isoq = Isoquinoline)

Magnetic interactions J in [Cr(isoq) 2 (NCS) 4 ] - -based charge-transfer salts, (BDH-TTP)[Cr(isoq) 2 (NCS) 4 ], (TMTTF)[Cr(isoq) 2 (NCS) 4 ], and (BO)[Cr(isoq) 2 (NCS) 4 ], are estimated on the basis of the extended Huckel molecular orbital calculations (BDH-TTP = 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene, BO = bis(ethylenedioxy)-tetrathiafulvalene, TMTTF = tetramethyltetrathiafulvalene, and isoq = isoquinoline). From this, experimental magnetic properties are discussed, and the magnetic structures are determined. (BDH-TTP)[Cr(isoq) 2 (NCS) 4 ] is composed of two-dimensional array of antiferromagnetically coupled ferrimagnetic chains. These two-dimensional arrays, however, have net magnetic moment perpendicular to the sheets and give rise to weak ferromagnetism. (TMTTF)[Cr(isoq) 2 (NCS) 4 ] and (BO)[Cr(isoq) 2 (NCS) 4 ] have similar “body-centered” molecular arrangement. The TMTTF salt has considerable πd-interaction in two directions, and this compound actually shows a magnetic transit...

Synthesis, redox properties, and X-ray diffraction structure of 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide

Refluxing 2,3-dichloromaleic anhydride with p-anisidine in benzene with water removal gives the condensation product 2,3-dichloro-N-(p-MeOC6H4)maleimide (1) 75% yield. This new maleimide compound reacts with added ethanethiol in the presence of Et3N or DBU to furnish the bidentate sulfide ligand 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide (2) in 85% yield. Each product has been characterized in solution by IR, NMR, and UV-vis spectroscopies, and the solid-state structure of 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide was unequivocally established by single-crystal X-ray diffraction analysis. 2,3-bis(Ethylthio)-N-(p-MeOC6H4)maleimide crystallizes in the monoclinic space group C2/c, a = 20.035(3)A, b = 9.188(1)A, c = 16.887(2)A, β = 93.696(2)°, V = 3102.3(8)A3, Z = 8, and Dcalcd = 1.385 mg/m3; R = 0.0268, Rw = 0.0676 for 2025 reflections with I > 2σ (I). The nature of the LUMO in 2,3-bis(ethylthio)-N-(p-MeOC6H4)maleimide (2) has been determined by extended Huckel molecular orbital calculations, and these data are discussed relative to the cyclic voltammetry results and other structurally relevant compounds prepared in our labs.

Reaction of dichloromaleic anhydride with 1,8-diaminonaphthalene: Synthesis and X-ray diffraction structure of 8,9-dichloropyrrolo[1,2-a]perimidin-10-one

The reaction of dichloromaleic anhydride with 1,8-diaminonaphthalene in refluxing toluene or 1,2-dichloroethane produces the new heterocyclic compound 8,9-dichloropyrrolo[1,2-a]perimidin-10-one in low yields. 8,9-Dichloropyrrolo[1,2-a]perimidin-10-one has been isolated by column chromatography and characterized in solution by IR, 1H NMR, and UV/vis spectroscopies. The solid-state structure was unequivocally established by single-crystal X-ray diffraction analysis. 8,9-Dichloropyrrolo[1,2-a]perimidin-10-one crystallizes in the monoclinic space group P2 1/c, a = 7.475(1)A, b = 10.650(2)A, c = 14.468(2)A, β = 94.478(2)°, V = 1148.3(3) A3, Z = 4, and d calc = 1.672 Mg/m3; R = 0.0289, R w = 0.0762 for 1644 reflections with I > 2σ(I). The nature of the HOMO and LUMO in 8,9-dichloropyrrolo[1,2-a]perimidin-10-one has been determined by extended Huckel molecular orbital calculations, and these data are discussed relative to the cyclic voltammetric data obtained at a platinum electrode.

Regioselective and Stereoselective Reactions of 2-Butyne Bound to a Resolved Chiral Tungsten(II) Center

Deprotonation of chiral complexes of the type Tp‘(CO)(I)W{RC⋮CCH2R‘} at the propargyl carbon followed by alkylation is regioselective and stereoselective. Photolysis liberates the elaborated alkynes from the metal complex. Resolution of metal diastereomers, with amido ligands, Tp‘(CO)(NHR)W{CH3C⋮CCH3} (NH2R = ((S)-(−)-α-methylbenzylamine), has been accomplished by fractional crystallization. Conversion to an amine complex followed by iodide replacement of H2NCHMePh produces separate enantiomers of Tp‘(CO)(I)W{CH3C⋮CCH3}. Methylation followed by benzylation of each enantiomer affords Tp‘(CO)(I)W{CH3C⋮CCHMeCH2Ph}. The alkynes (−)-CH3C⋮CCHMeCH2Ph and (+)-CH3C⋮CCHMeCH2Ph have been released from each of these enantiomerically enriched complexes in optically active form, as assayed by optical rotation and by 1H NMR with chiral shift reagents. The barrier to alkyne rotation in Tp‘(CO)(I)W{RC⋮CCH2R‘} complexes has been probed by variable-temperature 1H NMR, and extended Huckel molecular orbital calculations have been performed on model complexes. The origin of regioselectivity and stereoselectivity is considered in light of the X-ray structures of Tp‘(CO)(I)W{CH3C⋮CCHMe(CH2)4I} (7) and Tp‘(CO)(I)W{Me(PhCH2)HCC⋮CCHMeCH(OH)Ph} (11). The absolute configuration of Tp‘(CO)W{CH3C⋮CCH3}(NHCHMePh) ((+)-14(SS)) has been established by X-ray analysis with the resolving amine as reference.

Kinetics and mechanism of the oxidation of Mn(ii)aq by bromate and peroxomonosulfate in the presence of molybdate to form [MnIVMo9O32]6–

The oxidation of Mn(II) by both BrO3− and HSO5− in the presence of MoO42−, in weakly acidic solution over the pH ranges 3.9–5.5 and 4.4–5.5, respectively, results in the formation of the heteropolyoxomolybdate [MnMo9O32]6− in each case. The kinetics of oxidation were studied at 40.0 °C for BrO3− and 30.0 °C for HSO5−, along with temperature dependence studies, and for each oxidant were found to exhibit solution autocatalytic behaviour. For BrO3− the oxidation kinetics followed the expanded rate expression +d[MnMo9O326−]/dt = kAC(1)[Mn2+][MnMo9O326−][HMoO4−]2[BrO3−] based on an examination of the individual [BrO3−], pH and actual [MoO42−] dependences, with a value for kAC(1) of 9.09(34) × 106 dm12 mol−4 s−1. For HSO5− the oxidation kinetics followed the expanded two-term rate expression +d[MnMo9O326−]/dt = kAC(2a)[Mn2+][MnMo9O326−][HMoO4−]−5[SO5−] + kAC(2b)[Mn2+][MnMo9O326−][HMoO4−]−5[HSO5−] based on [HSO5−], pH and [MoO42−] dependences. The values of kAC(2a) and kAC(2b) are 4.6(4) × 10−11 dm−9 mol3 s−1 and 2.6(4) × 10−15 dm−9 mol3 s−1. For BrO3− oxidation, from the composition of the transition state, it is proposed that the product [MnMo9O32]6− species combines with Mn(II) and two HMoO4− ions to generate a Mn(II)-substituted lacunary [MnIIMnIVMo11O39]6− anion based on a Keggin structure, with the Mn(IV) located at the centre of the polyoxomolybdate framework. Extended-Huckel molecular orbital calculations have been used to investigate the stability of the proposed Mn(II)-substituted lacunary species, based on an α-Keggin structure, relative to the unassembled components. For HSO5− oxidation, the two parallel pathways indicate oxidation by both HSO5− and its deprotonated form SO5−. The two mechanisms reflect the differences in how BrO3− and HSO5− operate oxidatively and have been highlighted by the facile nature of polyoxomolybdate polymerization. In each case following oxidation of Mn(II) to Mn(IV) fast separation of the two Mn(IV) centres must subsequently occur, along with rapid assembly of the polyoxomolybdate frameworks around each centre to yield the product species.

Copper Concentration inside Guinier-Preston I Zones Formed in an Al–Cu Alloy

Abstract The copper concentration inside a Guinier-Preston (GPI) zone was studied by means of an extended Huckel molecular orbital (EHMO) calculation. The present EHMO calculation revealed that the Cu concentration inside a GPI zone depends on the size of the zone. The GPI zone comprises both Cu and Al in a small GPI zone, whereas the Cu concentration inside the GPI zone increases to nearly 100 at.% Cu in a large GPI zone. A compressive lattice distortion, which is associated with the zone, increases the stability, but when a GPI zone is 5 nm in diameter, the stabilizing effect of the zone is estimated to be not higher than 10% of the chemical energy due to Cu coalescence.

A π-Stacked Organometallic Propeller: Experimental and Theoretical Studies on Reactivity and Bonding in the π-Arene-Bridged Nickel Triple-Decker [{(η5-Me4EtC5)Ni}2(μ-η3:η3-decacyclene)

Synthesis, structure, reactivity, and theoretical studies of a new type of Ni triple-decker complexes are reported. A prominent feature of its molecular structure is the (η3:η3)-bis(enyl) coordination of two [(η5-Me4EtC5)Ni] fragments to the central arene ring of the polycondensed π-perimeter decacyclene. The central arene ring of decacyclene displays a distinct chair conformation with significant tilt angles of 25.5 and 26.1°. The three naphthalene units are twisted in a propeller-like fashion with torsion angles of 16°, 16.6°, and 19°, which is twice the twisting observed for the free decacyclene ligand. According to Extended Huckel Molecular Orbital (EHMO) calculations, a qualitative measure for arene lability in the title compound and a number of related arene-bridged metal complexes can be attributed to a subsequent filling of metal−arene antibonding orbitals. The experimentally observed reactivities of π-arene exchange in these complexes are reproduced nicely by the EHMO calculations. The title compound offers access to a number of organonickel chalcogenato complexes under mild conditions by cleavage of the S−S, Se−Se, or Te−Te bonds of diorganyl dichalcogenides R−E−E−R. The title compound is also capable of C−H activation of azobenzene and [(dimethylamino)methyl]ferrocene resulting in orthometallated complexes containing five-membered metallacycles. PES and semi-empirical ZINDO/S calculations were used to study the product of the orthometallation of the title compound with azobenzene. These studies reveal that the HOMO of the metallacycle (IE = 6.45 eV) has dominant Ni−Cp and Ni−Caryl π-antibonding character.

Experimental and theoretical investigations on the synthesis, structure, reactivity, and bonding of the stannylene-iron complex Bis bis(2-tert-butyl-4,5,6-trimethyl-phenyl)SnFe (eta6-toluene) (Sn-Fe-Sn)

The pi-(arene)bis(stannylene) complex bis(bis(2-tert-butyl-4,5,6-trimethylphenyl)SnFe(eta6-toluene) (Sn-Fe-Sn, 15) is accessible in high yields by a metal-atom-mediated synthesis between iron atoms, toluene, and the stannylene [bis(2-tert-butyl-4,5,6-trimethylphenyl)Sn](3). Complex 15 has a half-sandwich structure with short Fe -Sn bonds (2.432(1) A) and a trigonal-planar coordination at both the Fe and Sn atoms. The distance between the two Sn centers is 3.56 A. Complex 15 is stable under ambient conditions and displays a pi-arene lability, so far rarely observed for (arene)iron complexes; this leads to an irreversible substitution of the arene and formation of fivefold-coordinated zerovalent iron complexes. The pi-arene lability of the title compound is a result of the Fe-Sn bonding situation, which can be interpreted, on the basis of an extended Huckel molecular orbital calculation, as being solely a donation of the 5sigma lone-pair of Sn into empty or half-filled acceptor d orbitals on Fe. As the calculations reveal, there is little backbonding from the iron to the tin, and the strong sigma donation leads to an increased occupation of the pi-antibonding orbitals of the eta6-arene, which are mainly responsible for the experimentally observed arene lability. Fe and Sn Mossbauer spectra support the polar character of Sn(sigma+)-->Fe(sigma-) with strong sigma donation from tin to iron, but significantly low iron-to-tin pi backdonation.

Electron Localization in Polysilane Radical Ions

The localization mechanism for the excess electron and positive hole in the polysilane radical anion and cation has been investigated by means of both molecular dynamics (MD) and extended Huckel molecular orbital (EHMO) calculations. A linear oligosilane (Si nH2n+2 )( n ) 32 and 64) was chosen as the model of polysilane. The geometry optimization of the polysilane at the MM2 level gave a regular all-trans form as the most stable structure. The MD calculations, started from the optimized structure, showed that the conformation of the polysilane skeleton was gradually randomized as a function of time by thermal activation at 300 K. The conformations at 0.0, 0.05, 1.0, 1.5, and 2.0 ps were chosen as sampling points, and spin densities on the silicon atoms were calculated by the EHMO calculations at each sampling point. An excess electron and a positive hole were fully delocalized along the Si chain in the regular all-trans form (time ) 0.0 ps), whereas in the disordered conformations (time * 0) both electron and hole were completely localized on a few Si atoms (10-20 monomer units). The present calculations suggested that a continuous disorder of the Si main chain randomized by thermal energy (i.e., disordered dihedral angle of Si skeletons) is dominant in both electron and hole localization. The localization mechanism was discussed on the basis of theoretical results.

Electrophilic Methylplatinum Complexes: A Theoretical Study of the Mechanism of C−C and C−H Bond Formation and Activation

The reductive elimination of methane or ethane from the five-coordinate intermediate model complexes [PtHMe2L2]+, or [PtMe3L2]+ respectively, and the corresponding C−H or C−C bond activation from the alkane complexes [PtMe(CH4)L2]+ or [PtMe(C2H6)L2]+, respectively, have been studied by carrying out extended Huckel molecular orbital (EHMO) calculations and density functional theory (DFT) calculations on both the ground-state and transition-state structures with L = NH3 or PH3. The EHMO calculations on trans-[PtL2Me3]+, L = PH3, show that the regular trigonal-bipyramidal (TBP) structure has an orbitally degenerate ground state and should undergo distortion to either the square-pyramidal (SP) or pinched trigonal-bipyramidal (PTBP) structure. In the PTBP structure, two methyl groups are in close proximity (C−Pt−C ca. 70°) and tilted away from each other. Although the tilting leads to a close Pt···HC contact, no attractive agostic Pt···H bonding is indicated. The DFT calculations predict that C−H reductive eli...

Properties of a New 58-Electron Butterfly [Pt 4 (μ 2 -dpam) 3 (μ 2 -CO) 3 (η 1 -dpam)] 2+ Cluster

A new organometallic cluster [Pt4(μ2-dpam)3(μ2-CO)3(η1-dpam)] X2 (X=CF3CO2-; 4a) was prepared from a reaction between Pt(dpam)(CF3CO2)2 and CO in a methanol (water mixture). The PF6- salt 4b is obtained from an anion exchange with NH4PF6 in methanol. Dark red crystals suitable for X-ray crystallography (X=PF6-) revealed the commonly encountered butterfly structure for a 58-electron Pt cluster (angle between the two Pt3 planes =95.36(8)°, nonbonding Pt...Pt distance =3.094(1) A). The structure consists of two edge sharing Pt3 triangles, a small one 'Pt3(μ2-dpam)32+ and a larger one “Pt3(μ2-CO)3(L)3” where L=As group. The Extended Huckel Molecular Orbital calculations (EHMO) have been rationalized from interactions of two L and PtL22+ fragments onto the planar triangular Pt3(μ2-CO)3L3 cluster (L = AsH3). The compound is luminescent at 77 K with a structureless band located at 690 nm (τe=3.2 ±0.2μs). Finally, the 1H NMR spectra are interpreted, particularly with respect to fluxionality and presence of two isomers. X-ray data for 4b: orthorhombic, C c2a, a=26.320(9), b=27.613(6), c=28.891(4) A, V=20998(9) A3, Z=4, D(calc.)=1.925 Mg/m3.

Interaction of CO2 with small rutile crystallites-an EHMO study

Several possible adsorption sites and adsorption geometries of CO2 on small rutile fragments were studied by Extended Huckel Molecular Orbital (EHMO) calculations. The parameters for the rutile part were optimised to reproduce the experimental rutile bulk structure and were tested in several small clusters up to [(TiO2)31(OH)32]32−•6H2O, a 175 atoms cluster. It was found that the average experimental bond legth can be reproduced with good accuracy. However the slight distortion of the TiO6 octahedra is calculated with the wrong sign (four long and two short Ti−O bonds). The agreement for the angle αO-Ti-O is less satisfactory. The study shows that CO2 can adsorb on fivefold coordinated surface titanium sites as well as surface oxygen sites. This means that CO2 can act as either Lewis base or acid. In the case of binding as a Lewis base, CO2 can adsorb linearly forming a single Ti−OCO bond, or interact with two neighboring Ti4+ sites. A chelating structure forming two Ti−O bonds was found to be weakly stable at the most. When CO2 behaves as a Lewis acid, a carbonate-like structure is formed by interaction with either terminal oxygen ions or bridging oxygen centers.

Hydrocarbyl derivatives of [UCl2HB(pz)32]: Synthesis, characterization and reactivity studies towards protic substrates and ketones

Abstract The reaction of [UCl 2 HB(pz) 3 2 ] ( 1 ) with lithium alkyls LiR (R = Me, CH 2 SiMe 3 , C 6 H 4 - o -CH 2 NMe 2 ) in the 1:1 or 1:2 molar ratio affords the compounds [UCIRHB(pz) 3 2 ] (pz = C 3 H 3 N 2 , R = Me ( 2 ), CH 2 SiMe 3 ( 3 ), C 6 H 4 - o -CH 2 NMe 2 ( 4 )) and [UR 2 HB(pz) 3 2 ] (R = Me ( 5 ), CH 2 SiMe 3 ( 6 )) respectively in 60–80% yield. Complex 2 can also be obtained (60% yield) by redistribution at room temperature between the complexes 1 and 5 . Compounds 2, 3 and 4 react with pzH providing [UCl(pz)HB(pz) 3 2 ] ( 7 ) in almost quantitative yield and 5 and 6 react also with pzH leading to [U(pz) 2 HB(pz) 3 2 ] ( 8 ). The alkoxide [U(OC 6 H 4 - o -OMe) 2 HB(pz) 3 2 ] ( 9 ) was synthesized by reacting 5 or 6 with guaiacol. By reacting the chlorohydrocarbyls 2, 3 or 4 with excess of acetone the aldolate [UCl(OCMe 2 CH 2 (C=O)Me)HB(pz) 3 2 ] ( 11 ) was obtained, due to the activation of α -CH bond of acetone; however, for 3 the reaction is not clean and a mixture of 11 and [UCl(OCMe 2 CH 2 SiMe 3 )HB(pz) 3 2 ] ( 12 ) is always obtained. Stoichiometric amounts of acetone insert into the metal-carbon bonds of 2 and 5 yielding [UCl(O t Bu)HB(pz) 3 2 ] and [U(O t Bu) 2 HB(pz) 3 2 ] respectively, while the insertion product [U(OCMe 2 CH 2 SiMe 3 ) 2 HB(pz) 3 2 ] ( 10 ) can only be obtained when 6 reacts with excess of this substrate. 9 crystallizes from toluene/hexane in the triclinic space group P 1¯ with unit cell dimensions a = 12.295(2) A, b = 12.640(2) A, c = 13.994(2) A, α = 76.10(1)°, β = 72.50(1)°, γ = 80.71(1)°, V = 2004(2) A 3 and Z = 2. Recrystallization of a mixture containing [UCl(O t Bu)HB(pz) 32 ] and 11 led to a decomposition product which has been characterized by X-ray structural analysis as [UCl(Hpz)(O t Bu)(μ-O)B(μ-pz)(pz) 2 ] 2 ( 13 ): monoclinic P 2 1 / n , a = 13.701(3) A, b = 11.337(2) A, c = 14.857(4) A, β = 104.65(2)°, V = 2233(1) A 3 and Z = 2. Extended Huckel molecular orbital calculations provided some information on the bonding capabilities of the [UHB(pz) 3 2 ] fragment compared to [U(C 5 Me 5 ) 2 ] and on the vulnerability of the poly(pyrazolyl)borate ligands to nucleophilic attack.

Haptotropic Shifts in (C5H5)Fe and Mn(CO)3 Complexes of 4H-Cyclopenta[def]phenanthrene (cppH): X-ray Crystal Structure and NMR Fluxionality of (η1-cpp)Mn(CO)3(PEt3)2

The favored pathways for the haptotropic shifts from (η6-cpp)MLn to (η5-cpp)MLn, where cppH is 4H-cyclopenta[def]phenanthrene and MLn = Fe(C5H5) or Mn(CO)3, have been investigated by means of extended Huckel molecular orbital calculations, and energy hypersurfaces for these processes have been obtained. These data suggest the intermediacy of an exocyclic (η3-cpp)MLn species, stabilized by the presence of a naphthalene-type 10π aromatic system. In an attempt to generate (η3-cpp)Fe(CO)(C5H5), the corresponding (η1-cpp)Fe(CO)2(C5H5) complex was prepared and allowed to decompose; the major products were [(C5H5)Fe(CO)2]2 and the cpp trimer, C15H8(C15H9)2, 20, that was shown to adopt a rigid geometry with C2 symmetry. Treatment of (η5-cpp)Mn(CO)3 with triethylphosphine yields (η1-cpp)Mn(CO)3(PEt3)2, 23, which exhibits hindered rotation about the C(4)−Mn bond with a barrier of 16.5 kcal mol-1. The molecule [(η6-cppH)Fe(C5H5)]PF6, 2b, and also the manganese complex 23 have been characterized by X-ray crystallogra...