Qualitative MO Research Articles

Reversible Carbon−Carbon Double Bond Cleavage of a Ketene Ligand at a Single Iridium(I) Center: A Theoretical Study

Modeling the chemistry of ketene complex {κ2-(t-Bu)2PCH2P(t-Bu)2}IrCl[η2-(C,C)-Ph2CCO] (1), DFT studies have been carried out for (κ2-H2PCH2PH2)IrCl[η2-(C,C)-H2CCO] (A), for cation (κ2-H2PCH2PH2)Ir[η2-(C,C)-CH2CO]+ (B), for its carbene carbonyl isomer (κ2-H2PCH2PH2)Ir(CH2)(CO)+ (C), and for their interconversion by intramolecular CC double bond cleavage/formation. A qualitative MO analysis from extended Hückel calculations shows the CC cleavage and formation to be symmetry allowed at a d8-ML2 late transition metal template. For the two iridium model complexes B and C the process is calculated by DFT to be reversible, with activation barriers of 17.3 kcal mol-1 toward the more stable carbonyl carbene system and 25.1 kcal mol-1 for the reverse reaction, respectively. This is in line with experimental observations for 1, which generates {κ2-(t-Bu)2PCH2P(t-Bu)2}Ir(CPh2)(CO)+ (3) upon chloride abstraction and regenerates 1 after the addition of chloride. QM/MM calculations of the ONIOM type have been employed for the real systems 1 and 3, to take into account and to evaluate the role of steric effects and to allow a validation of theoretical results by comparing computed and X-ray-determined structures. Contrasting the iridium case, the analogous rhodium ketene complex (κ2-H2PCH2PH2)Rh[η2-(C,C)-CH2CO]+ (G) is computed to be favored by 8.0 kcal mol-1 compared to its carbene carbonyl isomer (κ2-H2PCH2PH2)Rh(CH2)(CO)+ (H), with a barrier of 22.9 kcal mol-1 for the endothermic CC cleavage step. Conceivable dynamic processes were treated theoretically for the temperature dependence of NMR line shapes of carbene complex {κ2-(t-Bu)2PCH2P(t-Bu)2}Ir(CPh2)(CO)+ (3). A comparison with the experimental data suggests a plausible pathway for the observed exchange of the two P centers.

Multiplicity of Planar Hexasilylbenzene Dianions: Effects of Substituents and Countercations

The electronic structure of various planar hexasilylbenzene dianions was investigated by ESR spectroscopy. Dilithium salts of a planar hexasilylbenzene (1,3,4,6,7,9-hexasila-1,1,3,3,4,4,6,6,7,7,9,9-dodecamethyltrindane) dianion (1a) show thermally accessible triplet ESR signals with the D value of 0.0963 cm-1. The D value as well as the singlet−triplet energy difference (ΔEST) in 1a is dependent on the countercations (M+ = Li+, K+, and Rb+), indicative of the significant ion-pair interaction. The related dilithium salts of hexasilylbenzene dianions 2a and 3a, where all CH2 bridges in 1a are replaced by O and NMe bridges, respectively, afford similar triplet ESR signals, but they are thermally less stable than 1a, suggesting that the thermal distortion of the ring structure occurs more easily in 2a and 3a than in 1a. The origin of the singlet ground state of 1a (M+ = Li+) is ascribed to the ion-pair interaction with the countercations on the basis of qualitative MO consideration.

Understanding Chemical Shifts in pi-Systems: 13C, 15N, 17O NMR

In the NMR spectra of nuclei other than 1H, pi-systems show strong shielding or deshielding effects that are not explained by the simple consideration of electron densities. This is mainly because, in the presence of a magnetic field, the low-lying unoccupied pi* orbital allows combinations with energy-rich occupied orbitals of appropriate spatial orientation. The underlying principles can be understood using simple qualitative MO discussions, which follow, in principle, the same reasoning as the very successful ab initio calculations. The typical argument is developed for three examples taken from the literature. First, in phenyllithium the ipso carbon atom is strongly deshielded in a direction in the ring plane perpendicular to the C-Li bond; this is due to the n-pi* orbital combination. Second, in carbonyl compounds R-CO-X the influence of X is systematically felt on the chemical shift of 17O, but not on the chemical shift of 13C; this is attributed to the ny-pi* orbital combination, magnetically acting at O in the direction of the C-O bond, but lacking at C. Third, in linear pi-systems (acetylene, CO2, isocyanate ion, etc.) all atoms are strongly shielded with respect to a nonlinear comparison; this is because, for reasons of orbital symmetry, a deshielding contribution in the direction of the molecular axis has vanished.

Metallkomplexe mit funktionalisierten Schwefelliganden. XIV. Synthese von Thiosulfinatokomplexen über die Oxidative Addition von Platin(0)-Komplexen an 2,3,4-Benzotrithiepin-1-oxid

The platinum(0) compounds L2Pt(η2-C2H4) [L = PPh3, 1/2 dppf, 1/2 (R,R)-(–)-DIOP] 3a–c react with the 2,3,4-benzotrithiepin 1-oxides 2a–c to give the platinum(II) thio-lato thiosulfinato complexes L2Pt[S–S(O)–CH2–C6R12R22-CH2–S] (R1 = H, R2 = CH3; R1 = CH3, R2 = H) 4a–6c. Treatment of 5a with an excess of PBu3 efficiently affords (Bu3P)2Pt[S–S(O)–CH2–C6Me2H2–CH2–S] 7 by substitution of PPh3. Complex 7 reacts in solution with PBu3 to the platinum(II) sulfenato thiolato complex (Bu3P)2Pt[S(O)–CH2–C6Me2H2–CH2–S] 8 as well as S=PBu3. The compounds 4a–6c have been characterized by analytical and spectroscopic methods. Simple qualitative MO calculations of the extended Huckel type by using the CACAO package indicate that the platinum(0) species acts as a Lewis acid attacking the nucleophilic sulfur atom S(3).

Molecular Structures of M(2)(CO)(9) and M(3)(CO)(12) (M = Fe, Ru, Os): New Theoretical Insights.

A different number of bridging carbonyls is found in bi- or trinuclear clusters having the title formulas. Comparative calculations at the SCF, MP2, and DFT levels of theory show that only the latter is able to describe properly the energetics of various isomers of the whole triad. For the first-row transition metal, DFT gives excellent agreement with the experimental structures, whereas the MP2 approach fails completely. Conversely for the second- and third-row metals, the best agreement with the experiment is obtained by the MP2 optimizations. The quantitative computational results, associated with a qualitative MO analysis, allow one to conclude that the structural preferences are determined by a critical balance of metal-bridge bonding, metal-metal bonding, and intermetallic repulsion. Although the M-M bond order is expected to be 1 in all cases, the bridge-supported bond is experimentally and computationally shorter than the unsupported one. By contrast, the trend for the overlap population (OP) is reversed, with even negative values for the shorter bridge bonds. For the latter, only a weak attractive interaction stems from the almost pure t(2g) orbitals, taken as metal lone pairs or eventually responsible for back-donation (formation of metal-bridge sigma bonds). Thus, the negative OP values are consistent with a prevailing repulsion between the latter levels. In the iron systems, with more contracted metal orbitals, the direct metal-metal repulsion is relatively weak while the metal-bridge bonds are sufficiently strong. This is not equally true for the more diffuse ruthenium and osmium orbitals, so the alternative nonbridged structure is preferred.

Photochemical switching through protonation in merocyanines

The ground- and excited-state energies for the planar and various 90-degree twisted structures of the merocyanine Me 2N–CHCH–CHCH–CHO and its protonated form + Me 2N CH–CHCH–CHCH–OH have been calculated by the AM1 method involving complete geometry optimization. On successively twisting bonds in the polymethine chain, the energies of the resulting twisted forms display a pronounced alternation which is opposite in the ground and the excited state. This trend in computed energy values as well as the reversed alternation pattern for the two types of polymethine compounds can be anticipated from simple qualitative MO considerations and in the topological long-chain approximation.

Metal–Metal Interactions in Dinuclear (triphos)Cobalt Complexes Exhibiting Mixed Valency

The spectroscopic and electrochemical properties of a series of dinuclear mixed-valence complexes containing two (triphos)Co units are reported: [(triphos)Co(L)Co(triphos)]+ (L = C6O4X2; × = H, Cl, Br, I, Me: 2a–e+; L = C14H4O4Me2: 4+). Complexes 2a–e+ are bridged by tetraoxybenzene ligands and exhibit very strong metal–metal interaction leading to delocalized class-III behaviour while in 4+ the extended tetraoxoanthracence bridging ligand leads to a partial electron localization. Additionally, the different oxidation behaviour of the parent dicationic complexes 1a–e2+ and 32+ have been investigated and are explained on the basis of a qualitative MO model.

Metal–Metal Interactions in Dinuclear (triphos)Cobalt Complexes Exhibiting Mixed Valency

The spectroscopic and electrochemical properties of a series of dinuclear mixed-valence complexes containing two (triphos)Co units are reported: [(triphos)Co(L)Co(triphos)]+ (L = C6O4X2; × = H, Cl, Br, I, Me: 2a–e+; L = C14H4O4Me2: 4+). Complexes 2a–e+ are bridged by tetraoxybenzene ligands and exhibit very strong metal–metal interaction leading to delocalized class-III behaviour while in 4+ the extended tetraoxoanthracence bridging ligand leads to a partial electron localization. Additionally, the different oxidation behaviour of the parent dicationic complexes 1a–e2+ and 32+ have been investigated and are explained on the basis of a qualitative MO model.

Synthesis and NMR characterisation of meso-ethylenebis-(4,7-dimethyl-1-indenyl) zirconium dimethyl

The new compound meso-ethylenebis (4,7-dimethyl-1-indenyl) zirconium dimethyl ( 1-Me 2) has been prepared and a detailed structural investigation by 2D-NMR has been performed, using COSYgs, NOESY, HBMCgs and HSQCgs spectra. These experiments allowed us to perform the full 1H and 13C NMR spectral assignments. We also detected long distance interactions between the methyl protons on the metal centre and carbon atoms in the indenyl system. A qualitative MO analysis has been performed in order to clarify the experimental evidence.

Redox behavior of the molybdenum and tungsten metallafullerenes M(η2-C60)(CO)2(phen)(dbm) (phen = 1,10-phenanthroline; dbm = dibutyl maleate): (spectro)electrochemistry and theoretical considerations

Electrochemistry of M(η2-C60)(CO)2(phen)(dbm) (M = W 1, Mo 2; phen = 1,10-phenanthroline; dbm = dibutyl maleate) shows that the complexes undergo four sequential reduction processes. As with free C60, the first three electrons add reversibly (even if the relevant potentials are shifted ca. 0.15 V toward negative values), whereas the fourth reduction features chemical irreversibility. Cyclic voltammetry gives evidence that, as a consequence of the latter process, the metal fragment decomplexes and [C60]3– is released. In good agreement with this picture, a qualitative MO approach shows four close LUMOs for the ground state structure of the uncharged complexes. The first three levels are delocalized over C60 (somewhat extended to the dmb π system), while the fourth one is metal–fullerene antibonding (back donation dπ → π* C60) and its occupation causes fulleride dissociation. The EPR spectra of the electrogenerated [1]– and [2]– monoanions are significantly different from that of [C60]– and seem suggestive of metal character for these radical species. At present, this result is unexpected in that the unpaired electron in the anions [1]– and [2]– should be intuitively centered on the coordinated fullerene.

Transition metal dicarbollide complexes: synthesis, molecular, crystal and electronic structures of [M(C2B9H11)(NMe2)3] (M = Nb or Ta) and their insertion reactions with CO2 and CS2 †

The homoleptic amides [M(NMe2)5] (M = Nb 1 or Ta 2; the latter is characterised by a structural study) reacted with the carborane nido-C2B9H13 to eliminate two equivalents of HNMe2 and generate the dicarbollide half-sandwich tris(dimethylamide) complexes [M(C2B9H11)(NMe2)3] (M = Nb 3 or Ta 4). The crystal structures of isomorphous 3 and 4 have been determined and reveal two NMe2 ligands in a vertical orientation and the third one in a horizontal orientation with respect to the η5-co-ordinated face of the C2B9H11 ligand. The electronic factors responsible for the amide ligand orientations in these complexes are explored using qualitative MO arguments. Complexes 3 and 4 reacted with CO2 and CS2 to yield the tris(carbamate) [M(C2B9H11)(O2CNMe2)3] (M = Nb 5 or Ta 7) and tris(dithiocarbamate) [M(C2B9H11)(S2CNMe2)3] (M = Nb 6 or Ta 8) complexes, respectively. The crystal structures of 6 and 7 show two (dithio)carbamate ligands in horizontal and one in vertical orientation, demonstrating the similarity between the σ,π-donor frontier orbitals of the ligands NMe2 and X2CNMe2 in 4 and 6 or 7 respectively.

Thioether–iodine charge-transfer complexes. Synthesis and low-temperature single-crystal structures of complexes of penta-, hexa- and octa-dentate homoleptic thioether macrocycles

Charge-transfer complexes 2([15]aneS5)·7I2 1 ([15]aneS5 = 1,4,7,10,13-pentathiacyclopentadecane), [18]aneS6·I2 2, [18]aneS6·4I2 3 ([18]aneS6 = 1,4,7,10,13,16-hexathiacyclooctadecane), [24]aneS8·I2 4 and [24]aneS8·6I2 5 ([24]aneS8 = 1,4,7,10,13,16,19,22-octathiacyclotetracosane) have been prepared and their structures and solution properties investigated. The compounds were prepared by slow evaporation of solutions containing I2 and the appropriate thioether macrocycle in CH2Cl2. The single-crystal structure determination of 1 shows three I2 molecules co-ordinated to three exo oriented S donors [S(1)–I(1) 2.797(3), I(1)–I(1′) 2.798(2), S(4)–I(4) 2.885(4), I(4)–I(4′) 2.764(2), S(7)–I(7) 2.828(3) and I(7)–I(7′) 2.779(2) A; S(1)–I(1)–I(1′) 178.39(8), S(4)–I(4)–I(4′) 171.12(8), S(7)–I(7)–I(7′) 178.80(8)°]. The fourth I2 molecule (with a site occupancy of 0.5) lies close to S(10) [I(10)–S(10) 2.839(5) A], the bond distance I(10)–I(10′) 2.674(3) A being unusually short. Compound 2 is an example of a 1∶1 I2∶macrocycle adduct and shows symmetrically bridging I2 molecules [S(1)–I(1) 3.099(2), I(1)–I(1i) 2.7881(10) A; S(1)–I(1)–I(1i) 178.68(4)°; i 1 – x, 1 – y, 1 – z] between [18]aneS6 macrocycles. Compound 3 is the first example of an adduct between I2 and a homoleptic thioether macrocycle which shows both exo [S(1)–I(1) 2.838(2), I(1)–I(1′) 2.7875(6) A; S(1)–I(1)–I(1′) 174.95(4)°] and endo [S(4)–I(4) 2.792(2), I(4)–I(4′) 2.8067(7) A; S(4)–I(4)–I(4′) 174.43(4)°] co-ordination of I2 molecules. The endo-oriented I2 molecules occupy space above and below the macrocyclic plane with the macrocycle adopting a sigmoid conformation. The single-crystal structure determination of the 1∶1 adduct 4 shows symmetrically bridging I2 molecules [S(1)–I(1) 3.215(2) and I(1)–I(1i) 2.758(2) A; S(1)–I(1)–I(1i) 172.75(3)°; i –x, –y, –z] which are a characteristic of this stoichiometry. Compound 5 contains endo- and exo-oriented S donors within the same adduct [I(1)–I(1′) 2.7861(8), I(4)–I(4′) 2.7937(8), I(7)–I(7′) 2.8345(8), S(1)–I(1) 2.821(2), S(4)–I(4) 2.815(2), S(7)–I(7) 2.741(2) A; S(1)–I(1)–I(1′) 170.15(5), S(4)–I(4)–I(4′) 177.41(5), S(7)–I(7)–I(7′) 177.24(5)°]. These results are discussed in the context of the stability and characteristics of thioether crown–iodine charge-transfer complexes, and a qualitative MO diagram is proposed to account for the shorter I–I distances in bridging I2 fragments compared to those in terminally bound I2.

Ab initio MO study of m-phenylenediamine and 2,4-diamino-1,3,5-triazine dication diradicals

Ab initio MO calculations of m-phenylenediamine ( 1) and 2,4-diamino-1,3,5-triazine ( 2) dication diradicals, which are the model molecules of two possible high-spin polymers (poly( m-aniline) and poly(imino-1,3,5-triazinediyl) cation radicals), have been carried out with the 6–31G * basis set at the ROHF, GVB and CASSCF levels. In accordance with qualitative MO considerations, the electronic structures of 1 2+ and 2 2+ were quite different from each other. In the planar C 2V geometry, the singlet-triplet splittings (ΔE S-T) of 1 2+ and 2 2+ were calculated to be 0.7 and 12.5 kcal/mol, respectively, at the CASPT2 level. m-1,3,5-triazinediyl is another effective ferromagnetic coupler for the aza-substituted systems.

Aspects of organic chemistry: structure

Introduction in organic chemistry the structural model of classical organic chemistry optical activity, chirality, and symmetry of molecules configurational analysis (demonstrated using carbohydrates examples) conformational analysis (demonstrated using steroid examples) macromolecular and supramolocular chemistry the qualitative MO model documentation and retrieval of chemical knowledge symmetry point groups and space groups determination of absolute configuration NMR-spectroscopy the special case of benzene base-pairing in biology and chemistry.

Ferromagnetic Interaction between Aminium Radical Centers through m-Phenylene and m-1,3,5-Triazinediyl.

Ab initio MO study (at the ROHF, GVB, and CASSCF levels) of m-phenylenediamine (1) and 2,4-diamino-1,3,5-triazine (2) dication diradicals, which are model molecules of two possible high-spin polymers, poly(m-aniline) and poly(imino-1,3,5-triazinediyl) cation radicals, is presented. On the basis of qualitative MO considerations, it was seen that the electronic structures of 1(2+) and 2(2+) differed considerably. The singlet-triplet splittings (DeltaE(S)(-)(T)) of 1(2+) and 2(2+) were calculated to be 0.7 and 12.5 kcal/mol, respectively, at the CASPT2 level. m-1,3,5-Triazinediyl is shown to be a strong ferromagnetic coupler for aza-substituted systems.

Wechselwirkungen in Kristallen 99. Alkalimetall-Kontaktionenmultipel von acenaphthylen- und Fluoranthen-Dianionen: Fünfring- und Sechsring-Koordination

Stimulated by qualitative MO considerations, the non-alternant π-hydrocarbons acenaphthylene and fluoranthene have been reduced in their ether solutions under aprotic conditions by alkali metals to their dianions. These crystallize as contact ion multiples of different structures: the (15-crown-5)-solvated sodium salt of acenaphthylene is a monomeric contact ion triple [(H 2CH 2CO) 5Na ⊕C 12H 8 ⊝⊝Na ⊕-(OCH 2CH 2) 5] 1 with the ten-fold coordinated Na ⊕ counter cations above and below the cyclopentadienyl ring. In contrast, the DME- or diglyme-solvated Na ⊕- and Li ⊕-salts of fluoranthene dianion [(R 2O) 2Me ⊕C 16H 10 ⊝⊝Me ⊕(OR 2) 2]∞ crystallize in polymer strings containing eight-fold coordinated Li ⊕ or ten-fold coordinated Na ⊕ counter cations above and below different of the two naphthalene six-membered rings. For these rather unexpected coordination differences, MNDO calculations based on the structural data provide a plausible rationale: the negative charge in the acenaphthylene dianion is predominantly located within the five-membered ring and in the fluoranthene dianion within the naphthalene six-membered rings.

Electronic absorption spectra of rhenium(VII) compounds: The effect of the coordination number

Changes in electronic absorption spectra in the following series: MeRe(L2)O3 ← McReO3 ← ReO4− → ReO65−, have been studied in terms of the qualitative MO scheme.

A molecular orbital approach towards designing ligand constants

It is shown that by a judicious mixing of ionisation potential and electron affinity, a constant can be devised for a monodentate ligand as a measure of the electronic effects exerted by the ligand. A qualitative MO treatment is presented to justify the approach.

Cofacial and antarafacial indenyl bimetallic isomers: a descriptive MO picture and implications for the indenyl effect on ligand substitution reactions

The hetero-bimetallic Ind complexes (Ind=indenyl anion) with the formula (CO) 3Cr(μ-Ind)RhL 2 (L=CO, ethylene; L 2=COD, COT, NBD) have two possible conformers, one with the metals at antipodal sides of the condensed bicyclic polyene (antarafacial, anti) and the other with both metals on the same side (cofacial, syn). These systems can be considered to contain 34 valence electrons by assuming that Ind is capable of donating all of its ten π electrons to the metals. A qualitative MO analysis, based on EHMO calculations, is presented. The anti conformer is compared to the homometallic 34e species, namely [CpRu(μ-Ind)RuCp] +. Here, the pseudo-symmetrical relationship between the two identical metal fragments strongly suggests that one Ind σ-bonding MO donates part of its electron density to each of the two metals, which thus become formally saturated (36e). This argument is extended to the anti CrRh derivative in which a critical role is played by an empty FMO of the (CO) 2Rh(I) fragment with π ‖ symmetry (i.e. lying in the molecular mirror plane). The latter, in spit the large π CO ∗ contributions, has good acceptor capabilities at the metal.The calculations suggest that the so-called indenyl effect, namely the increased rate of substitution of one CO ligand by another σ donor (e.g. a phosphine), although a structurally dynamic process, also depends on the electron density which accumulates on the aforementioned π ‖ FMO. In the series ( η 5-C 5H 5)Rh(CO) 2, ( η 5-C 7H 9Rh(CO) 2 and { η 5-[(CO) 3CrC 7H 9]}-Rh(CO) 2 the progressively reduced donor capabilities of the cyclic polyene toward the (CO) 2Rh(I) fragment induce a larger electrophilicity in rhodium, in good agreement with the experimental data on substitution reactivity. Finally, the electronic features of the cofacial conformer (CO) 3Cr(μ-Ind)Rh(CO) 2 are examined. In spite of the long CrRh separation ( > 3 A ̊ ) the graphically illustrated MO analysis indicates a direct, heterodox, intermetallic linkage, which helps the metals to achieve a formal electronic saturation. In this case, the limited propensity of the complex toward any CO substitution reactions is likely attributable to the hindered mobility of the (CO) 2Rh(I) fragment.

Coordination of B in K2O-SiO2-B2O3-P2O5glasses using B K-edge XANES

High-resolution B K-edge X-ray absorption near-edge structure (XANES) spectra of K{sub 2}O-SiO{sub 2}-B{sub 2}O{sub 3}-P{sub 2}O{sub 5} glasses are reported using synchrotron radiation. Two prominent features, peak a at about 194.0 eV and peak b at about 198.0 eV, are observed. In the basis of the qualitative MO diagrams of BO{sub 3}{sup 3{minus}} and BO{sub 4}{sup 5{minus}} clusters, peak a is assigned to the transition of B 1s electrons to the unoccupied B 2p{sub z} ({pi}*) states for threefold-coordinated B ({sup [3]}B), and peak b is assigned to the transition of B 1s electrons to the unoccupied B {sigma}* states for fourfold-coordinated B({sup [4]}B). B K-edge XANES spectroscopy is established as a method for {open_quotes}fingerprinting{close_quotes} {sup [3]}B and {sup [4]}B in borate and borosilicate minerals, glasses, and melts. Also, the relative proportions of {sup [4]}B and {sup [3]}B in the borosilicate glasses are determined from the integrated peak areas for {sup [3]}B and {sup [4]}B edge peaks and are shown to be generally in good agreement with recent {sup 11}B MAS NMR measurements. However, the surface and near-surface structure of powder particles of perboric glasses containing P{sub 2}O{sub 5} appears to have a lower proportion of {sup [3]}B entities thanmore » the bulk. 27 refs., 4 figs., 1 tab.« less