Resonance Character of Copper/Silver/Gold Bonding in Small Molecule⋅⋅⋅MX (X=F,Cl, Br,CH3, CF3) Complexes

The resonance character of Cu/Ag/Au bonding is investigated in B???M?X (M=Cu, Ag, Au; X=F, Cl, Br, CH₃, CF₃; B=CO, H₂O, H₂S, C₂H₂, C₂H₄) complexes. The natural bond orbital/natural resonance theory results strongly support the general resonance‐type three‐center/four‐electron (3c/4e) picture of Cu/Ag/Au bonding, B:M?X?B⁺?M:X^?, which mainly arises from hyperconjugation interactions. On the basis of such resonance‐type bonding mechanisms, the ligand effects in the more strongly bound OC???M?X series are analyzed, and distinct competition between CO and the axial ligand X is observed. This competitive bonding picture directly explains why CO in OC???Au?CF₃ can be readily replaced by a number of other ligands. Additionally, conservation of the bond order indicates that the idealized relationship b_B???M+b_MX=1 should be suitably generalized for intermolecular bonding, especially if there is additional partial multiple bonding at one end of the 3c/4e hyperbonded triad.     

Structural Variety of Defect Perovskite Variants M3E2X9 (M = Rb,Tl, E = Bi,Sb, X = Br,I)

The compounds Rb₃Sb₂Br₉, Rb₃Sb₂I₉, Rb₃Bi₂Br₉, Rb₃Bi₂I₉, and Tl₃Bi₂Br₉ were synthesized and their crystal structures determined from single crystal X‐ray diffraction data. The compounds Rb₃Sb₂Br₉, Rb₃Sb₂I₉, and Rb₃Bi₂I₉ crystallize in the Tl₃Bi₂I₉ type of structure (space group P2₁/n, no. 14). Rb₃Bi₂Br₉ and Tl₃Bi₂Br₉ crystallize in a new but closely related type of structure (space group P2₁/a, no. 14). Both structure types feature characteristic double layers comprising corner‐sharing EX₆ octahedra. The space groups are set in a way that the stacking direction of the layers is the [001] direction. The group‐subgroup relations to cubic perovskite ABO₃ are discussed. Differences between M₃E₂X₉ types are attributed to distortions of the underlying MX₃ close packing. Depending on the atomic size ratio, the distortions are quantified by an order parameter.     

Inverse sandwich complexes based on low‐valent group 13 elements and cyclobutadiene: A theoretical investigation on E‐C4H4‐E (E = Al,Ga, In,Tl)

The chemistry of the low‐valent Group 13 elements (E = B, Al, Ga, In, Tl) has formed the recent hot topic. Recently, a series of low‐valent Group 13‐based compounds have been synthesized, i.e., [E‐Cp*‐E]⁺ (E = Al, Ga, In, Tl) cations, which have been termed as the interesting “inverse sandwich” complexes. To enrich the family of inverse sandwiches, we report our theoretical design of a new type of inverse sandwiches E‐C₄H₄‐E (E = Al, Ga, In, Tl) for stabilizing the low‐valent Group 13 elements. The calculated dissociation energies indicate that unlike [E‐Cp‐E]⁺ that dissociates via loss of the charged atom E⁺, E‐C₄H₄‐E dissociates via loss of the neutral atom E with the bond strengths of Al > Ga > In > Tl. Moreover, E‐C₄H₄‐E are more stable in dissociation than [E‐Cp‐E]⁺ cations. By comparing with other various isomers, we found that the inverted E‐C₄H₄‐E should be kinetically quite stable with the least conversion barriers of 33.5, 33.5, 35.2, and 36.9 kcal/mol for E = Al, Ga, In, and Tl, respectively. Furthermore, replacement of cyclobutadiene‐H atoms by the highly electron‐positive groups such as SiH₃ and Si(CH₃)₃ could significantly stabilize the inverted form in thermodynamics. Possible synthetic routes are proposed for E‐C₄H₄‐E. With no need of counterions, the newly designed neutral complexes E‐C₄H₄‐E welcome future synthesis. © 2012 Wiley Periodicals, Inc.     

Exploring the Potential Energy Surface of E2P4 Clusters (E=Group 13 Element): The Quest for Inverse Carbon‐Free Sandwiches

Inverse carbon‐free sandwich structures with formula E₂P₄ (E=Al, Ga, In, Tl) have been proposed as a promising new target in main‐group chemistry. Our computational exploration of their corresponding potential‐energy surfaces at the S12h/TZ2P level shows that indeed stable carbon‐free inverse‐sandwiches can be obtained if one chooses an appropriate Group 13 element for E. The boron analogue B₂P₄ does not form the D_4h‐symmetric inverse‐sandwich structure, but instead prefers a D_2d structure of two perpendicular BP₂ units with the formation of a double B?B bond. For the other elements of Group 13, Al–Tl, the most favorable isomer is the D_4h inverse‐sandwich structure. The preference for the D_2d isomer for B₂P₄ and D_4h for their heavier analogues has been rationalized in terms of an isomerization‐energy decomposition analysis, and further corroborated by determination of aromaticity of these species.     

Reaction Mechanism of the Symmetry‐Forbidden [2+2] Addition of Ethylene and Acetylene to Amido‐Substituted Digermynes and Distannynes Ph2NEENPh2, (E=Ge,Sn): A Theoretical Study

Quantum chemical calculations of reaction mechanisms for the formal [2+2] addition of ethylene and acetylene to the amido‐substituted digermyne and distannyne Ph₂N?EE?NPh₂ (E=Ge, Sn) have been carried out by using density functional theory at the BP86/def2‐TZVPP level. The nature and bonding situations were studied with the NBO method and with the charge and energy decomposition analysis EDA‐NOCV. The addition of ethylene to Ph₂N?EE?NPh₂ takes place through an initial [2+1] addition to one metal atom and consecutive rearrangement to four‐membered cyclic species, which feature a weak E?E bond. Rotation about the C?C bond with concomitant rupture of the E?E bond leads to the 1,2‐disubstituted ethanes, which have terminal E(NPh₂) groups. The overall reaction Ph₂N?EE?NPh₂+C₂H₄→(Ph₂N)E?C₂H₄?E(NPh₂) has very low activation barriers and is slightly exergonic for E=Ge but slightly endergonic for E=Sn. The analysis of the electronic structure shows that there is charge donation of nearly one electron to the ethylene moiety already in the first part of the reaction. The energy partitioning analysis suggests that the HOMO(Ph₂N?EE?NPh₂)→LUMO(C₂H₄) interaction has a similar strength as the HOMO(C₂H₄)→LUMO(Ph₂N?EE?NPh₂) interaction. The [2+2] addition of acetylene to Ph₂N?EE?NPh₂ also takes place through an initial [2+1] approach, which eventually leads to 1,2‐disubstituted olefins (Ph₂N)E?C₂H₂?E(NPh₂). The formation of the energetically lowest lying conformations of cis‐(Ph₂N)E?C₂H₂?E(NPh₂), which occurs with very low activation barriers, is clearly exergonic for the germanium and the tin compound. The trans‐coordinated isomers of (Ph₂N)E?C₂H₂?E(NPh₂) are slightly lower in energy than the cis form but they are separated by a substantial energy barrier for the rotation about the C?C bond. The energy decomposition analysis indicates that the initial reaction takes place under formation of electron‐sharing bonds between triplet fragments rather than HOMO–LUMO interactions.     

HB9X9˙ and H2B9X9 (X = Cl,Br, I): Neutral closo-Nonaboranes in the Novel Series BnHn+1, and BnHn+2 – Syntheses,Ab Initio Calculations,and Electronic Structures

Chemical reduction of B₉X₉ (X = Cl, Br, I) with gaseous HI proceeds stepwise to give the neutral paramagnetic clusters HB₉X₉ ^· , and the corresponding diamagnetic clusters H₂B₉X₉. Together they comprise the first neutral derivatives in the series B_nH_n+1 and B_nH_n+2 with n = 9. The EPR spectra of the paramagnetic HB₉X₉ ^· (X = Cl, Br, I) in glassy frozen CH₂Cl₂ solutions showed increasing g anisotropy for the heavier halogen derivatives, illustrating significant halogen participation at the singly occupied MO due to the larger spin-orbit coupling contributions. Temperature dependent ¹H NMR spectra of H₂B₉X₉ (in CD₃CN, X = Cl, Br) indicate the presence of H₂B₉X₉, [HB₉X₉]^–, and [CD₃CNH]⁺ with H₂B₉X₉ acting as a Brønsted acid. The corresponding ¹¹B NMR spectra (in CD₃CN) show the presence of the dianions [B₉X₉]^2– as a result of the protonation of CD₃CN. The ¹¹B resonances of the species H₂B₉X₉ and [HB₉X₉]^– are obscured by superimposition of the two resonance lines of the dianions [B₉X₉]^2–. Temperature dependent ¹¹B{¹H} MAS-NMR spectra of H₂B₉Br₉ show coalescence at 410 K and hence dynamic behaviour of the neutral B₉-cluster in the solid. Cyclic voltammetry experiments of H₂B₉Br₉ in CH₃CN solvent) are compatible with the redox sequence [B₉Br₉]^2––[B₉Br₉] ^{· –}–B₉Br₉. Quantum chemical calculations with the electron localization function (ELF) are described.     

Microwave spectrum of N2D4, 14N quadrupole coupling constants,and the barrier to inversion of the amino group

The microwave spectrum of N₂D₄ has been observed and analyzed. Based on five low-J rotational transitions the effective rotational constants are: A = 74712.9 ± 1.9 MHz, B = 18500.42 ± 0.46 MHz, and C = 18439.91 ± 0.46 MHz. The quadrupole coupling constants of the ¹⁴N nuclei are X_aa = 4.23 ± 0.04 MHz, X_bb = 1.98 ± 0.05 MHz, and X_cc = ?2.25 ± 0.05 MHz. Using the observed ground state inversion splittings for N₂D₄ and N₂H₄ the barrier to inversion of a single amino group is computed to be 5.00 kcal mol^?1.     

Determination of the stability constant and thermodynamic parameters between Tl<Superscript>+</Superscript>, Ag<Superscript>+</Superscript> and Pb<Superscript>2+</Superscript> cations with 2,6-di(furyl-2yl)-4-(4-methoxy phenyl)pyridine as a new synthesis ligand

The complexation reaction between Tl⁺, Ag⁺ and Pb²⁺ cations with 2,6-di(furyl-2yl)-4-(4-methoxy phenyl)pyridine as a new synthesis ligand in acetonitrile (ACN)–H₂O and methanol (MeOH)–H₂O binary solutions has been studied at different temperatures using conductometric method. The conductometric data show that the stoichiometry of the complexes is 1: 1 [M: L] and the stability constant of complexes changes with the binary solutions identity. Also, the structure of the resulting 1: 1 complexes was optimized using the LanL2dz basis set at the B3LYP level of theory using GAUSSIAN03 software. The results show that the change of logK_f for (DFMP.Pb)²⁺ and (DFMP.Ag)⁺ complexes with the mole ratio of acetonitrile and for (DFMP.Ag)⁺ and (DFMP.Tl)⁺ complexes with the mole ratio of methanol have a linear behavior, while the change of logK_f of (DFMP.Tl)⁺complex in ACN–H₂O binary solutions (with a minimum in X_ACN = 0.5) and (DFMP.Ag)⁺ complex in MeOH–H₂O binary solutions (with a minimum in X_MeOH = 0.75) show a non-linear behavior. The selectivity order of DFMP ligand for these cations in mol % CAN = 25 and 75 obtain Tl⁺ > Pb²⁺ > Ag⁺ but in mol % CAN = 50, the selectivity order observe Pb²⁺ > Tl⁺ > Ag⁺. Also, this selectivity sequence of DFMP in MeOH–H₂O (mol % MeOH = 75 and 100) and (mol % MeOH = 50) is obtained Pb²⁺ > Ag⁺ and Tl⁺ > Ag⁺ > Pb²⁺ respectively. The values of thermodynamic parameters show that these values are influenced by the nature and the composition of binary solution. In all cases, the resulting complexes are enthalpy stabilized and entropy destabilized. The TΔS_C^° versus ΔH_C^° plot of all obtained thermodynamic data shows a fairly good linear correlation which indicates the existence of enthalpy-entropy compensation in the complexation reactions.     

Schwingungsspektren der Clusterverbindungen (M6X)X · 8 H2O,M = Nb,Ta; Xi = CI,Br; Xa = CI,Br, I

Vibrational Spectra of the Cluster Compounds (M₆X₁₂ⁱ) · 8H₂O, M = Nb, Ta; Xⁱ = Cl, Br; X^a = Cl, Br, I IR and, for the first time, Raman spectra at 80 K of the cluster compounds (M₆X)X · 8H₂O; M = Nb, Ta; Xⁱ = Cl, Br; X^a = Cl, Br, I, have been recorded, characterized by typical frequencies of the (M₆X) unit, which are only slightly influenced by the terminal X^a ligands. The most intense line with the depolarisation ≈? 0.2 in all Raman spectra is caused by inphase movement of all atoms and assigned to the symmetric metal-metal vibration v₁, observed for the clusters (Nb₆Cl) at 233–234, for (Nb₆Br) at 186–187, for (Ta₆Cl) at 199–203, and for (Ta₆Br) at 176–179 cm^?1. The IR spectra exhibit in the same series intense bands at 233, 204, 207, and 179 cm^?1, assigned to the antisymmetric metal-metal vibration. The metal-metal frequencies are significantly higher than discussed before. The tantalum clusters show on excitation with the krypton line 647.1 nm in the region of a d–d transition at 645 nm a resonance Raman effect with series of overtones and combination bands. In case of (Ta₆Br) another polarisized band is observed at 229 cm^?1 and assigned to the Ta? Brⁱ vibration v₂. From the progressions of v₁ and v₂ anharmonicity constants of about ?3 cm^?1 are calculated indicating a strong distortion of the potential curves.     

The Gauche Effect in XCH2CH2X Revisited

We have quantum chemically investigated the rotational isomerism of 1,2-dihaloethanes XCH₂CH₂X (X = F, Cl, Br, I) at ZORA-BP86-D3(BJ)/QZ4P. Our Kohn-Sham molecular orbital (KS-MO) analyses reveal that hyperconjugative orbital interactions favor the gauche conformation in all cases (X = F−I), not only for X = F as in the current model of this so-called gauche effect. We show that, instead, it is the interplay of hyperconjugation with Pauli repulsion between lone-pair-type orbitals on the halogen substituents that constitutes the causal mechanism for the gauche effect. Thus, only in the case of the relatively small fluorine atoms, steric Pauli repulsion is too weak to overrule the gauche preference of the hyperconjugative orbital interactions. For the larger halogens, X⋅⋅⋅X steric Pauli repulsion becomes sufficiently destabilizing to shift the energetic preference from gauche to anti, despite the opposite preference of hyperconjugation.     

Structure and stability of X4Y4H4 (XY=CC,BN)

Ab initio calculations have been carried out to study the structures and relative stabilities of the planar eight‐membered ring B₄N₄H₄ and its isoelectronic species C₈H₄ at the HF/6‐31G*, MP2/6‐31G*, MP2/6‐311G**, and MP4SDQ/6‐31G* levels. The analyses of Milliken population, vibration frequencies, π‐molecular orbital components, and orbital energy levels were used to evaluate the relative stabilities of these two similar systems. The homodesmotic reactions were also taken to be a useful index of relative stability for X₄Y₄H₄ (XY=CC, BN) and gave the resonance energies with MP4SDQ/6‐31G* of C₈H₄ (?37.2 kcal/mol) < B₄N₄H₄ (?29.2 kcal/mol). Furthermore, we calculated the thermodynamic functions of these reactions to discuss the influence of temperature. It is concluded that B₄N₄H₄ may exist in theory and could be a little more stable than C₈H₄. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 293–298, 2001     

Theoretical investigation on identical anionic halide‐exchange SN2 reaction processes on N‐haloammonium cation NH3X+ (X = F,Cl, Br,and I)

CCSD(T) calculations have been used for identically nucleophilic substitution reactions on N‐haloammonium cation, X^? + NH₃X⁺ (X = F, Cl, Br, and I), with comparison of classic anionic S_N2 reactions, X^? + CH₃X. The described S_N2 reactions are characterized to a double curve potential, and separated charged reactants proceed to form transition state through a stronger complexation and a charge neutralization process. For title reactions X^? + NH₃X⁺, charge distributions, geometries, energy barriers, and their correlations have been investigated. Central barriers ΔE for X^? + NH₃X⁺ are found to be lower and lie within a relatively narrow range, decreasing in the following order: Cl (21.1 kJ/mol) > F (19.7 kJ/mol) > Br (10.9 kJ/mol) > I (9.1 kJ/mol). The overall barriers ΔE relative to the reactants are negative for all halogens: ?626.0 kJ/mol (F), ?494.1 kJ/mol (Cl), ?484.9 kJ/mol (Br), and ?458.5 kJ/mol (I). Stability energies of the ion–ion complexes ΔE_comp decrease in the order F (645.6 kJ/mol) > Cl (515.2 kJ/mol) > Br (495.8 kJ/mol) > I (467.6 kJ/mol), and are found to correlate well with halogen Mulliken electronegativities (R² = 0.972) and proton affinity of halogen anions X^? (R² = 0.996). Based on polarizable continuum model, solvent effects have investigated, which indicates solvents, especially polar and protic solvents lower the complexation energy dramatically, due to dually solvated reactant ions, and even character of double well potential in reactions X^? + CH₃X has disappeared. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical study of the gas‐phase SN2 reactions of X− with CH3OY (X,Y = Cl,Br, I)

The gas‐phase nucleophilic substitution reactions at saturated oxygen X^? + CH₃OY (X, Y = Cl, Br, I) have been investigated at the level of CCSD(T)/6‐311+G(2df,p)//B3LYP/6‐311+G(2df,p). The calculated results indicate that X^? preferably attacks oxygen atom of CH₃OY via a S_N2 pathway. The central barriers and overall barriers are respectively in good agreement with both the predictions of Marcus equation and its modification, respectively. Central barrier heights (ΔH and ΔH) correlate well with the charges (Q) of the leaving groups (Y), Wiberg bond orders (BO) and the elongation of the bonds (O? Y and O? X) in the transition structures. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12X11]− (X=F,Cl, Br,I, CN)

Electrophilic anions of type [B₁₂X₁₁]⁻ posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B₁₂X₁₁]⁻ with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N₂ as σ-donor-π-acceptors. Temperature-dependent formation of [B₁₂X₁₁NG]⁻ indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B₁₂X₁₁]⁻ and steric effects. The binding of CO and N₂ to [B₁₂X₁₁]⁻ is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B₁₂X₁₁CO]⁻ and [B₁₂X₁₁N₂]⁻ were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N₂ stretching frequencies and , respectively. Observed shifts of and are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π-backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B₁₂X₁₁]⁻ reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions.     

Insertion of the p-complex structure of silylenoid H2SiLiF into X–H bonds (X = C,Si, N,P, O,S, and F)

The insertion reactions of the p-complex structure (A) of silylenoid H₂SiLiF into XH_n molecules (X = C, Si, N, P, O, S, and F; n = 1–4) have been studied by ab initio calculations at the G3(MP2) level. The results indicate that the insertion reactions of A into X–H bonds proceed via three reaction paths, I, II, and III, forming the same products, substituted silanes H₃SiXH_n − 1 with dissociation of LiF, respectively, and all insertion reactions are exothermic. All the seven X–H bonds can undergo insertion reactions with A via path I and II, but only four of them, C–H, Si–H, P–H, and S–H, undergo insertion reactions via path III. The following conclusions emerge from this work: (i) the X–H insertion reactions of A occur in a concerted manner via a three-membered ring transition state; (ii) for path I and II, the stabilization energies of the A–XH_n complexes decrease in the order HF > H₂O > H₂S > NH₃ > SiH₄ > CH₄; (iii) for path I and II, the greater the atomic number of heteroatom (X) in a given row, the easier the insertion reaction of XH_n hydrides and the larger the exothermicity, and for the second-row hydrides, the reaction barriers are lower than for the first-row hydrides; (iv) The barriers of path I are lowest in those of three pathways with the exception of A + SiH₄ system, which barrier of path III is lowest. Moreover, the present study demonstrates that both electronic and steric effects play major roles in the course of insertion reactions of A into X–H bonds.     

Theoretical Studies of Inorganic Compounds. 19 1) Quantum Chemical Investigations of the Phosphane Complexes X3B‐PY3 and X3Al‐PY3 (X = H,F, Cl; Y = F,Cl, Me,CN)

We report about quantum chemical ab initio calculations at the MP2/6‐311+G(2d)//MP2/6‐31G(d) level and DFT calculations at BP86/TZP of the geometries and bond dissociation energies of the borane‐phosphane complexes X₃B‐PY₃ and the alane‐phosphane complexes X₃Al‐PY₃ (X = H, F, Cl; Y = F, Cl, Me, CN). The nature of the B‐P and Al‐P bonds is analyzed with a bond energy partitioning method. The calculated bond dissociation energies D_e of the borane adducts X₃B‐PY₃ show for the phosphane ligands the trend PMe₃ > PCl₃ ∼ PF₃ > P(CN)₃. A similar trend PMe₃ > PCl₃ > PF₃ > P(CN)₃ is predicted for the alane complexes X₃Al‐PY₃. The order of the Lewis acid strength of the boranes depends on the phosphane Lewis base. The boranes show with PMe₃ and PCl₃ the trend BH₃ > BCl₃ > BF₃ but with PF₃ and P(CN)₃ the order is BH₃ > BF₃ > BCl₃. The bond energies of the alane complexes show always the trend AlCl₃ ≥ AlF₃ > AlH₃. The bonding analysis shows that it is generally not possible to correlate the trend of the bond energies with one single factor which determines the bond strength. The preparation energy which is necessary to deform the Lewis acid and Lewis base from the equilibrium form to the geometry in the complex may have a strong influence on the bond energies. The intrinsic interaction energies may have a different order than the bond dissociation energies. The trend of the interaction energies are sometimes determined by a single factor (Pauli repulsion, electrostatic attraction or covalent bonding) but sometimes all components are important. The higher Lewis acid strength of BCl₃ compared with BF₃ in strongly bonded complexes is not caused by the deformation energy of the fragments but it is rather caused by the intrinsic interaction energy. P(CN)₃ is a weaker Lewis base than PF₃, PCl₃ and PMe₃ mainly because of its weaker electrostatic attraction. The complex H₃B‐P(CN)₃ is predicted to have a bond dissociation energy D_o = 14.8 kcal/mol which should be sufficient to synthesize the compound as the first adduct with the ligand P(CN)₃. The calculated bond energies at the BP86 level are in most cases very similar to the MP2 results. In a few cases significantly different absolute values have been found which are caused by the method and not by the quality of the basis set.     

Conformational preferences of 34 valence electron A2X4 molecules: An ab initio Study of B2F4, B2Cl4, N2O4, and C2O

Ab initio molecular orbital structures and energies of B₂F₄, B₂Cl₄, N₂O₄, and C₂O have been calculated for both perpendicular D_2d and planar D_2h rotamers. The experimental trend toward greater preference for the D_2d forms in going from B₂F₄ to B₂Cl₄ is reproduced. N₂O₄ favors the planar conformation, although the rotation barrier is overestimated at the theoretical levels used. The oxalate dianion is calculated to be more stable in the D_2d conformation; the experimental planar arrangement in the solid may be due to crystal packing forces. The preferences for one conformation over another are small; analysis indicates that different effects may predominate in each case: π stabilization for B₂F₄, hyperconjugation for B₂Cl₄, lone-pair interactions for N₂O₄, and electrostatic repulsions for C₂O.     

The closo-Cluster Triad: B9X9, [B9X9]· –, and [B9X9]2– with Tricapped Trigonal Prisms (X = Cl,Br, I). Syntheses,Crystal and Electronic Structures

Halogenation of nido-B₁₀H₁₄ with C₂H₂Cl₄, C₂Cl₆, Br₂, or I₂, produces by cluster degradation the (2 n)-closo-clusters B₉X₉ (X = Cl, Br, I). The synthesis of salts of the perhalogenated radical anions of the type (2 n + 1)-closo-[B₉X₉]^{· –} and of the corresponding dianions (2 n + 2)-closo-[B₉X₉]^2– from neutral B₉X₉ is described [n is the number of cluster atoms; (2 n), (2 n + 1), and (2 n + 2) is the number of cluster electrons]. Molecular and crystal structures of B₉Cl₉, B₉Br₉, [(C₆H₅)₄P][B₉Br₉] · CH₂Cl₂, and [(C₄H₉)₄N]₂[B₉Br₉] · CH₂Cl₂ have been determined via X-ray diffraction. All three oxidation states of the cluster retain the tricapped trigonal prism. The reduction of the clusters B₉X₉ was shown by cyclic voltammetry in CH₂Cl₂ to proceed via two successive one-electron reversible steps, separated by at least 0.4 V. The paramagnetic radical anions [B₉X₉]^{· –} (X = Cl, Br) were further characterized by magnetic susceptibility measurements of [Cp₂Fe][B₉X₉] and [Cp₂Co][B₉X₉], respectively. The EPR spectra of [B₉X₉]^{· –} (X = Cl, Br, I) in glassy frozen CH₂Cl₂ solutions showed increasing g anisotropy for the heavier halogen derivatives, illustrating significant halogen participation at the singly occupied MO. The ¹¹B NMR spectra of CD₂Cl₂ solutions of the neutral clusters B₉X₉ exhibit only one sharp resonance, indicating that the boron atoms are highly fluxional in solution. In contrast, two different boron resonances as expected for a rigid tricapped trigonal prism are clearly observed for the [B₉X₉]^2– dianions in solutions and for solid B₉Br₉ in the ¹¹B MAS NMR spectra. Temperature dependent ¹¹B MAS NMR experiments on B₉Br₉ and [B₉Br₉]^2– in the solid state show a reversible coalescence of the two resonances at higher temperature. ¹¹B MAS NMR spectra and DTA measurements of [B₉Br₉]^2– showed a phase transition.     

The isotypic hydrogen phosphate and arsenate dihydrates M2HXO4·2H2O (M = Rb,Cs; X = P,As)

The four isotypic alkaline metal monohydrogen arsenate(V) and phosphate(V) dihydrates M₂HXO₄·2H₂O (M = Rb, Cs; X = P, As) [namely dicaesium monohydrogen arsenate(V) dihydrate, Cs₂HAsO₄·2H₂O, dicaesium monohydrogen phosphate(V) dihydrate, Cs₂HPO₄·2H₂O, dirubidium monohydrogen arsenate(V) dihydrate, Rb₂HAsO₄·2H₂O, and dirubidium monohydrogen phosphate(V) dihydrate, Rb₂HPO₄·2H₂O] were synthesized by reaction of an aqueous H₃XO₄ solution with one equivalent of aqueous M₂CO₃. Their crystal structures are made up of undulating chains extending along [001] of tetrahedral [XO₃(OH)]⁻ anions connected via strong O—H...O hydrogen bonds. These chains are in turn connected into a three‐dimensional network via medium‐strength hydrogen bonding involving the water molecules. Two crystallographically different M⁺ cations are located in channels running along [001] or in the free space of the [XO₃(OH)]⁻ chains, respectively. They are coordinated by eight and twelve O atoms forming irregular polyhedra. The structures possess pseudosymmetry. Due to the ordering of the protons in the [XO₃(OH)]⁻ chains in the actual structures, the symmetry is reduced from C2/c to P2₁/c. Nevertheless, the deviation from C2/c symmetry is minute.     

Geometries,vibrational frequencies,and electron affinities of X2Cl (X=C,Si,Ge) clusters

Ab initio quantum chemical calculations have been performed on X₂Cl^? and X₂Cl (X = C, Si, Ge) clusters. The geometrical structures, vibrational frequencies, electronic properties and dissociation energies are investigated at the Hartree–Fock (HF), Møller–Plesset second‐ and fourth‐order (MP2, MP4), CCSD(T) level with the 6‐311+G(d) basis set. The X₂Cl (X = C, Si, Ge) and X₂Cl^? (X = Si, Ge) take a bent shape obtained at the ground state, while C₂Cl^? has a linear structure. The impact on internal electron transfer between the X₂Cl and the corresponding anional clusters is studied. The three different types of electron affinities (EAs) at the CCSD(T) are reported. The most reliable adiabatic electronic affinities, obtained at the CCSD(T)/cc‐pvqz level of theory, are predicted to be 3.30, 2.62, and 1.98 eV for C₂Cl, Si₂Cl, and Ge₂Cl, respectively. The calculated EAs of C₂Cl and Ge₂Cl are in good agreement with theoretical results reported. The correlation effects and basis sets effects on the geometrical structures and dissociation energies are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007