Dπ Interaction Research Articles

Theoretical Study of Terminal Vanadium(V) Chalcogenido Complexes Bearing Chlorido and Methoxido Ligands

Solvent (methanol) coordinated vanadium(V) chalcogenido complexes bearing chlorido and methoxido ligands have been studied computationally by means of density functional (DFT) methods. The gas phase complexes were fully optimized using B3LYP/GEN functionals with 6-31+G⁎⁎ and LANL2DZ basis sets. The optimized complexes show distorted octahedral geometries around the central vanadium atom. The ligand pπ-vanadium dπ interactions were analyzed by natural bond order (NBO) and natural population analyses (NPA). These results show strong stabilization of the V=O bond as was further confirmed by the analyses of the frontier molecular orbitals (FMOs). Second-order perturbation analyses also revealed substantial delocalization of lone pair electrons from the oxido ligand into vacant non-Lewis (Rydberg) orbitals as compared to the sulfido and seleno analogues. These results show significant ligand-to-metal charge transfer (LMCT) interactions. Full interaction map (FIM) of the reference complex confirms hydrogen bond interactions involving the methanol (O-H) and the chlorido ligand.

Oxygenation of a Ruthenium Complex Bearing a PBP-Pincer Ligand Inducing the Formation of a Boronato Ligand with a Weak Ru–O Bond

A ruthenium hydride complex having a boron-containing PBP pincer ligand was synthesized. Oxygenation of this complex by using NMO formed the corresponding boronate complex with a nearly 90° B–O–Ru angle and a long Ru–O bond. A detailed structural and theoretical study revealed B═O double-bond character and a very small contribution of a pπ–dπ interaction between Ru and O.

Description of the [formula omitted] resonance as the Dπ atom

We discuss the possibility that the recently reported resonance in the Dsπ0 spectrum can be described in terms of residual Dπ interactions.

Charge-Density Study of Methane Di(triimido)sulfonic Acid H2C{S(NtBu)2(NHtBu)}2—the NR Analogue of H2C{S(O)2(OH)}2 This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. The discussions in the Würzburger Graduiertenkolleg “Elektronendichte” were very stimulating. We thank one of the referees for valuable comments.

Short bonds explained at long last: The charge-density distribution in the SN bonds of H2C{S(NtBu)2(NHtBu)}2, the NR analogue of methane disulfonic acid H2C{S(O)2(OH)}2, was studied by high-resolution X-ray diffraction and subsequent multipole structure refinement (see picture). This experiment reveals that all three nitrogen atoms at each sulfur atom are sp3 hybridized. The formal SN double bonds are shortened by electrostatic reinforcement and not by pπ–dπ interaction.

Tristhiolatomolybdenum nitrides, (RS)(3)Mo[triple bond]N where R = (i)Pr and (t)Bu, preparation, characterization and comparisons with related trialkoxymolybdenumnitrides.

The addition of thiols to ((t)BuO)(3)Mo[triple bond]N in toluene leads to the formation of (RS)(3)Mo[triple bond]N compounds as yellow, air-sensitive compounds, where R = (i)Pr and (t)Bu. The single-crystal structure of ((t)BuS)(3)Mo[triple bond]N reveals a weakly associated dimeric structure where two ((t)BuS)(3)Mo[triple bond]N units (Mo-N = 1.61 A, Mo-S = 2.31 A (av)) are linked via thiolate sulfur bridges with long 3.03 A (av) Mo-S interactions. Density functional theory calculations employing Gaussian 98 B3LYP (LANL2DZ for Mo and 6-31G* for N, O, S, and H) have been carried out for model compounds (HE)(3)Mo[triple bond]N and (HE)(3)MoNO, where E = O and S. A comparison of the structure and bonding within the related series ((t)BuE)(3)Mo[triple bond]N and ((t)BuE)(3)MoNO is made for E = O and S. In the thiolate compounds, the highest energy orbitals are sulfur lone-pair combinations. In the alkoxides, the HOMO is the N 2p lone-pair which has M-N sigma and M-O pi* character for the nitride. As a result of greater O p pi to Mo pi interactions, the M-N pi orbitals of the Mo-N triple bond are destabilized with respect to their thiolate counterpart. For the nitrosyl compounds, the greater O p pi to Mo d pi interaction favors greater back-bonding to the nitrosyl pi* orbitals for the alkoxides relative to the thiolates. The results of the calculations are correlated with the observed structural features and spectroscopic properties of the related alkoxide and thiolate compounds.

Mechanistic and Theoretical Analysis of the Oxidative Addition of H2 to Six-Coordinate Molybdenum and Tungsten Complexes M(PMe3)4X2 (M = Mo, W; X = F, Cl, Br, I): An Inverse Equilibrium Isotope Effect and an Unprecedented Halide Dependence

Experimental observations, together with a theoretical analysis, indicate that the energetics of the oxidative addition of H2 to the six-coordinate molybdenum and tungsten complexes trans-M(PMe3)4X2 (M = Mo, W; X = F, Cl, Br, I) depend very strongly on the nature of both the metal and the halogen. Specifically, the exothermicity of the reaction increases in the sequences Mo < W and I < Br < Cl < F. Of most interest, this halogen dependence provides a striking contrast to that reported for oxidative addition of H2 to the Vaska system, trans-Ir(PPh3)2(CO)X. A theoretical analysis suggests that the halide dependence for trans-M(PMe3)4X2 is a result of both steric and electronic factors, the components of which serve to reinforce each other. Oxidative addition is thus favored sterically for the fluoride derivatives since the increased steric interactions upon forming the eight-coordinate complexes M(PMe3)4H2X2 would be minimized for the smallest halogen. The electronic component of the energetics is associated with the extent that π-donation from X raises the energy of the doubly occupied 3e*, π-antibonding, dxz and dyz pair of orbitals in trans-M(PMe3)4X2. Consequently, with F as the strongest π-donor, trans-M(PMe3)4X2 is destabilized with respect to M(PMe3)4H2X2 by pπ−dπ interaction to the greatest extent for the fluoride complex, so that oxidative addition becomes most favored for this derivative. Equilibrium studies of the oxidative addition of H2 to trans-W(PMe3)4I2 have allowed the average W−H bond dissociation energy (BDE) in W(PMe3)4H2I2 to be determined [D(W−H) = 62.0(6) kcal mol-1]. The corresponding average W−D BDE [D(W−D) = 63.8(7) kcal mol-1] is substantially greater than the W−H BDE, to the extent that the oxidative addition reaction is characterized by an inverse equilibrium deuterium isotope effect [KH/KD = 0.63(5) at 60 °C]. The inverse nature of the equilibrium isotope effect is associated with the large number (six) of isotope-sensitive vibrational modes in the product, compared to the single isotope-sensitive vibrational mode in reactant H2. A mechanistic study reveals that the latter reaction proceeds via initial dissociation of PMe3, followed by oxidative addition to five-coordinate [W(PMe3)3I2], rather than direct oxidative addition to trans-W(PMe3)4I2. Conversely, reductive elimination of H2 does not occur directly from W(PMe3)4H2I2 but rather by a sequence that involves dissociation of PMe3 and elimination from the seven-coordinate species [W(PMe3)3H2I2].

ChemInform Abstract: Reactions of Heteroarylhydrosilanes with Dichlorocarbene Under Phase‐Transfer Conditions.

Abstract The reactions of dimethyl(2-thienyl)-, methyl[di(2-thienyl)]-, tri(2-thienyl)- and dimethyl(2-furyl)silanes (I–IV, respectively) with dichlorocarbene generated from sodium trichloroacetate under solid-liquid phase-transfer conditions afford the corresponding dichloromethylsilanes in 38–66% yield. The reactivity of the thienylsilanes decreases with the number of electron-accepting thienyl substituents at the silicon atom. Using the competition reactions, the relative rate constants for the reaction of dichlorocarbene insertion into the SiH bond of thienyl- and furyl-silanes (I and IV) were measured. The reactivity of these silanes was found to be greater than had been expected taking as a basis the Taft σ* constants for the substituent at the silicon atom. This is apparently due to the existence of a pπ—dπ interaction between the heterocyclic π-system and the d-orbitals of silicon.

Paramagnetic effects in magnetic circular dichroism spectra of high-spin ferrous hemoproteins in the visible and near infrared spectral regions

Magnetic circular dichroism spectra of high-spin ferrous hemoproteins (deoxyhemoglobin, deoxymyoglobin, ferroperoxidase and reduced cytochrome c oxidase) are compared for spectral region 350–800 nm at temperatures 293 - 10 K. At the lowest temperatures the MCD spectra over the whole spectral region are shown to be composed predominantly of the temperature-dependent C terms. The clear evidence are given for paramagnetic origin of positive MCD bands at about 760 and 680 nm and negative MCD band at about 630 nm associated with CT or d→d transitions. The C terms associated with these bands as well as those associated with π−π transitions in the visible Q band region are sensitive to the peculiarities in heme environment induced by protein conformation. In contrast, the Soret region MCD spectra of these proteins are very similar. The origin of C terms for porphyrin π−π* transitions is discussed in the framework of π−d π interaction suggested earlier for the explanation of C terms in the visible and Soret MCD of low-spin ferric hemoproteins [Mineyev AP, Sharonov YA (1978) Theor Chim Acta 49: 295–307]. The results and discussion illustrate the capability of C terms in near infrared and visible MCD in probing the active center in high-spin ferrous hemoproteins.

Nuclear magnetic resonance studies on the anions [M(SnCl3)5Cl]4− (M = Ru, Os) and [Ru(SnCl3)5(MeCN)]3−: the X-ray crystal structure of [N(C2H5)4]4[Ru(SnCl3)5Cl

The octahedral anions [M(SnCl3)5Cl]4− (M = Ru, Os) have been fully characterized by 119Sn FT nmr spectroscopy. For M = Ru, 117Sn and 115Sn nmr spectra were also recorded, and an X-ray crystallographic study was carried out on the tetraethylammonium salt, isolated as a disolvate from acetonitrile. The Ru—Sn bond lengths indicate some degree of dπ–dπ interactions. The slight distortions from octahedral geometry are discussed in connection with the packing of the chlorine atoms. The Sn nmr spectra reveal the first observed coupling to a 99Ru nucleus (I = 5/2, 12.7% natural abundance), very large 2J(119Sn—117Sn) coupling constants, and the first observed second-order effects on a heteronuclear system. The octahedral anion [Ru(SnCl3)5(MeCN)]3− was also synthesized as the tetraethylammonium salt and characterized spectroscopically.

31P NMR chemical shifts of p-nitrophenyl esters of phosphorus acids

1. We measured the31P chemical shifts of some p-nitrophenyl esters of substituted phosphinic, phosphonic, and phosphoric acids. The contribution made by pπ−dπ interaction to the δ31P value was estimated for the p-NO2C6H4O and p-ClC6H4O groups. 2. The rate-for the alkaline hydrolysis of the p-nitrophenyl esters was studied.

Notizen: Synthesis and Crystal Structure of the Hydrogen Bonded Complex of Tri-paratoluidylphosphazenyl Oxide with Ethyl Alcohol

The title compound was synthesised and its structure determined by X-ray crystallography. The structure contains a hydrogen bond between the OH group of the alcohol and the phosphoryl oxygen atom. The P–N bond lengths are significantly different and the differences can be attributed to varying ρπ—dπ interactions along the bonds.

Heteroarylphosphorus compounds: Nickel(II) and cobalt(II) complexes of thienylphosphines

Complexes of a series of tertiary phosphines bearing 2-thienyl groups with nickel(II) and cobalt(II) halides and thiocyanates have been prepared and their magnetic and spectroscopic properties are presented. The nickel(II) halide and cobalt(II) halide and thiocyanate complexes have a distorted tetrahedral structure, whereas the nickel(II) thiocyanate complexes are square planar. Consideration of the spectroscopic data for the tetrahedral nickel(II) halide complexes of the thienylphosphines indicates that a 2-thienyl substituent is more electron-releasing towards phosphorus than is a phenyl group when the phosphorus is coordinated to the metal, suggesting that in this situation effects of pπ− dπ interactions between the heterocyclic ring system and phosphorus may be significant in influencing the donor properties of the phosphine. Calculation of the Racah parameters for the nickel(II) complexes indicates that the nephelauxetic effects of the thienylphosphines are different from those of triphenylphosphine and that there is a greater degree of covalency in the metal-ligand bond in the case of the thienylphosphines.

Bonding studies of compounds of Group 3–5 elements. Part XVIII. He(I) photoelectron spectra of bivalent homoleptic alkyls and amides, especially of Group 4 elements, and of tin(II) chloride and bromide

He(I) photoelectron (p.e.) spectraof (i) themonomeric, presumed-bent, compounds MR2, M(NR′2)2, and M(NR′R″)2[M = Ge, Sn, or Pb; R = CH(SiMe3)2, R′= SiMe3, and R″= CMe3], (ii) the linear HgR2 and M′(NR′2)2(M = Zn or Hg), and (iii) the bent momomeric SnCl2 and SnBr2 have been recorded, as well as the reference compounds RH, R′2NH, and R′R″NH. For the Group 4 metal(II) alkyls the first vertical ionisation potential (i.p.) is close to that of the free metal; the metal lone-pair orbital is progressively higher in M(NR′R″), and M(NR′2)2, in which compounds it is the second band, being preceded by the antibonding b2 molecular orbital (m.o.) formed by the nitrogen lone-pair atomic orbitals (a.o.s). Confirmation of this assignment is also provided by comparison of data for the bent and linear metal(II) amides; in the latter, nitrogen lone-pair interactions are negligible. Assignments, on the basis of trends and, for MX2, of CNDO calculations, are prooosed and trends noted. There is evidence of substantial N→Si pπ–dπ interaction in the amides.

Nuclear quadrupole resonance in organosilicon nitrogen compounds

Based on the14N NQR data, the pπ − dπ interaction in dimethylaminosilanes is weaker than in the amides of phosphorus acids.

Photoelectron spectra and the question of pπ → dπ interactions in silane and germanes

Photoelectron spectra and the question of pπ → dπ interactions in silane and germanes

14N and 35Cl NQR studies of organophosphorus compounds

Abstract14N and 35Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the NP bond are calculated according to the Townes and Dailey method. The experimental data are interpreted assuming a partial double bond character of the NP bond due to the pπdπ interaction and pπσ conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the NP bond populations is observed in X  PRn (R2N)3‐n molecules (where X is O, S, Se, or lone pair electrons and n = 0, 1, 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence PSe &gt; PS &gt; PO &gt; P.

Silicon–transition-metal compounds. Part I. Silyltetracarbonylcobalt and related compounds

Iodosilane and di-iodosilane react with NaCo(CO)4 in ether at low temperatures to give silyltetracarbonylcobalt, SiH3Co(CO)4, and silylenebis(tetracarbonylcobalt), SiH2[Co(CO)4]2, respectively. Physical properties of both compounds are reported, together with i.r., mass, and n.m.r. spectra of SiH3Co(CO)4. Pyrolysis of gaseous SiH3Co(CO)4 at 120° yields H2, CO, SiH4, and a dark-coloured inert solid; some HCo(CO)4 is also present in the early stages. The stoicheiometry and probable cross-linked polymeric nature of the solid product are inferred, and its metal coating properties are outlined. The Si–Co bond in SiH3Co(CO)4 is broken by HX (X = F, Cl, O/2) to give SiH3X and HCo(CO)4, also by HgX2(X = Cl, I) to give SiH3X and Hg[Co(CO)4]2. SiH2[Co(CO)4]2 behaves similarly. No insertion of CO into the Si–Co bond has been achieved; PPh3 reacts slowly with SiH3Co(CO)4 to displace CO and probably yields trans-SiH3Co(CO)3(PPh3). Evidence for dπ–dπ interaction in the silicon–metal bond is discussed.

Stability of Some Bivalent Metal Complexes of Benzoyl-thiobenzoylmethane in Aqueous Dioxane

Abstract Concentration equilibrium constants of benzoyl-thiobenzoylmethane (SdbmH) complexes of bivalent magnesium, cobalt, nickel, copper and zinc have been measured in 75% (v/v) aqueous dioxane containing 0.05 m sodium perchlorate. The values for the transition metal ions are greater than those of dibenzoylmethane (dbmH) complexes measured under the same conditions, although the pK value of SdbmH is smaller than that of dbmH. Contribution of dπ–dπ interaction seems to be responsible for such an unusual relationship.

A kinetic effect of dπ- dπ interactions in polysilanes

A kinetic effect of dπ- dπ interactions in polysilanes

Physical properties and chemical constitution. Part XLVIII. The electric dipole moments of R3XOMe, (R3X)2O, R3XSMe, and (R3X)2S where X is C, Si, Ge, or Sn

The electric dipole moments of compounds with bonds between O or S atoms and C, Si, Ge, or Sn atoms have been measured in benzene solution at 25·00°. The Me3XO– and Me3XS– group moments are deduced, and evidence obtained for pπ–dπ interaction in the Si–O bond.