The pressure tunning Raman and IR spectral studies on the multinuclear metal carbyne complexes

The Raman and infrared (IR) spectra of four tungsten metal carbyne complexes I, II, IV and V [Cl(CO)2(L)W[triple bond]CC6H4[triple bond](C[triple bond]CC6H4)n[triple bond]N[triple bond]C[triple bond]]2M (L = TMEDA, n = 0, M = PdI2 or ReCl(CO)3; L = DPPE, n = 1, M = PdI2 or ReCl(CO)3) were studied at high external pressure. Their pressure-induced phase transitions were observed near 20kbar (complexes I), 15 kbar (complexes II), 25 kbar (complex IV) and 30 kbar (complex V). The pressure-induced phase transition likely is first order in complex I and the pressure-induced phase transitions of complexes II, IV and V are mostly second order. The pressure sensitivities d nu/dp of nu(W[triple bond]C) are high in the low-pressure phase area and very low in the high-pressure phase area due to the pressure strengthening pi back-bonding from metal W to pi* orbital of C[triple bond]O in fragment Cl(CO)2(L)W[triple bond]C. The pressure strengthening metal pi back-bonding from metal Re or Pd to pi* orbital of C[triple bond]O or C[triple bond]N also happened to both of central metal centers of NCPd(I2)CN in complex I and NCReCl(CO)3CN in complex II.     

Transition metal-carbon complexes. A theoretical study

The equilibrium geometries and bond dissociation energies of 16VE and 18VE complexes of ruthenium and iron with a naked carbon ligand are reported using density functional theory at the BP86/TZ2P level. Bond energies were also calculated at CCSD(T) using TZ2P quality basis sets. The calculations of [Cl2(PMe3)2Ru(C)] (1Ru), [Cl2(PMe3)2Fe(C)] (1Fe), [(CO)2(PMe3)2Ru(C)] (2Ru), [(CO)2(PMe3)2Fe(C)] (2Fe), [(CO)4Ru(C)] (3Ru), and [(CO)4Fe(C)] (3Fe) show that 1Ru has a very strong Ru-C bond which is stronger than the Fe-C bond in 1Fe. The metal-carbon bonds in the 18VE complexes 2Ru-3Fe are weaker than those in the 16VE species. Calculations of the related carbonyl complexes [(PMe3)2Cl2Ru(CO)] (4Ru), [(PMe3)2Cl2Fe(CO)] (4Fe), [(PMe3)2Ru(CO)3] (5Ru), [(PMe3)2Fe(CO)3] (5Fe), [Ru(CO)5] (6Ru), and [Fe(CO)5] (6Fe) show that the metal-CO bonds are much weaker than the metal-C bonds. The 18VE iron complexes have a larger BDE than the 18VE ruthenium complexes, while the opposite trend is calculated for the 16VE compounds. Charge and energy decomposition analyses (EDA) have been carried out for the calculated compounds. The Ru-C and Fe-C bonds in 1Ru and 1Fe are best described in terms of two electron-sharing bonds with sigma and pi symmetry and one donor-acceptor pi bond. The bonding situation in the 18 VE complexes 2Ru-3Fe is better described in terms of closed shell donor-acceptor interactions in accordance with the Dewar-Chatt-Duncanson model. The bonding analysis clearly shows that the 16VE carbon complexes 1Ru and 1Fe are much more strongly stabilized by metal-C sigma interactions than the 18VE complexes which is probably the reason why the substituted homologue of 1Ru could become isolated. The EDA calculations show that the nature of the TM-C and TM-CO binding interactions resembles each other. The absolute values for the energy terms which contribute to Delta(Eint) are much larger for the carbon complexes than for the carbonyl complexes, but the relative strengths of the energy terms are not very different from each other. The pi bonding contribution to the orbital interactions in the carbon complexes is always stronger than sigma bonding. There is no particular bonding component which is responsible for the reversal of the relative bond dissociation energies of the Ru and Fe complexes when one goes from the 16VE complexes to the 18VE species. That the 18 VE compounds have longer and weaker TM-C and TM-CO bonds than the respective 16 VE compounds holds for all complexes. This is because the LUMO in the 16 VE species is a sigma-antibonding orbital which becomes occupied in the 18 VE species.     

P2 addition to terminal phosphide M[triple bond]P triple bonds: a rational synthesis of cyclo-P3 complexes

The diphosphaazide complex (Mes*NPP)Nb(N[Np]Ar)3 (Mes* = 2,4,6-tri-tert-butylphenyl, Np = neopentyl, Ar = 3,5-Me2C6H3), 1, has previously been reported to lose the P2 unit upon gentle heating, to form (Mes*N)Nb(N[Np]Ar)3, 2. The first-order activation parameters for this process have been estimated here using an Eyring analysis to have the values Delta H(double dagger) = 19.6(2) kcal/mol and Delta S(double dagger) = -14.2(5) eu. The eliminated P2 unit can be transferred to the terminal phosphide complexes P[triple bond]M(N[(i)Pr]Ar)3, 3-M (M = Mo, W), and [P[triple bond]Nb(N[Np]Ar)3](-), 3-Nb, to give the cyclo-P3 complexes (P3)M(N[(i)Pr]Ar)3 and [(P3)Nb(N[Np]Ar)3](-). These reactions represent the formal addition of a P[triple bond]P triple bond across a M[triple bond]P triple bond and are the first efficient transfers of the P2 unit to substrates present in stoichiometric quantities. The related complex (OC)5W(Mes*NPP)Nb(N[Np]Ar)3, 1-W(CO)5, was used to transfer the (P2)W(CO)5 unit in an analogous manner to the substrates 3-M (M = Mo, W, Nb) as well as to [(OC)5WP[triple bond]Nb(N[Np]Ar)3](-). The rate constants for the fragmentation of 1 and 1-W(CO)5 were unchanged in the presence of the terminal phosphide 3-Mo, supporting the hypothesis that molecular P2 and (P2)W(CO)5, respectively, are reactive intermediates. In a reaction related to the combination of P[triple bond]P and M[triple bond]P triple bonds, the phosphaalkyne AdC[triple bond]P (Ad = 1-adamantyl) was observed to react with 3-Mo to generate the cyclo-CP2 complex (AdCP2)Mo(N[(i)Pr]Ar)3. Reactions of the electrophiles Ph3SnCl, Mes*NPCl, and AdC(O)Cl with the anionic, nucleophilic complexes [(OC)5W(P3)Nb(N[Np]Ar)3](-) and [{(OC)5W}2(P3)Nb(N[Np]Ar)3](-) yielded coordinated eta(2)-triphosphirene ligands. The Mes*NPW(CO)5 group of one such product engages in a fluxional ring-migration process, according to NMR spectroscopic data. The structures of (OC)5W(P3)W(N[(i)Pr]Ar)3, [(Et2O)Na][{(OC)5W}2(P3)Nb(N[Np]Ar)3], (AdCP2)Mo(N[(i)Pr]Ar)3, (OC)5W(Ph3SnP3)Nb(N[Np]Ar)3, Mes*NP(W(CO)5)P3Nb(N[Np]Ar)3, and {(OC)5W}2AdC(O)P3Nb(N[Np]Ar)3, as determined by X-ray crystallography, are discussed in detail.     

Spectroscopic studies and structures of trans-ruthenium(II) and ruthenium(III) bis(cyanide) complexes supported by a tetradentate macrocyclic tertiary amine ligand

trans-[Ru(16-TMC)(C[triple bond]N)2] (1; 16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane) was prepared by the reaction of trans-[Ru(16-TMC)Cl2]Cl with KCN in the presence of zinc powder. The oxidation of 1 with bromine gave trans-[Ru(16-TMC)(CN)2]+ isolated as PF6 salt (2.PF6). The Ru-C/C-N distances are 2.061(4)/1.130(5) and 2.069(5)/1.140(7) A for 1 and 2, respectively. Both complexes show a Ru(III/II) couple at 0.10 V versus FeCp2+/0. The UV-vis absorption spectrum of 1 is dominated by an intense high-energy absorption at lambda(max) = 230 nm, which is mainly originated from dpi(RuII) --> pi*(N[triple bond]C-Ru-C[triple bond]N) charge-transfer transition. Complex 2 shows intense absorption bands at lambda(max) pi*(N[triple bond]C-Ru-C[triple bond]N) and sigma(-CN) --> d(RuIII) charge-transfer transition, respectively. Density functional theory and time-dependent density-functional theory calculations have been performed on trans-[(NH3)4Ru(C[triple bond]N)2] (1') and trans-[(NH3)4Ru(C[triple bond]N)2]+ (2') to examine the Ru-cyanide interaction and the nature of associated electronic transition(s). The 230 nm band of 1 has been probed by resonance Raman spectroscopy. Simulations of the absorption band and the resonance Raman intensities show that the nominal nuC[triple bond]N stretch mode accounts for ca. 66% of the total vibrational reorganization energy. A change of nominal bond order for the cyanide ligand from 3 to 2.5 is estimated upon the electronic excitation.     

Accounting for the differences in the structures and relative energies of the highly homoatomic np pi-np pi (n > or = 3)-bonded S2I4 2+, the Se-I pi-bonded Se2I4 2+, and their higher-energy isomers by AIM, MO, NBO, and VB methodologies

The bonding in the highly homoatomic np pi-np pi (n > or = 3)-bonded S2I42+ (three sigma + two pi bonds), the Se-I pi-bonded Se2I42+ (four sigma + one pi bonds), and their higher-energy isomers have been studied using modern DFT and ab initio calculations and theoretical analysis methods: atoms in molecules (AIM), molecular orbital (MO), natural bond orbital (NBO), and valence bond (VB) analyses, giving their relative energies, theoretical bond orders, and atomic charges. The aim of this work was to seek theory-based answers to four main questions: (1) Are the previously proposed simple pi*-pi* bonding models valid for S2I42+ and Se2I42+? (2) What accounts for the difference in the structures of S2I42+ and Se2I42+? (3) Why are the classically bonded isolobal P2I4 and As2I4 structures not adopted? (4) Is the high experimentally observed S-S bond order supported by theoretical bond orders, and how does it relate to high bond orders between other heavier main group elements? The AIM analysis confirmed the high bond orders and established that the weak bonds observed in S2I42+ and Se2I42+ are real and the bonding in these cations is covalent in nature. The full MO analysis confirmed that S2I42+ contains three sigma and two pi bonds, that the positive charge is essentially equally distributed over all atoms, that the bonding between S2 and two I2+ units in S2I42+ is best described by two mutually perpendicular 4c2e pi*-pi* bonds, and that in Se2I42+, two SeI2+ moieties are joined by a 6c2e pi*-pi* bond, both in agreement with previously suggested models. The VB treatment provided a complementary approach to MO analysis and provided insight how the formation of the weak bonds affects the other bonds. The NBO analysis and the calculated AIM charges showed that the minimization of the electrostatic repulsion between EI2+ units (E = S, Se) and the delocalization of the positive charge are the main factors that explain why the nonclassical structures are favored for S2I42+ and Se2I42+. The difference in the structures of S2I42+ and Se2I42+ is related to the high strength of the S-S pi bond compared to the weak S-I sigma bond and the additional stabilization from increased delocalization of positive charge in the structure of S2I42+ compared to the structure of Se2I42+. The investigation of the E2X42+ series (E = S, Se, Te; X = Cl, Br, I) revealed that only S2I42+ adopts the highly np pi-np pi (n > or = 3)-bonded structure, while all other dications favor the pi-bonded Se2I42+ structure. Theoretical bond order calculations for S2I42+ confirm the previously presented experimentally based bond orders for S-S (2.1-2.3) and I-I (1.3-1.5) bonds. The S-S bond is determined to have the highest reported S-S bond order in an isolated compound and has a bond order that is either similar to or slightly less than the Si-Si bond order in the proposed triply bonded [(Me3Si)2CH]2(iPr)SiSi triple bond SiSi(iPr)[CH(SiMe3)2]2 depending on the definition of bond orders used.     

Probing ruthenium-acetylide bonding interactions: synthesis, electrochemistry, and spectroscopic studies of acetylide-ruthenium complexes supported by tetradentate macrocyclic amine and diphosphine ligands

The synthesis and spectroscopic properties of trans-[RuL4(C[triple bond]CAr)2] (L4 = two 1,2-bis(dimethylphosphino)ethane, (dmpe)2; 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane, 16-TMC; 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane, N2O2) are described. Investigations into the effects of varying the [RuL4] core, acetylide ligands, and acetylide chain length for the [(-)C[triple bond]C(C6H4C[triple bond]C)(n-1)Ph] and [(-)C[triple bond]C(C6H4)(n-1)Ph] (n = 1-3) series upon the electronic and electrochemical characteristics of trans-[RuL4(C[triple bond]CAr)2](0/+) are presented. DFT and TD-DFT calculations have been performed on trans-[Ru(L')4(C[triple bond]CAr)2](0/+) (L' = PH3 and NH3) to examine the metal-acetylide pi-interaction and the nature of the associated electronic transition(s). It was observed that (1) the relationship between the transition energy and 1/n for trans-[Ru(dmpe)2{C[triple bond]C(C6H4C[triple bond]C)(n-1)Ph}2] (n = 1-3) is linear, and (2) the sum of the d(pi)(Ru(II)) --> pi*(C[triple bond]CAr) MLCT energy for trans-[Ru(16-TMC or N2O2)(C[triple bond]CAr)2] and the pi(C[triple bond]CAr) --> d(pi)(Ru(III)) LMCT energy for trans-[Ru(16-TMC or N2O2)(C[triple bond]CAr)2]+ corresponds to the intraligand pi pi* absorption energy for trans-[Ru(16-TMC or N2O2)(C[triple bond]CAr)2]. The crystal structure of trans-[Ru(dmpe)2{C[triple bond]C(C6H4C[triple bond]C)2Ph}2] shows that the two edges of the molecule are separated by 41.7 A. The electrochemical and spectroscopic properties of these complexes can be systematically tuned by modifying L4 and Ar to give E(1/2) values for oxidation of trans-[RuL4(C[triple bond]CAr)2] that span over 870 mV and lambda(max) values of trans-[RuL4(C[triple bond]CAr)2] that range from 19,230 to 31,750 cm(-1). The overall experimental findings suggest that the pi-back-bonding interaction in trans-[RuL4(C[triple bond]CAr)2] is weak and the [RuL4] moiety in these molecules may be considered to be playing a "dopant" role in a linear rigid pi-conjugated rod.     

The structure and chemical bonding in the N(2)-CuX and N(2)...XCu (X = F, Cl, Br) systems studied by means of the molecular orbital and Quantum Chemical Topology methods

Ab initio studies carried out at the MP2(full)/6-311+G(2df) and MP2(full)/aug-cc-pVTZ-PP computational levels reveals that dinitrogen (N(2)) and cuprous halides (CuX, X = F, Cl, Br) form three types of systems with the side-on and end-on coordination of N(2): N[triple bond]N-CuX (C(infinity v)), N(2)-CuX (C(2v)) stabilized by the donor-acceptor bonds and weak van der Waals complexes N(2)...XCu (C(2v)) with dominant dispersive forces. An electron density transfer between the N(2) and CuX depends on type of the N(2) coordination and a comparison of the NPA charges yields the [N[triple bond]N](delta+)-[CuX](delta-) and [N(2)](delta-)-[CuX](delta+) formula. According to the NBO analysis, the Cu-N coordinate bonds are governed by predominant LP(N2)-->sigma*(Cu-X) "2e-delocalization" in the most stable N[triple bond]N-CuX systems, meanwhile back donation LP(Cu)-->pi*(N-N) prevails in less stable N(2)-CuX molecules. A topological analysis of the electron density (AIM) presents single BCP between the Cu and N nuclei in the N[triple bond]N-CuX, two BCPs corresponding to two donor-acceptor Cu-N bonds in the N(2)-CuX and single BCP between electron density maximum of the N[triple bond]N bond and halogen nucleus in the van der Waals complexes N(2)...XCu. In all systems values of the Laplacian nabla(2)rho(r)(r(BCP)) are positive and they decrease following a trend of the complex stability i.e. N[triple bond]N-CuX (C(infinity v)) > N(2)-CuX (C(2v)) > N(2)...XCu (C(2v)). A topological analysis of the electron localization function (ELF) reveals strongly ionic bond in isolated CuF and a contribution of covalent character in the Cu-Cl and Cu-Br bonds. The donor-acceptor bonds Cu-N are characterized by bonding disynaptic basins V(Cu,N) with attractors localized at positions corresponding to slightly distorted lone pairs V(N) in isolated N(2). In the N[triple bond]N-CuX systems, there were no creation of any new bonding attractors in regions where classically the donor-acceptor bonds are expected and there is no sign of typical covalent bond Cu-N with the bonding pair. Calculations carried out for the N[triple bond]N-CuX reveal small polarization of the electron density in the N[triple bond]N bond, which is reflected by the bond polarity index being in range of 0.14 (F) to 0.11 (Cl).     

Triimidosulfonic acid and organometallic triimidosulfonates: S(+)-N(-) versus S=N bonding

Sulfonic acids RSO(2)OH and their metal salts MO(3)SR are versatile catalysts in large-scale industrial cyclization and polymerization processes. Isoelectronic replacement of the oxygen atoms by NR imido groups gives triimidosulfonic acid and triimidosulfonates. The salts form nonaggregated soluble molecules rather than infinite solid-state lattices such as their oxo analogues. In this paper, we present the synthesis and structure of the basic starting material MeS(N(t)Bu)(3)H (1), the metal complexes [Me(2)Al(N(t)Bu)(3)SMe] (2) and [Zn[(N(t)Bu)(3)SMe](2)] (3), and the mixed metal adduct [(thf)Li[(N(t)Bu)(3)SMe].ZnMe(2)] (4). The chelating coordination, rather than the tripodal coordination, cannot be attributed to steric effects of the S-bonded methyl group, as the less demanding Ph-C triple bond C-alkynyl substituent at sulfur in [(thf)(2)Li[(N(t)Bu)(3)SCCPh]] (5) causes the same conformation. S-N bond shortening to the pendant imido group has to be attributed to closed-shell electrostatic attraction rather than to S-N double bonding by valence expansion at the central sulfur atom. Coordination to an additional N-->Zn dative bond in 4 widens the bond length to values normally interpreted as S-N single bonds. We take this fact as experimental evidence that S-N bonding is predominantly governed by electrostatic interaction rather than by valence expansion employing d-orbitals. This was predicted by theoreticians more than a decade ago.     

Completion of the series of M2(hpp)4Cl2 compounds from W to Pt: the W, Os, and Pt compounds

The series of M2(hpp)4Cl2 complexes (hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) from M = W to M = Pt has been completed by the preparation and characterization of those with M = W, Os, and Pt. W(hpp)4Cl2 (1) has a W-W distance of 2.250(2) A, is diamagnetic, and can be assigned a W-W triple bond based on a sigma 2 pi 4 electron configuration. Os2(hpp)4Cl2 (2) has an Os-Os distance of 2.379(2) A and displays a temperature-independent paramagnetism. It can be assigned a sigma 2 pi 4 delta 2 delta*2 configuration. Pt2(hpp)4Cl2 has a Pt-Pt distance of 2.440(1) A and is diamagnetic. A bond order of 1, based on a configuration in which only the sigma* orbital is empty, is consistent with these data.     

New direction in organopalladium chemistry: structure and reactivity of unsaturated hydrocarbon ligands bound to multipalladium units

Examination of the manner of interaction between Pd(0) and allylpalladium(II) complexes, both being involved as key intermediates in Pd-catalyzed allylic coupling, led us to discover a new role for such combinations in affecting the stereochemistry of the transformations. A similar investigation of the system involving Pd(0) and allenyl/propargyl complexes of Pd(II) led to the discovery of dinuclear Pd(I)bond;Pd(I) complexes containing bridging allenyl/propargyl ligands, which exhibited novel structural and reactivity aspects of great synthetic significance. A systematic comparison was made between the structure, stability, and reactivity of allyl and allenyl/propargyl ligands in dinuclear complexes and those in mononuclear counterparts. On the basis of MO calculations, coordination behavior specific to the ligands of the dinuclear complex is attributed to the occurrence of the back-donating interaction from filled Pdbond;Pd bonding orbitals to vacant ligand pi* orbitals. Similar bonding features are the origin of the ready synthesis of novel one-dimensional sandwich complexes composed of conjugated polyene ligands and linear polypalladium chains. A substitutionally labile dipalladium complex reacts with an equimolar amount of trienes or alkynes to give formal [4pi + 2sigma] or [2pi + 2sigma] adducts, respectively, which undergo further unique transformations with additional unsaturated substrates.     

Carbonyl analogues? Analysis of Fe-E (E=B, Al, Ga) bonding in cationic terminal diyl complexes by density functional theory

A series of DFT calculations has been carried out with the aim of characterizing the metal-group 13 element interaction in the novel cationic borylene complex [(eta5-C2Me5)Fe(CO)2(BMes)]+ (1) and related species of the type [eta5-C5R5)M(L)2(EX)]n+. In addition, comparisons have been made with charge neutral borylene complexes and with related group 14 based ligand systems (e.g. cationic metal carbonyls, carbenes and vinylidenes) for which models of bonding have previously been established. In this regard particular attention has been focused on the interpretation of (i) molecular orbital composition; (ii) bond dissociation energies (BDEs) and the ratio of ionic to covalent contributions (DeltaEelstat/DeltaEorb); and (iii) sigma and pi symmetry covalent contributions. The molecular orbital compositions for the prototype borylene complex 1 and for related cationic and neutral systems [e.g.[(eta5-C5H5)Fe(PMe3)2(BMes)]+ and (eta5-C5H5)Mn(CO2(BMes)]] are consistent with the presence of bonding interactions between metal and borylene fragments of both sigma and pi symmetry. Furthermore, on the basis of BDEs, DeltaEorb values and sigma/pi covalent ratios, the bonding in cationic terminal borylene complexes such as 1 appears to have as much right to be termed a M=E double bond as does that in archetypal Fischer carbene and related complexes such as [(eta5-C5R5)Fe(CO)2(CCMe2)]+ and [(eta5-C5R5)Fe(CO)2(CH2)]+.     

Aminoborylene complexes of group 6 elements and iron: a synthetic, structural, and quantum chemical study

Transition-metal-borylene complexes of the type [(OC)(5)M=BR] {M=Cr, Mo, W; R=N(SiMe(3))(2), 1a-3a, Si(SiMe(3))(3), 4a} and [(OC)(4)Fe=B=N(SiMe(3))(2)] (8) were prepared by salt elimination reactions. Synthesis of the latter complex was accompanied by the formation of substantial amounts of an unusual dinuclear iron complex [Fe(2){mu-C(2)O(2)(BN(SiMe(3))(2))}(2)(CO)(6)] (9). The aminoborylene complexes of Group 6 metals were converted to trans-[(Cy(3)P)(CO)(4)M=B=N(SiMe(3))(2)] (5a-7a) by irradiation in the presence of PCy(3). Structural and spectroscopic parameters were discussed with respect to the trans-effect of the borylene ligand and the degree of M-B d(pi)-p(pi)-backbonding. Computational studies were performed on Group 6-borylene complexes. The population and topological analyses as well as the molecular orbital composition are consistent with the presence of both sigma-and pi-type interactions. There are, however, indications that the d(pi)-p(pi)-backbonding in the silylborylene complex is significantly more pronounced than in the aminoborylene complexes.     

The behavior of transition metal nitrido bonds towards protonation rationalized by means of localized bonding schemes and their weights

A new computational scheme is applied to rationalize the different protonation behaviors of the nitrido complexes [L'Mn(V)(N)(acac)](+), [LCr(V)(N)(acac)](+), and [LV(V)(N)(acac)](+). L and L' represent the macrocycles 1,4,7-triazacyclononane and its N-methylated derivative, respectively, and acac is the bidentate monoanion pentane-2,4-dionate. The bonds of the complexes are partitioned into bonds to be investigated and bonds of lesser interest. The investigated bonds are the transition metal nitrido bonds M(V)[triple chemical bond]N| (M = Mn, Cr, and V) and the bonds of lesser interest are located in the ligands. The ligand bonds are described by means of the strongly occupied natural bond orbitals. The electrons in the M(V)[triple chemical bond]N| nitrido bonds, however, are treated more accurately. A full configuration interaction procedure is applied in the space spanned by the strongly occupied natural bond orbitals and their corresponding antibonding orbitals. Localized bonding schemes and their weights are obtained for the d(pi)-p(pi) bonds of interest. This is achieved by representing the two-center natural bond orbitals for a d(pi)-p(pi) bond by the one-center natural hybrid orbitals localized at the bond atoms. The obtained bonding schemes are close to orthogonal valence bond structures. Their weights indicate that the nitrido nitrogen in [LV(V)(N)(acac)](+) is more easily protonated than the nitrido nitrogens in [L'Mn(V)(N)(acac)](+) and [LCr(V)(N)(acac)](+). This result is in good accord with experiment.     

Density functional study of the sigma and pi bond activation at the Pd=X (X = Sn, Si, C) bonds of the (H2PC2H4PH2)Pd=XH2 complexes. Is the bond cleavage homolytic or heterolytic?

The mechanism for the activation of the sigma bonds, the O-H of H2O, C-H of CH4, and the H-H of H2, and the pi bonds, the C[triple bond]C of C2H2, C=C of C2H4, and the C=O of HCHO, at the Pd=X (X = Sn, Si, C) bonds of the model complexes (H2PC2H4PH2)Pd=XH2 5 has been theoretically investigated using a density functional method (B3LYP). The reaction is significantly affected by the electronic nature of the Pd=X bond, and the mechanism is changed depending on the atom X. The activation of the O-H bond with the lone pair electron is heterolytic at the Pd=X (X = Sn, Si) bonds, while it is homolytic at the Pd=C bond. The C-H and H-H bonds without the lone pair electron are also heterolytically activated at the Pd=X bonds independent of the atom X, where the hydrogen is extracted as a proton by the Pd atom in the case of X = Sn, Si and by the C atom in the case of X=C because the nucleophile is switched between the Pd and X atoms depending on the atom X. In contrast, the pi bond activation of C[triple bond]C and C=C at the Pd=Sn bond proceeds homolytically, and is accompanied by the rotation of the (H2PC2H4PH2)Pd group around the Pd-Sn axis to successfully complete the reaction by both the electron donation from the pi orbital to Sn p orbital and the back-donation from the Pd dpi orbital to the pi orbital. On the other hand, the activation of the C=O pi bond with the lone pair electron at the Pd=Sn bond has two reaction pathways: one is homolytic with the rotation of the (H2PC2H4PH2)Pd group and the other is heterolytic without the rotation. The role of the ligands controlling the activation mechanism, which is heterolytic or homolytic, is discussed.     

Lewis adducts of the side-on end-on dinitrogen-bridged complex [{(NPN)Ta}2(mu-H)2(mu-eta 1:eta 2-N2)] with AlMe3, GaMe3, and B(C6F5)3: synthesis, structure, and spectroscopic properties

Reaction of the side-on end-on dinitrogen complex [{(NPN)Ta}(2)(mu-H)(2)(mu-eta(1):eta(2)-N(2))] (1; in which NPN=(PhNSiMe(2)CH(2))(2)PPh), with the Lewis acids XR(3) results in the adducts [{(NPN)Ta}(2)(mu-H)(2)(mu-eta(1):eta(2)-NNXR(3))], XR(3)=GaMe(3) (2), AlMe(3) (3), and B(C(6)F(5))(3) (4). The solid-state molecular structures of 2, 3, and 4 demonstrate that the N-N bond length increases relative to those found in 1 by 0.036, 0.043, and 0.073 A, respectively. In solution complexes 2-4 are fluxional as evidenced by variable-temperature (1)H NMR spectroscopy. The (15)N{(1)H} NMR spectra of 2-4 are reported; furthermore, their vibrational properties and electronic structures are evaluated. The vibrational structures are found to be closely related to that of the parent complex 1. Detailed spectroscopic analysis on 2-4 leads to the identification of the theoretically expected six normal modes of the Ta(2)N(2) core. On the basis of experimental frequencies and the QCB-NCA procedure, the force constants are determined. Importantly, the N-N force constant decreases from 2.430 mdyn A(-1) in 1 to 1.876 (2), 1.729 (3), and 1.515 mdyn A(-1) (4), in line with the sequence of N-N bond lengths determined crystallographically. DFT calculations on a generic model of the Lewis acid adducts 2-4 reveal that the major donor interaction between the terminal nitrogen atom and the Lewis acid is mediated by a sigma/pi hybrid molecular orbital of N(2), corresponding to a sigma bond. Charge analysis performed for the adducts indicates that the negative charge on the terminal nitrogen atom of the dinitrogen ligand increases with respect to 1. The lengthening of the N-N bond observed for the Lewis adducts is therefore explained by the fact that charge donation from the complex fragment into the pi* orbitals of dinitrogen is increased, while electron density from the N-N bonding orbitals p(sigma) and pi(h) is withdrawn due to the sigma interaction with the Lewis acid.     

Metathesis of nitrogen atoms within triple bonds involving carbon, tungsten, and molybdenum

(ButO)3Mo triple bond N and W2(OBut)6(M triple bond M) react in hydrocarbons to form Mo2(OBut)6(M triple bond M) and (ButO)3W triple bond N via the reactive intermediate MoW(OBut)6(M triple bond M). (ButO)3W triple bond N and CH3C triple bond N15 react in tetrahydrofuran (THF) at room temperature to give an equilibrium mixture involving (ButO)3W triple bond N15 and CH3C triple bond N. The (ButO)3W triple bond N compound is similarly shown to act as a catalyst for N15-atom scrambling between MeC13 triple bond N15 and PhC triple bond N to give a mixture of MeC13 triple bond N and PhC triple bond N15. From studies of degenerate scrambling of N atoms involving (ButO)3W triple bond N and MeC13 triple bond N in THF-d8 by 13C(1H) NMR spectroscopy, the reaction was found to be first order in acetonitrile and the activation parameters were estimated to be DeltaH = 13.4(7) kcal/mol and DeltaS = -32(2) eu. A similar reaction is observed for (ButO)3Mo triple bond N and CH3C triple bond N15 upon heating in THF-d8. The reaction is suppressed in pyridine solutions and not observed for the dimeric [(ButMe2SiO)3W triple bond N]2. The reaction pathway has been investigated by calculations employing density functional theory on the model compounds (MeO)3M triple bond N and CH3C triple bond N where M = Mo and W. The transition state was found to involve a product of the 2 + 2 cycloaddition of M triple bond N and C triple bond N, a planar metalladiazacyclobutadiene. This resembles the pathway calculated for alkyne metathesis involving (MeO)3W triple bond CMe, which modeled the metathesis of (ButO)3W triple bond CBut. The calculations also predict that the energy of the transition state is notably higher for M = Mo relative to M = W.     

On the paucity of molecular actinide complexes with unsupported metal-metal bonds: a comparative investigation of the electronic structure and metal-metal bonding in U2X6 (X = Cl, F, OH, NH2, CH3) complexes and d-block analogues

Density functional calculations have been performed on M2X6 complexes (where M = U, W, and Mo and X = Cl, F, OH, NH2, and CH3) to investigate general aspects of their electronic structures and explore the similarities and differences in metal-metal bonding between f-block and d-block elements. A detailed analysis of the metal-metal interactions has been conducted using molecular orbital theory and energy decomposition methods. Multiple (sigma and pi) bonding is predicted for all species investigated, with predominant f-f and d-d metal orbital character, respectively, for U and W or Mo complexes. The energy decomposition analysis involves contributions from orbital interactions (mixing of occupied and unoccupied orbitals), electrostatic effects (Coulombic attraction and repulsion), and Pauli repulsion (associated with four-electron two-orbital interactions). The general results suggest that the overall metal-metal interaction is stronger in the Mo and W species, relative to the U analogues, as a consequence of a significantly less destabilizing contribution from the combined Pauli and electrostatic ("pre-relaxation") effects. Although the orbital-mixing ("post-relaxation") contribution to the total bonding energy is predicted to have a larger magnitude in the U complexes, this is not sufficiently strong to compensate for the comparatively greater destabilization that originates from the Pauli-plus-electrostatic effects. Of the pre-relaxation terms, the Pauli repulsion is comparable in analogous U and d-block compounds, contrary to the electrostatic term, which is (much) less favorable in the U systems than in the W and Mo systems. This generally weak electrostatic stabilization accounts for the large pre-relaxation destabilization in the U complexes and, ultimately, for the relative weakness of the U-U bonds. The origin of the small electrostatic term in the U compounds is traced primarily to MX(3) fragment overlap effects.     

Nucleophilic reactivity and oxo/sulfido substitution reactions of MVIO3 Groups (M = Mo, W)

The nucleophilic reactivity of oxo ligands in the groups M(VI)O(3) in the trigonal complexes [(Me(3)tacn)MO(3)] (M = Mo (1), W (10)) and [(Bu(t)(3)tach)MO(3)] (M = Mo (5), W (14)) has been investigated. Complexes 1/10 can be alkylated with MeOTf to give [(Me(3)tacn)MO(2)(OMe)](1+) (2/11), silylated with Pr(i)(3)SiOTf to form [(Me(3)tacn)MO(2)(OSiPr(i)(3))](+) (3/12), and protonated with HOTf to yield [(Me(3)tacn)MoO(2)(OH)](+) (4). Similarly, complexes 5/14 can be silylated to [(Bu(t)(3)tach)MO(2)(OSiPr(i)(3))](+) (6/15) and protonated to [(Bu(t)(3)tach)MO(2)(OH)](+) (7/16). Products were isolated as triflate salts in yields exceeding 70%. When excess acid was used, the dinuclear mu-oxo species [(Bu(t)(3)tach)(2)M(2)O(5)](2+) (8/17) were obtained. X-ray structures are reported for 2-4, 6-8, 12, and 15-17. All mononuclear complexes have dominant trigonal symmetry with a rhombic distortion owing to a M[bond]OR bond (R = Me, SiPr(i)(3), H), which is longer than M[double bond]O oxo interactions; the latter exert a substantial trans influence on M[bond]N bond lengths. Oxo ligands in 5/14 undergo replacement with sulfide. Lawesson's reagent effects formation of [(Bu(t)(3)tach)MS(3)] (9/18), 14 with excess B(2)S(3) yields incompletely substituted [(Bu(t)(3)tach)WOS(2)] (20), and 5 with excess B(2)S(3) yields [(Bu(t)(3)tach)Mo(IV)O(S(4))] (19). The structures of 9, 19, and 20 are reported. Precedents for M(VI)S(3) groups in five- and six-coordinate molecules are limited. This investigation is the first detailed study of the behavior of M(VI)O(3) groups in nucleophilic and oxo/sulfido substitution reactions and should be useful in synthetic approaches to the active sites of the xanthine oxidase enzyme family and of certain tungstoenzymes. (Bu(t)(3)tach = 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane, Me(3)tacn = 1,4,7-trimethyl-1,4,7-triazacyclonane; OTf = triflate).     

A combined theoretical and experimental study of simple terminal group 6 nitride and phosphide N[triple bond]MX3 and P[triple bond]MX3 molecules

Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF 3, PF 3, and PCl 3 to form the simple lowest energy N[triple bond]MF 3, and P[triple bond]MX 3 products following insertion and halogen transfer, with the exception of P[triple bond]CrF3, which is a higher energy species and is not observed. The E[triple bond]MX3 pnictide metal trihalide molecules are identified from both argon and neon matrix infrared spectra and frequencies calculated by density functional theory and multiconfigurational second-order perturbation theory (CASSCF/CASPT2). These simple terminal nitrides involve strong triple bonds, which range from 2.80 to 2.77 to 2.59 natural bond order for M = W, Mo, and Cr, respectively, as computed by CASSCF/CASPT2, and the M[triple bond]N stretching frequencies also follow this order. The terminal phosphides are weaker with bond orders 2.74, 2.67, and 2.18, respectively, as the more diffuse 3p orbitals are less effective for bonding to the more compact metal valence d orbitals.