Structural Trans Effect Research Articles

Synthesis and Crystal and Molecular Structure of the Solvated Complex [MoO2(L) · C5H5N], Where L2– Is the 2-[N-(2-Hydroxynaphthalidene)amino]propane-1,2,3-Triol Anion

A new dioxomolybdenum(VI) compound [MoO2(L) · C5H5N], where L2– is 2-[N-(2-hydroxynaphthalidene)amino]propane-1,2,3-triol, was synthesized and its structure was determined by IR spectroscopy and X-ray diffraction. The Mo atom is in an octahedral coordination environment formed by two oxo ligands in cis positions to each other, two O atoms and the N(2) atom of the tridentate bis(chelate) ligand L, and the N(1) atom of the pyridine (Py) molecule. The N(2)(L) and N(1)(Py) atoms are in trans positions to the O(oxo) ligands at distances that are elongated due to the structural trans effect of the multiply bonded oxo ligands (Mo–N(1), 2.476 A; Mo–N(2), 2.272 A).

Kinetics and mechanistic study of the reduction of $$\hbox {Mn}^{\mathrm{III}}$$ Mn III by oxalate in Salophen scaffold: relevance to oxalate oxidase

The trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $${\hbox {OH}_{2})_{2}}^{+}$$ and bioxalate ( $$\hbox {HOX}^{-})$$ in aqueous medium equilibrate rapidly to trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $$\hbox {OH}_{2})$$ (HOX) followed by the acid dissociation equilibrium to the (aqua) mono oxalato complex. The slow redox reactions of trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $$\hbox {OH}_{2})$$ (HOX/OX) $$^{0/-}$$ with $$\hbox {H}_{2}\hbox {OX}$$ , $$\hbox {HOX}^{-}$$ , $$\hbox {OX}^{2-}$$ obey second order kinetics satisfying 2:1 stoichiometry $$([\hbox {Mn}^{{\mathrm{III}}}]_{\mathrm{T}}/[\hbox {OX}]_{\mathrm{T}} = 2/1)$$ . The products are $$\hbox {Mn}^{{\mathrm{II}}}$$ and $$\hbox {CO}_{2}$$ . Acrylamide monomer has no effect on the rate constant and the reaction does not induce its polymerization. The rate and activation parameters for the various rate limiting paths are reported. The intramolecular reduction of $$\hbox {Mn}^{{\mathrm{III}}}$$ by the coordinated $$\hbox {HOX}^{-}$$ and $$\hbox {OX}^{2-}$$ in trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $$\hbox {OH}_{2})(\hbox {HOX/OX})^{0/-}$$ could not be detected. Contrary to our expectation, it is observed that $$\hbox {H}_{2}\hbox {OX}$$ is a better reducing agent than $$\hbox {HOX}^{-}$$ for trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $$\hbox {OH}_{2})(\hbox {HOX})$$ , the slowest being the redox reaction of $$\hbox {OX}^{2-}$$ with trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $$\hbox {OH}_{2})(\hbox {OX})^{-}$$ . The molecular modelling by DFT depicts the structural trans effect in the oxalato complexes, it being maximum for trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $$\hbox {OH}_{2})(\hbox {OX})^{-}$$ . The observed sequence of the redox activity of the oxalato complexes reflects the potential role of non-covalent interaction i.e. H-bonding, governing the proton controlled electron transfer process (PCET). The $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen/Salen) complexes may turn out to be good substitute candidates for Oxalo Oxidase (OXO) enzyme in alleviating the oxalate overload in plants and animal biochemistry. The trans- $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)( $$\hbox {OH}_{2})_{2}^{+}$$ undergoes equilibrium pre-association in aqueous medium with bioxalate forming $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)(HOX) $$(\hbox {OH}_{2})$$ and $$\hbox {Mn}^{{\mathrm{III}}}$$ (Salophen)(OX) $$(\hbox {OH}_{2})^{-}$$ which are inert to intramolecular reduction but undergo reduction via second-order paths by $$\hbox {H}_{2}\hbox {OX}$$ , $$\hbox {HOX}^{-}$$ and $$\hbox {OX}^{2-}$$ with decreasing reactivity sequence implying proton controlled process.

What Can We Learn from the Crystal Structures of Metallacarboranes?

The determination of the molecular structures of metallacarboranes by X-ray diffraction remains critical to the development of the field, in some cases being the only viable way in which the overall architecture and the isomeric form of the molecule can be established. In such studies one problem frequently met is how to distinguish correctly {BH} and {CH} vertices, and this review begins by describing two relatively new methods, the Vertex-Centroid Distance (VCD) and Boron-Hydrogen Distance (BHD) methods, that have been developed to overcome the problem. Once the cage C atoms are located correctly, the resulting metallacarborane structure can frequently be analysed on the basis that cage B has a greater Structural Trans Effect (STE) than does cage C. In the absence of significant competing effects this gives rise to unequal M–L distances for a homogeneous ligand set and to a preferred Exopolyhedral Ligand Orientation (ELO) for a heterogeneous ligand set. ELO considerations can be used, amongst other things, to rank order the STEs of ligands and to identify suspect (in terms of cage C atom positions) metallacarborane structures.

Further studies of the Enhanced Structural Carborane Effect: tricarbonylruthenium and related derivatives of benzocarborane, dihydrobenzocarborane and biphenylcarborane.

Detailed comparison of the molecular structures of [1,2-μ-(C4H4)-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (1) and [1,2-μ-(C4H6)-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (2) reveals evidence for an Enhanced Structural Carborane Effect in 1 arising from the involvement of the cage pπ orbitals in the exopolyhedral ring to some degree. A minor co-product in the synthesis of 2 is [η-{1,2-μ-(C4H6)}-3,3-(CO)2-3,1,2-closo-RuC2B9H9] (3). Compounds 2 and 3 are readily interconverted, since heating 2 to reflux in THF or reaction with Me3NO affords 3 which readily reacts with CO to regenerate 2. The η-ene bonding in 3 is also displaced by PMe3, P(OMe)3 and t-BuNC to yield [1,2-μ-(C4H6)-3,3-(CO)2-3-PMe3-3,1,2-closo-RuC2B9H9] (4), [1,2-μ-(C4H6)-3,3-(CO)2-3-P(OMe)3-3,1,2-closo-RuC2B9H9] (5) and [1,2-μ-(C4H6)-3,3-(CO)2-3-t-BuNC-3,1,2-closo-RuC2B9H9] (6), respectively. Structural studies of 4-6, focussing on the Exopolyhedral Ligand Orientation of the {Ru(CO)2L} fragment relative to the C2B3 carborane face, are discussed in terms of the structural trans effects of PMe3, P(OMe)3 and t-BuNC relative to that of CO. An improved synthesis of [1,2-μ-(C6H4)2-1,2-closo-C2B10H10], "biphenylcarborane", is reported following which the first transition-metal derivatives of this species, [1,2-μ-(C6H4)2-3-Cp-3,1,2-closo-CoC2B9H9] (7) and [1,2-μ-(C6H4)2-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (8), are prepared. Comparisons of the structures of 7 and 8 with the corresponding benzocarborane derivatives [1,2-μ-(C4H4)-3-Cp-3,1,2-closo-CoC2B9H9] and 1, respectively, suggest that Clar's rule for aromaticity can be applied to polycyclic aromatic hydrocarbons fused onto carborane cages.

1,1′-Bis(ortho-carborane) as a κ2 co-ligand

The compounds (dmpe)Ni{κ2-2,2′-[1-(1′-1′,2′-closo-C2B10H10)-1,2-closo-C2B10H10]} (1) and (dppe)Ni{κ2-2,2′-[1-(1′-1′,2′-closo-C2B10H10)-1,2-closo-C2B10H10]} (2) have been synthesised and fully characterised, including crystallographic studies. In all three structures studied (1, 2 and 2·2THF) the metal atom is slightly distorted from square-planar towards tetrahedral (τ4 values [Houser et al., 2007] of 0.30, 0.33 and 0.11, respectively). Consideration of these molecular structures together with those of related species in the literature allows us to tentatively conclude that, in square-planar NiII compounds, the structural trans effect (trans influence) of σ-bonded carborane is greater than that of Cl and sp2 N but less than that of sp2 C and sp3 C.

Electronic Control of Metal-Centered Chirality in η5:κS-Indenyl-Sulfanyl and -Sulfinyl Rhodacycles of 2-Phenylpyridine

Cyclometalation of 2-phenylpyridine with the η5:κS-indenyl-sulfanyl and -sulfinyl rhodium dichloride complexes rac-8, (pR*,RS*)-9, and (pR*,SS*)-9 in the presence of AgSbF6 provided the rhodacyclic complexes 10, 11, and 12, respectively, in yields ranging from 72% to 92%. The complexes were obtained as mixtures of chiral-at-metal epimers, with ratios of the epimer with the phenyl ligand anti to the indenyl benzo-ring [(pR*,SRh*)] to the epimer with the phenyl ligand syn to the benzo-ring [(pR*,RRh*)] of 3:2, 10:1, and 2:5 for complexes 10, 11, and 12, respectively, establishing that sulfoxide chirality has a significant role in stereocontrol of metal-centered chirality in the initially formed complexes. Equilibration of the rhodacyclic complexes takes place at 120 °C in 1,1,2,2-tetrachloroethane-d2 (C2D2Cl4) solution or at 60 °C in C2D2Cl4 solution in the presence of catalytic 4-ethylpyridine. Kinetic studies demonstrate that under the former conditions, epimerization is catalyzed by adventitious water and that epimerization occurs through an associative activation mechanism. The thermodynamic ratios for the SRh*/RRh* epimers of the complexes were >100:1, >100:1, and 25:1 for complexes 10, 11, and 12, respectively, consistent with a structural indenyl effect favoring the epimer where the stronger structural trans effect phenyl ligand is anti to the indenyl benzo-ring. This is supported by 13C NMR and X-ray diffraction studies which show greater slip-fold distortions toward η3-coordination of the indenyl ligand in the SRh* epimers of the rhodacyclic complexes.

Reaction of 2-Pyridylmethylthiourea Derivatives with [(en)2Co(OSO2CF3)2]+ Induces Hypodentate Coordination of an Ethylenediamine Ligand

Pyridylmethylthiourea derivatives coordinate with [(en)2Co(OSO2CF3)2]+ in a tridentate manner resulting in the formation of a hypodentate ethylenediamine ligand. Four ligands were studied: N-(R)phenyl-N′-2-pyridylmethylthiourea (R = H (1a), CH3 (1b), OCH3 (1c)) and N-benzyl-N′-2-pyridylmethylthiourea (2). These bind through the sulfur, a deprotonated exo nitrogen, and the pyridyl nitrogen atoms forming four and five-membered rings, respectively. The ligand also coordinates in a bidentate manner through the sulfur and deprotonated endo or exo nitrogen atoms, forming two additional coordination isomers. The solid state structure (X-ray) of one of the bidentate isomers of Co-1b2+ (endo isomer) shows that the coordinated thiourea sulfur induces a structural trans effect of 0.035 Å on the trans Co–N bond while that of the tridentate isomer of Co-1a3+ confirms the coordination mode of the ligand and the presence of a protonated hypodentate ethylenediamine ligand as suggested by 1H and 13C NMR spectroscopy.

Synthesis and Characterisation of Sigma‐ and Pi‐Bonded Metallaphosphacarboranes

AbstractTwo new metallaphosphacarboranes have been synthesised from [7, 8‐nido‐CPB9H11]– by deprotonation followed by treatment with a metal fragment. Starting with [HNMe3][7, 8‐nido‐CPB9H11] deprotonation and reaction with a source of {(indenyl)Co}2+ affords the η‐bonded cobaltaphosphacarborane 3‐(η‐C9H7)‐3,1,2‐closo‐CoCPB9H10 (1) the first “half‐sandwich“ 3,1,2‐metallaphosphacarborane. The indenyl ligand conformation in 1 (cisoid between the indenyl bridgehead carbon atoms and cage heteroatoms, with the cage carbon atom lying below the indenyl C–C bond) reveals that the structural trans effect in a phosphacarborane ligand is weakest for carbon and therefore varies in the order B > P > C. Starting with [HNC5H11][7, 8‐nido‐CPB9H11] deprotonation and reaction with half an equivalent of [Ru(p‐cymene)Cl2]2 affords the σ‐bonded ruthenaphosphacarborane 8‐{Ru(p‐cymene)Cl(C5H11N)}‐7, 8‐nido‐CPB9H11 (2), the first example of a molecule with a phosphacarborane σ‐bonded to a metal to be crystallographically characterised.

Synthesis and Characterisation of [(en)2Co]3+ Complexes Coordinated by Substituted Thiourea Ligands

Substituted thiourea ligands bind in a bidentate manner forming a four-membered ring through the sulfur atom and a deprotonated thiourea nitrogen atom when reacted with [(en)2Co(OSO2CF3)2]+ in tetramethylene sulfone. Reaction of unsymmetrical ligands results in the formation of coordination isomers, some of which can be separated by column chromatography using Sephadex SPC-25. Coordination isomers are easily distinguishable based on visible and 1H NMR spectroscopy . Twelve para-substituted and one meta-substituted ligands were studied: N,N′-dibenzylthiourea (1a); N-(R)phenyl-N′-benzylthiourea {R = H (2a), NO2 (2b), CH3 (2c)}; N-(R)phenyl-N′-(R′)phenylthiourea {R, R′: H, H (3a), H, CH3 (3b), OCH3, NO2 (3c), CH3, NO2 (3d)}; N-methyl-N′-(R)phenylthiourea {R = H (4a), CH3 (4b), OCH3 (4c), NO2 (4d), 3-CH3 (4e)}. The solid state structure (X-ray) of one isomer of Co-4a as its perchlorate salt confirms the coordination mode suggested by 1H NMR spectroscopy and shows that the Co–N bond trans to the coordinated thiourea sulfur induces a structural trans effect of 0.019 Å.

On-surface coordination chemistry of planar molecular spin systems: novel magnetochemical effects induced by axial ligands

Paramagnetic transition-metal complexes assembled on surfaces are of great interest for potential applications in organic spintronics. The magnetochemical interactions of the spin of the metal centers with both ferromagnetic surfaces and optional axial ligands are yet to be understood. We use a combination of X-ray magnetic circular dichroism (XMCD) and quantum-chemical simulations based on density functional theory (DFT + U) to investigate these metal–organic interfaces with chemically tunable magnetization. The interplay between an optional axial ligand (NO, spin S = 1/2 or NH3, S = 0) and Ni and Co ferromagnetic surfaces affecting the spin of Co(II) tetraphenylporphyrin (d7, S = 1/2), Fe(II) tetraphenylporphyrin (d6, S = 1), Mn(II) tetraphenylporphyrin (d5, S = 5/2) and Mn(II) phthalocyanine (d5, S = 3/2) is studied. We find that the structural trans effect on the surface rules the molecular spin state, as well as the sign and strength of the exchange interaction with the substrate. We refer to this observation as the surface spin-trans effect.

ChemInform Abstract: Trans-Effects in Octahedral Transition Metal Complexes

Abstract An extensive survey of the occurrences and origins of both structural trans-effects (STEs) and kinetic trans-effects (KTEs) in octahedral d-transition metal complexes is presented. This allows the identification of general STE classes into which the majority of common ligands fit: (a) very large STE ligands (STE vs. Cl − >ca. 0.20 A): SiR 3 − , NO − , N 3− , O 2− , S 2− , RC 3− ; (b) large STE ligands (ca. 0.20>STE vs. Cl − >ca. 0.10 A): H − , R − , η 1 -alkenyl, η 1 -Ph, RCO − , RN 2 − ; (c) moderate STE ligands (ca. 0.10 A>STE vs. Cl − >0.00 A): CO, CN − , CNR, η 1 -acetylide, R 2 C, NO 2 − , NS + , RN 2 + , SO 3 2− , RSO 2 − , PR 3 , P(OR) 3 , RNH − , RS − , η 1 -thiones. The NO + ligand best illustrates the mutual nature of STEs, since it shows moderate STEs when trans to π-acceptor ligands, negligible STEs when trans to purely σ-donor ligands, and inverse STEs when trans to π-donors. STEs can sometimes show a marked dependency upon the electronic properties of the complexed metal centre, e.g. π-accepting RNC and PR 3 ligands generally give moderate STEs, but in d 0 complexes their STEs are weaker than that of Cl − . This may be attributed to an absence of π-back-bonding in such complexes. Also, the STEs of π-donating RN 2– ligands show an extremely wide variation which partially correlates with the metal d-configuration. The relationship between STEs and KTEs depends upon ligand substitution mechanisms, and because such reactions in octahedral complexes are generally dissociatively activated, there is often a close correlation between STEs and KTEs. For example, N 3− causes very large STEs and KTEs, whilst SO 3 2− gives moderate STEs and large KTEs. Since both of these ligands cause STEs primarily via powerful electron donation, the ground state destabilisations implied by STEs are likely to be accompanied by stabilisation of the electron-deficient five-co-ordinate transition states. By contrast, π-acceptor ligands such as CO or RNC generally exert moderate STEs, but cause pronounced delabilisation of trans metalligand bonds due to destabilisation of transition states.

Transinfluence in amer-octahedral triiodidolanthanide: triiodidotris(tetrahydrofuran-κO)ytterbium(III)

The structure of the six-coordinate title complex, [YbI(3)(C(4)H(8)O)(3)], is the first mer-octahedral form of an LnI(3)L(3) lanthanide (Ln) compound with neutral L ligands, and is closely related to that of several of the seven-coordinate LnX(3)L(4) series of compounds, where X = Cl, Br or I and L = tetrahydrofuran (THF), isopropanol, pyridine or water. A structural trans effect can be assigned to YbI(3)(THF)(3), in contrast to the LnX(3)L(4) compounds, where steric and crystal packing effects are significant. The Yb-I bond lengths are 2.9543 (4) and 2.9151 (6) A for I trans and cis to I, respectively, and the Yb-O bond lengths are 2.299 (5) and 2.251 (3) A for O trans and cis to I, respectively. The crystal packing allows for six contact distances as weak C-H...I interactions in the range 3.10-3.24 A. The title molecule has a crystallographic twofold axis passing through a THF O atom, the trans I atom and the Yb atom.

Investigation on the reactivity of dithiophosphonato/dithiophosphato NiII complexes towards 2,4,6-tris-2-pyridyl-1,3,5-triazine: developments and new perspectives

The reactions between tptz and differently substituted dithiophosphonato [Ni(ROpdt)2] [ROpdt = (RO)(4-MeOC6H4)PS2-; R = Et (2); Pr (3); i-Pr (4); Bu (5)] and dithiophosphato [Ni((EtO)2PS2)2] (6) Ni(II) complexes have been investigated, and the characterisation of the resulting neutral mixed complexes (2.tptz)-(6.tptz) is reported. In all these complexes, tptz forces one of the two dithiophosphonato/dithiophosphato ligands to behave as a monodentate ligand, a coordination mode rarely found in analogous Ni(II) phosphorodithioato complexes. A comparison has been performed between the Ni-S bond distances of the new complexes and those of isologous dithiophosphonato, dithiophosphato and dithiophosphito Ni(II) square-planar complexes, and of their penta- and hexa-coordinated adducts. The results, also supported by DFT calculations, are discussed and explained in terms of the structural trans-effect (STE). The reactivity of 2.tptz towards AgNO3 and CuSO4 to yield the complex [Ni(EtOpdt)(tptz)(H2O)]NO3 (7), and the dimer [(Ni(tptz)(mu-SO4)(H2O)]2.6H2O (8), respectively, is consistent with the proposed bonding models.

The first structurally characterized nitrosyl heme thiolate model complex

The NO ligand in the formally {FeNO}6 compound [Fe(oep)(NO)(thiolate)] is bent, and does not impart a significant structural trans effect to the Fe-S bond.

Effect of the sixth axial ligand in CS-ligated iron(II)octaethylporphyrinates: structural and Mössbauer studies.

The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been investigated. Six-coordinate complexes have been synthesized and characterized by Mössbauer and infrared spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate parent complex. The crystal structures of [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)] are reported and discussed. The 1-methylimidazole and pyridine derivatives exhibit Fe-C(CS) bond distances of 1.703(4) and 1.706(2) A that are significantly longer than the 1.662(3) A reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208). The trans Fe-N(ligand) distances of 2.112(3) and 2.1550(15) A observed for the 1-methylimidazole and pyridine complex are approximately 0.13 A longer than those observed for analogous bis-ligated complexes and are consistent with a significant structural trans effect for the CS ligand. Mössbauer investigations carried out for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features. All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS. The five-coordinate derivative has delta(Fe) = 0.08 mm/s, and the six-coordinate complexes exhibit delta(Fe) = 0.14 to 0.19 mm/s at 4.2 K. The five-coordinate complex shows a large quadrupole splitting (DeltaE(q) = 1.93 mm/s at 4.2 K) which is reduced on coordination of the sixth ligand (DeltaE(q) = 0.42-0.80 mm/s at 4.2 K). Addition of a sixth ligand also leads to a small decrease in the value of nu(CS). Correlations in structural, IR, and Mössbauer results suggest that the sixth ligand effect is primarily induced by changes in sigma-bonding. The structure of [Fe(OEP)(CS)(CH(3)OH)] is briefly reported. Crystal data: [Fe(OEP)(CS)(1-MeIm)] crystallizes in the monoclinic system, space group P2(1)/n, Z = 4, a = 9.5906(5) A, b = 16.704(4) A, c = 23.1417(6) A, beta = 100.453(7) degrees. [Fe(OEP)(CS)(Py)] crystallizes in the triclinic system, space group P1, Z = 5, a = 13.9073(6) A, b = 16.2624(7) A, c = 22.0709(9) A, alpha = 70.586(1) degrees, beta = 77.242(1) degrees, gamma = 77.959(1) degrees. [Fe(OEP)(CS)(CH(3)OH)] crystallizes in the triclinic system, space group P1, Z = 1, a = 9.0599(5) A, b = 9.4389(5) A, c = 11.0676(6) A, alpha = 90.261(1) degrees, beta = 100.362(1) degrees, gamma = 114.664(1) degrees.

Synthesis and structure of some cobalt(II), cobalt(III) and one nickel(II) monomeric, monodentate( S) thiosulfato complexes. Trans and cis structural effects in the cobalt(III) complexes.

The crystal structures of five monomeric complexes with monodentate thiosulfato- S ligation have been determined. A, (NH 4) 6[Co(II)(S 2O 3) 4]·H 2O, contains a novel [Co(II)(S 2O 3) 4] 6− anion in which the cobalt has a distorted tetrahedral coordination environment and is bonded to four discrete thiosulfate ligands: (Co–S, 2.330(3)–2.351(4) Å). B, trans-(N(CH 3) 4) 2[Co(II)(H 2O) 4(S 2O 3) 2], and C, trans-(N(CH 3) 4) 2[Ni(II)(H 2O) 4(S 2O 3) 2], are isomorphous and contain the novel anions trans-[M(II)(H 2O) 4(S 2O 3) 2] 2- [M=Co(II) and Ni(II)]. The anions are centrosymmetric with the metals in octahedral environments; [Co(II)–S, 2.488(2); Co(II)–O, 2.104(3), 2.120(3); Ni(II)–S, 2.452(1); Ni(II)–O, 2.080(2), 2.100(2) Å]. D, trans-Na[Co(III)(en) 2(S 2O 3) 2], and E, trans-NH 4[Co(III)(en) 2(S 2O 3) 2]·2H 2O contain the trans-[Co(III)(en) 2(S 2O 3) 2] 1− anion with Na + and NH 4 + cations, respectively. These anions are centrosymmetric with cobalt in an octahedral environment; [ D Co(III)–S, 2.340(3); Co–N, 1.982(6), 2.002(6); E Co(III)–S, 2.322(1); Co–N, 1.974(5) Å]. In D and E, there are structural trans effects; with mutually trans thiosulfato ligands, the Co(III)–thiosulfate bond is lengthened, by 0.061(3) and 0.043(2) Å for D and E, respectively. This structural trans effect correlates with the general labilising of ligands trans to thiosulfate ligands, but is not consistent with the stability of the anion in D and E to nucleophilic substitution. This stability is attributed to four intramolecular hydrogen bonds (N–H⋯O–S) between the ethylenediamine and thiosulfate ligands. In C and D, the Co–N bond lengths cis to the thiosulfate ligand are slightly longer than expected: for D by 0.041 and 0.021Å, and in E by 0.013 Å. This cis lengthening may be associated with the intramolecular (N–H⋯O–S) hydrogen bonds, but there is no direct correlation between the cis lengthening and the shortness of the hydrogen bond.

Syntheses, Characterization, and Structural Studies of Several (Nitro)(nitrosyl)iron(III) Porphyrinates: [Fe(Porph)(NO2)(NO)

The preparation and characterization of several (nitro)(nitrosyl)iron(III) complexes, [Fe(Porph)(NO2)(NO)], where Porph = tetraphenylporphyrin, octaethylporphyrin, tetra-p-methoxyphenylporphyrin, or picket fence porphyrin, are described. Crystal data: [Fe(Tp-OCH3PP)(NO2)(NO)]·CH2Cl2, tetragonal, space group I41/a, Z = 4, with a = 15.620(3) Å, c = 18.649(2) Å; [Fe(TpivPP)(NO2)(NO)]·C6H5Cl (1), tetragonal, P4/ncc, Z = 4, with a = 18.094(3) Å, c = 18.939(4) Å, and (2) with a = 18.117(2) Å, c = 19.0838(8) Å; [Fe(TpivPP)(NO2)(NO)] (3), monoclinic, C2/c, Z = 2, with a = 16.1559(5) Å, b = 18.6920(8) Å, c = 19.7779(10) Å, and β = 90.971(6)°; [Fe(TpivPP)(NO2)(NO)]·C7H8 (4), monoclinic, P21/n, Z = 4, with a = 12.7764(10) Å, b = 27.192(2) Å, c = 19.1716(8) Å, and β = 105.063(5)°. Of the several crystal structures described, two (out of four characterized) picket fence porphyrin derivatives exhibited no crystallographic disorder that led to problems defining the axial ligand geometry. In these two derivatives the Fe−axial ligand distances are Fe−N(NO) = 1.668(2) or 1.671(2) Å and Fe−N(NO2) = 2.002(2) or 1.998(2) Å; NO does not cause a significant structural trans effect. The average Fe−Np distance of 1.998(4) Å is consistent with a low-spin state for iron. The linear or near-linear (169.3(2)°) Fe−N−O angle observed is that expected for {FeNO}6 (i.e., iron(III) nitrosyl) species. The solid-state NO stretching frequencies (1871−1893 cm-1) are also consistent with a linear Fe−N−O group. Mössbauer spectra show that the complexes have quadrupole splittings ΔEq = 1.35−1.50 mm/s and isomer shifts δ = 0.0−0.11 mm/s; the isomer shifts display a strong temperature dependence with lower values at high temperature. Mössbauer spectra in applied magnetic field at 4.2 K show that the molecules have diamagnetic ground states which is presumed to result from antiferromagnetic coupling between the low-spin iron(III) and the S = 1/2 nitric oxide center.

Syntheses, Molecular Structures, and Spectroscopy of Gold(III) Dithiolate Complexes.

Two Au(III) dithiolate complexes, [Au(dbbpy)(tdt)]PF(6) and Au(eta(2)-C,N-ppy)(tdt), (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; tdt = 3,4-toluenedithiolate; ppy = C-deprotonated 2-phenylpyridine), have been prepared and structurally characterized by X-ray crystallography. The complexes have low-energy absorption bands that exhibit mild solvatochromism (lambda(max) = 444 nm (epsilon, 2310 M(-)(1) cm(-)(1)) in CH(2)Cl(2) and 406 nm (epsilon, 3170 M(-)(1) cm(-)(1)) in DMSO) and are tentatively assigned to a charge-transfer-to-diimine transition. This transition occurs at higher energy than the analogous charge-transfer transition in related Pt(II) complexes (e.g., Pt(dbbpy)(tdt), lambda(max) = 563 nm (epsilon, 7200 M(-)(1) cm(-)(1)). Whereas neither Au(III) complex is emissive, their respective dichloride precursors, [Au(dbbpy)Cl(2)]PF(6) and Au(eta(2)-C,N-ppy)Cl(2), luminesce in low-temperature glass matrixes from an excited state that is tentatively assigned as intraligand pi-pi. The neutral complex, Au(eta(2)-C,N-ppy)(tdt), is more easily oxidized (E(ox) = 0.925 V vs 1.589 V (vs NHE)) and less easily reduced (E(red) = -1.339 V vs -0.255 V (vs NHE)) than the cationic complex, [Au(dbbpy)(tdt)]PF(6). Both dithiolate complexes exhibit approximately square planar coordination. Yellow crystals of [Au(dbbpy)(tdt)]PF(6) (C(25)H(30)AuF(6)N(2)PS(2)) are triclinic, space group P&onemacr; (No. 2), with a = 7.1977(2) Å, b = 11.7292(1) Å, c = 17.5820(5) Å, alpha = 104.537(2) degrees, beta = 96.592(2) degrees, gamma = 102.455(2) degrees, V = 1380.41(6) Å(3), Z = 2, and final R = 0.050 (R(w) = 0.1062) for 3446 unique reflections. Orange crystals of Au(eta(2)-C,N-ppy)(tdt) (C(18)H(14)AuNS(2)) are monoclinic, space group P2(1)/c (No. 14), with a = 8.852(1) Å, b = 11.726(2) Å, c = 15.499(5) Å, beta = 101.34(2) degrees, V = 1577.5(6) Å(3), Z = 4, and final R = 0.0438 (R(w) = 0.0759) for 3685 unique reflections. A structural trans effect in Au(eta(2)-C,N-ppy)(tdt) results in a significantly longer Au-S distance (ca. 0.1 Å) trans to the Au-C bond than that trans to the Au-N bond. In the solid state, the [Au(dbbpy)(tdt)](+) cations are arranged in stacks with alternating intermolecular Au.Au separations of 3.60 and 3.75 Å while the Au(eta(2)-C,N-ppy)(tdt) molecules form stacks with an intermolecular Au.Au separation of 3.81 Å.

Oxygen Abstraction Reactions of N-Substituted Hydroxamic Acids with Molybdenum(V) and Vanadium(III) and -(IV) Compounds.

A wide range of N-substituted mono- and dihydroxamic acids undergo oxygen abstraction on reaction with V(III), V(IV), and Mo(V) compounds to form hydroxamates of V(V) and Mo(VI) respectively together with the corresponding amides and diamides. The molybdenyl and vanadyl hydroxamates form metal-oxygen clusters under FABMS conditions. The X-ray crystal structures of [MoO(2){CH(3)(CH(2))(n)()C(O)N(C(6)H(5))O}(2) (1 and 2) (n = 4, 5) show monomeric structures with structural trans effects and consequent weakening of the Mo-O(ligand) bonds which may account for the tendency to form clusters in FABMS. In constrast, the electrospray MS of the vanadyl dihydroxamates, VO(OH)[PhN(O)C(O)(CH(2))(n)()C(O)N(O)Ph] (n = 3, 5) and VO(OH)[p-CH(3)C(6)H(4)N(O)C(O) (CH(2))(n)()C(O)N(O)C(6)H(4)-CH(3)) (n = 2, 4) show the presence of dimers in solution.

Some new dinuclear rhodium compounds: preparations and structures

The following seven dinuclear rhodium compounds have been prepared, characterized spectroscopically and their crystal and molecular structures determined: 1,5,6-Rh 2Cl 6(PEt 3) 3 ( 1), 1,3,6,8-Rh 2Cl 6(PEt 3) 4 ( 2), 1,5,6-Rh 2Cl 6(PPr n 3) 3 ( 3), 1,5,6-Rh 2Br 6(PEt 3) 3 ( 4), 1,5,6-Rh 2Br 6(PPr n 3) 3 ( 5), 1,5,6-Rh 2Br 6(PBu n 3) 3 ( 6), 1,3,6,8-Rh 2Br 6(PBu n 3) 4 ( 7). In each case there are marked structural trans effects such that RhX br bonds trans to PR 3 ligands are 0.12–0.17 Å (av. 0.15 Å) longer than those trans to an RhX bond. The fact that for the face-sharing bioctahedral (FSBO) molecules only one of the two possible isomers was formed and for the edge-sharing bioctahedral (ESBO) molecules only one of the nine possible isomers was formed is discussed and some principles that largely account for these observations are invoked. These principles are (i) that no μ-X ligand will have both of its RhX bonds trans to PR 3 and (ii) that in ESBO species phosphine ligands on two adjacent axial positions are sterically disfavored.