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A simple, inexpensive, and essentially foolproof apparatus is described for obtaining a constant temperature, constant flow of cold nitrogen gas for use in low-temperature diffraction studies.

Utilisation des deplacements chimiques RMN de 95 Mo pour identifier divers ions en solution, du type [(XCu) n MoS 4 ] 2− , et determination des rapports CuX:MoS 4 pour des complexes inconnus. On montre que les clusters Cu-Mo-S ne se dissocient pas en solution car chaque complexe donne seulement une resonance de 95 Mo

The qualitative features of the electronic states of the {CoNO}8 group are investigated by calculating the electrostatic energies of the (dz2, dxz, dyz)2, (dxz, dyz)2, and (dz2)2 electron configurations in D′4 symmetry. The geometries of the {CoNO}8 group are correlated with the behavior of the singlet electronic states in the D′4 ligand field.

EPR spectra of fluid and frozen toluene solutions of MoOLX 2 , where L = hydrotris(3,5-dimethylpyrazolyl(borate), and X = Cl, SEt, OMe or NCS, at both X - and Q -band frequencies, have been simulated on the basic of C s -symmetry for the molecules. The non-coincidence of the g -and A -tensor axes are discussed in terms of the probable d -orbital mixings caused by the low-symmetry ligand fields in these molecules.

[Mo(C15H22BN6)(C6H5S)2(NO)], Mr = 641.5, monoclinic, P2(1)/n, a = 10.898 (1), b = 18.484 (2), c = 15.989 (2) A, beta = 109.81 (1) degree, V = 3030.2 A3, Z = 4, Dx = 1.41, Dm = 1.41 g cm-3, lambda(Mo K alpha) = 0.7173 A, mu = 5.9 cm-1, F(000) = 1320, T = 293 K, wR = 0.047 for 3997 observed reflections. In each complex, the Mo atom is six-coordinate, ligated by a terminal NO group, a tridentate pyrazolylborate ligand and two benzenethiolate ligands. The structure is compared with that of similar complexes and the Mo-S bonding is discussed.

Molybdenum-95 NMR has been used to directly detect the two diastereomers differing only in the Mo configuration in the square pyramidal molybdenum(II) complex [C 5 H 5 Mo(CO) 2 NN★]PF 6 , where NN★ is the chiral pyridine-2-carbaldimine ligand derived from pyridine-2-carbaldehyde and (S)(-)-1-phenylethylamine. The simplicity of the spectra clearly reveals the optical purity of the complex with respect to the metal center.

Aerial recrystallization of the mononuclear molybdenum(V) complex {HB(Me 2 pz) 3 }MoO(OC 6 H 4 - o -S) produced the novel binuclear Mo(VI) complex [{HB(Me 2 pz) 3 }MoO 2 (OC 6 H 4 - o -S)] 2 which contains a disulfide bond. The dimer crystallizes as the dioxane solvate in the space group C 2/ c with cell parameters a =23.46(2), b =11.100(4), c =21.571(8) A, β=104.23(4)°, Z =4. Final R w =0.062 (2236 reflections with F o >3σ( F o ), 301 variable parameters). The dimer contains two crystallographically identical distorted octahedral MoO 2 2+ centers. One face of each octahedron is occupied by two oxo ligands and a phenolate oxygen atom; the opposite face is occupied by three nitrogens of the HB(Me 2 pz) 3 − ligand. The two Mo(VI) centers of the dimers are linked by a disulfide bond formed upon oxidation of the 2-mercaptophenolate ligand of the original molybdenum(V) compound.

The reaction of cis-[MoO2(C5H10NO)2] (C5H10NOH = 1-piperidinol) with benzene 1,2-diol(catechol) in CHCl3 leads to the formation of [MoO(C5H10NO)2(C6H4O2)], which has been characterized by X-ray crystallography, NMR spectroscopy and cyclic voltammetry.

The electronic structures of transition metal complexes of NO are controlled by the stereochemistry about the metal atom (stereochemical control of valence). The six-coordinate complex, trans -[CoNO(NCS)-(C 6 H 4 [As(CH 3 ) 2 ] 2 ) 2 ] + , consists of [Co(III)-(N=O - )] 2+ (angle Co-N-O = 135°), while the pentacoordinate trigonal bipyramidal complex, [CoNO(C 6 H 4 [As(CH 3 ) 2 ] 2 ) 2 ] 2+ , is best formulated as [Co(I)-(N≡O + ] 2+ (angle Co-N-O = 179°). Evidence indicates that complexes of (NO) + and N 2 are electronically similar. Hence, the principles of stereochemical control of valence may be applied to metal complexes of N 2 . In a linearly coordinated M n ˜m(N≡N) complex, valence electrons can be transferred from the metal to the N 2 ligand producing a bent, protonated, and/or metallated M n+2 -(N=N 2- ) complex. This reduction of N 2 can be effected by the addition of an appropriate ligand to M or by a change in the coordination geometry about M. Stereo-chemical control of valence leads to the rejection of one of the previously proposed mechanisms for reduction by nitrogenase.

C27H32BMoN6O3, Mr = 595.34, monoclinic, P2(1)/n, a = 16.376 (6), b = 10.438 (5), c = 17.016 (8) A, beta = 107.25 (3) degrees, V = 2777.8 A3, Z = 4, Dm = 1.37, Dx = 1.43 g cm-3, Mo K alpha,lambda = 0.71073 A, mu = 5.02 cm-1, F(000) = 1228, T = 296 K, R = 0.038, wR = 0.045 for 2457 reflections. This molecule is the first structurally characterized mononuclear molybdenum complex containing a terminal aryl oxide ligand. The central molybdenum atom adopts a distorted octahedral coordination geometry with one face of the octahedron occupied by the tridentate pyrazolylborate ligand and the opposite face by the oxo and the two phenolato ligands. The structure is compared with that of the analogous benzenethiolate complex.

Molybdenum-containing enzymes are vitally important in nitrogen uptake, nitrogen metabolism and several biochemical reactions.1,2 The detailed coordination environment about the molybdenum atoms of such enzymes is still unknown because no molybdoenzyme has had its structure determined by x-ray crystallography. However, the molybdenum centers of several enzymes have been probed directly by electron paramagnetic resonance (EPR) spectroscopy1 and by x-ray absorption spectroscopy (EXAFS).3–5 Comparison of the spectral data from enzymes with spectral data from coordination compounds suggests that sulfur atoms are coordinated to molybdenum in the enzymes. EXAFS data from xanthine oxidase4 and sulfite oxidase5 in their oxidized forms also indicate the presence of terminal oxo groups attached to the molybdenum. In order to effectively interpret the spectral results from enzymes, it is important to have available a series of compounds whose stoichiometry and stereochemistry have been definitively established by x-ray structure determination. Of particular interest are compounds of sulfur donor ligands and oxomolybdenum(V) species.

This chapter reviews the structural chemistry of complexes of reduced nitrogen ligands. The scope is limited to complexes of molybdenum and tungsten with emphasis on results appearing since 1978. References to other reviews and earlier research can be found elsewhere.1, 2 Special attention is given to compounds which contain sulfur ligands because EXAFS data indicate that the molybdenum atoms in nitrogenase are ligated by sulfur atoms.3

Preparation de MoO 2 (dttd), MoOCl(dttd) et MoO(PPh 2 Et)(dttd). On donne la structure des complexes prepares et on les etudie electrochimiquement. Spectre RPE de MoOCl(dttd). dttd=le dibenzotetrathiadecane et PPh 2 Et=la diphenylethylphosphine

The synthesis, crystal structure, spectroscopic and electrochemical properties of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)3]2−, are reported. This complex undergoes reversible protonation on the ligand nitrogen atoms and the triply protonated species [Mo(Hqdt)3]+ has also been isolated and characterized as its chloride salt.

Le complexe du titre avec E=O cristallise dans le systeme monoclinique avec le groupe d'espace P2 1 /c (R=0,037). Mo est dans un environnement de coordination octaedrique deforme. Le complexe avec E=S et solvate par CH 2 Cl 2 cristallise dans le systeme orthorhombique avec le groupe d'espace P2 1 2 1 2 1 (R=0,035). La geometrie de coordination est analogue a celle du precedent