Alina Samotus

Dioxotetracyano-complexes of molybdenum (IV) and tungsten (IV) and their vibrational spectra

Abstract A series of molybdenum(IV) and tungsten(IV) complexes of the type Me4[MO2(CN)4]x H2O (where Me = Na, K; M = Mo, W) has been synthesized. The solids obtained were characterized by chemical analysis, thermogravimetric and differential thermal analysis, i.r. and visible spectrophotometry, X-ray powder diffraction technique and magnetic susceptibility measurements. The analysis of the i.r. spectra has led to an assignment of the fundamental vibrations of the [MO2(CN)4]4− ion which is supported by studies of i.r. spectra of deuterated and anhydrous salts. The results obtained appeared to deviate considerably from previous assignments and a discussion of the discrepencies is given.

Complexation between molybdenum(VI) and citrate: structural characterisation of a tetrameric complex, K4[(MoO2)4O3(cit)2]·6H2O

The complex anion obtained by crystallisation from an aqueous solution of molybdate and citric acid (H3cit) at pH ca. 3 is of the type (i), [(MoO2)4O3(cit)2]4−, as determined by X-ray analysis of the salt K4[Mo4O11(cit)2]·6H2O. Principal dimensions [ranges or means; estimated standard deviation (e.s.d.) 0.005 A] are: MoO 1.692–1.734; Mo–O(bridging) 1.886–1.948; Mo–O(hydroxy) 1.972(5); Mo–O(carboxylate) (unidentate) 2.185–2.318, (bridging) 2.296–2.333 A. The crystals are triclinic, space group P, a = 8.966(4), b = 12.319(6), c = 15.962(9) A, α = 89.34(4), β = 99.80(4), γ = 107.04(4)°, R = 0.041 for 4 399 observed [I/σ(I) 2.0] reflections. The complex is tetranuclear with the citrato-ligands co-ordinated through the deprotonated hydroxy group and the α-carboxy group to form five-membered chelate rings. One of the oxygens of the β-carboxylate groups bridges between two Mo atoms [Mo(1)–Mo(2) and Mo(3)–Mo(4)] whereas the other oxygen bridges between these dimers. The other β-carboxylate group is not co-ordinated to molybdenum but is protonated. The i.r. and Raman solid-state spectra are in agreement with this structure as well as a study in solution (Raman, 1H and 13C n.m.r. spectroscopy, molecular weight determination).

1:1 Molybdenum(VI) citric acid complexes

SummaryThe complexes obtained by crystallisation from an aqueous solution of molybdate and citric acid (H4cit) at pH 4–8 have Mo:cit ratio 1∶1. The new complexes have been characterised by a variety of methods in the solid state and in solution. On this basis two dimeric species, [(MoO2)2O(citH)2]4− and [(MoO2)2O(cit)2]6−, and a monomeric species [MoO3(cit)OH2]4− are proposed.

Oxopentacyano-complexes of molybdenum(IV) and tungsten(IV) in photolysis of octacyanometalates(IV)

SummaryLigand-field (LF) photolysis of aqueous alkaline solutions of K4[M(CN)8] (M = Mo or W) containing KCN produces [MO(CN)5]3− species. NaCs2]MO(CN)5] was isolated and characterised by u.v.-vis., i.r. and Raman spectroscopy. In addition, the reactions of [MO(OH)(CN)4]3− with free CN− are described and the relations between octa-, penta- and tetra-cyanocomplexes are summarised.

Preparation and characterisation of [M(CN)4O(pz)]2− complexes (M=Mo or W) and their reactivity towards molecular oxygen

Abstract The synthesis and characterisation of (PPh4)2[M(CN)4O(pz)]·3H2O (M=Mo or W; pz=pyrazine) are presented. The salts are reactive towards molecular oxygen, both in solution and in the solid state, with formation of (PPh4)2[M(CN)4O(O2)]. The X-ray crystal structure of the molybdenum compound confirmed the presence of a peroxo ligand cis to the MO bond; the OO bond distance is 1.41 A. The IR spectra exhibit two absorption bands in the 950–850 cm−1 region assigned to the terminal MO group [917 (Mo) and 933 (W) cm−1] and the peroxo group [893 (Mo) and 871 (W) cm−1]. The possible mechanism of molecular oxygen uptake by pyrazine complexes is discussed.

pH dependent photolysis of octacyanotungstate(IV) and kinetics of thermal reactions of photoproducts

The kinetics of thermal reactions of photochemically generated aquoheptacyanotungstate(IV) and its protonated and deprotonated forms have been studied by conventional spectrophotometry in buffer solutions of pH = 1–13, I = 0.2 M, at 20°C. A pseudo-first-order rate constant k1 equal to 9.8 × 10−5 s−1 was found for the basic hydrolysis of [W(CN)7OH]4− and k3 equal to 3.64 × 10−4 s−1 for reaction of [HW(CN)7H2O]2− in acidic solution. The second-order rate constant k2, equal to 2.90 M−1 s−1, was found for the anation reaction of [W(CN)7H2O]3− in the pH range 6–8. A general reaction scheme for the photochemical and thermal reactions in the irradiated [W(CN)8]4− system as well as the over-all kinetic equations for the thermal reactions of the photoproducts are proposed. The quantum yields of each photoproduct are pH dependent but the total quantum yield is constant over the entire pH range and equal to one.

Monomeric and dimeric products of protonation of dioxotetracyanometalates(IV)—III The oxoaquotetracyanocomplexes of molybdenum and tungsten☆

Abstract The preparation and characterization of cadmium salts of diprotonated dioxotetracyanometalates(IV) (where metal = Mo, W) are described. Visible, IR and Raman spectral data are consistent with C4ν molecular geometry and support the formulation of the solids obtained as Cd[MoO(OH2)(CN)4] 6H2O and Cd[WO(OH2) (CN)4] 6H2O. The increasing values of ν(MO) and ν(MO) and ν(CN) in the monomeric trans dioxo-, oxohydroxo- and oxoaquotetracyanometalates(IV) are discussed.

Kinetics of oxidation of vanadyl acetylacetonate by oxygen in methanolic solution

The kinetics of oxidation of vanadyl acetylacetonate to VO(OH)(OMe)(acac) by molecular O2 in MeOH have been studied spectrophotometrically. The reaction, which is pseudo first-order with respect to [VO(acac)2] and [O2], is inhibited by Hacac and a vanadium(V) complex. The rate data were used to calculate the thermodynamic activation parameters. A mechanism for the reaction is discussed.

Quantum yields of [W(CN)8]4− formation in charge-transfer photochemistry of octacyanotungstate (V)

SummaryThe reactions of monochromatically photolyzed aqueous [W(CN)8]3− solutions in the pH range 1–13 were found to consist of a two-step sequence involving a primary inter-molecular redox process and a consecutive thermal chain process, both leading to the formation of octacyanotungstate(IV) as the main reaction product. The initial quantum yields, φ0, are practically constant for the entire pH range and equal to 0.81±0.05 mole/Einstein. The final quantum yields, φf, for both neutral and alkaline solution, were found to be greater than 1, having also an approximately constant value of 3.42±0.25. The empirical rate law for the post-irradiation reaction under excess of [W(CN)8]3− and at constant pH is given by: rate = kobs[R]2[W(CN)84−]−2 where R is a one-electron reductant. Kinetic data suggest a mechanism of the Haber-Weiss type for the thermal reduction of [W(CN)8]3− by H2O2.

Crystal structures of mixed-ligand oxocyano complexes of molybdenum(IV) and tungsten(IV) and their reactivity towards molecular oxygen studied by IR spectroscopy

The series of compounds, (PPh4)2[M(CN)4O(L)]·xH2O [M = Mo, W; L = pyrazine (pz), pyridine (py); Ph = phenyl group; x = 0, 2, 3], (PPh4)3[Mo(CN)5O]·7H2O and K(PPh4)2[Mo(CN)5O]·5H2O, which are able to bind molecular oxygen giving peroxo complexes (PPh4)2[M(CN)4O(O2)], have been synthesised. The substrates were characterised by thermogravimetric analysis, vibrational and UV-VIS spectroscopy and the crystal structure determination of (PPh4)2[M(CN)4O(pz)]·3H2O. The latter salts are isomorphous and consist of anions of approximately octahedral geometry, forming a three-dimensional hydrogen-bonded network with channels that enable the penetration of dioxygen. The solid state reactions of all the salts with O2 have been studied by measuring their infrared spectra. The integrated intensities of the O–O, pz or py, MO and CN bands change with time according to pseudo-first-order kinetics. The rate constants kobs range from 0.37 × 10−4 to 1.04 × 10−4 s−1 at 323 K. A detailed study of the function of temperature has been undertaken for (PPh4)2[W(CN)4O(pz)]·3H2O. The activation parameters, ΔH# = 68 ± 13 kJ mol−1 and ΔS# = −117 ± 39 J K−1 mol−1, were determined and an associative mechanism of dioxygen uptake was assumed.