Molybdenum complexes thiocyanates

Discussion. Molybdenum(VI) in acid solution when treated with tin(II) chloride [best in the presence of a little iron(II) ion] is converted largely into molybdenum(V) this forms a complex with thiocyanate ion, probably largely Mo(SCN)5, which is red in colour. The latter may be extracted with solvents possessing donor oxygen atoms (3-methylbutanol is preferred). The colour depends upon the acid concentration (optimum concentration 1M) and the concentration of the thiocyanate ion (1 per cent, but colour intensity is constant in the range 2-10 per cent) it is little influenced by excess of tin(II) chloride. The molybdenum complex has maximum absorption at 465 nm. 

NSC, Thiocyanate, gold complex, 26 90 NS2C5H11, Dithiocarbamic acid, diethyl-, molybdenum complex, 28 145 NSiC,H , Ethanamine, 1,1-dimethyl-A -(trimethylsilyl)-, 27 327 NSi2C4H , Silanamine, 1,1,1-trimethyl- -(trimethylsilyl)-, ytterbium complex, 27 148... 

Molybdenum Plant Thiocyanate complex Spectrophotometry 0.05-1.0 mg r Liquid-liquid extraction... 

Arsonium salts have found considerable use in analytical chemistry. One such use involves the extraction of a metal complex in aqueous solution with tetraphenyiarsonium chloride in an organic solvent. Titanium(IV) thiocyanate [35787-79-2] (157) and copper(II) thiocyanate [15192-76-4] (158) in hydrochloric acid solution have been extracted using tetraphenyiarsonium chloride in chloroform solution in this manner, and the Ti(IV) and Cu(II) thiocyanates deterrnined spectrophotometricaHy. Cobalt, palladium, tungsten, niobium, and molybdenum have been deterrnined in a similar manner. In addition to their use for the deterrnination of metals, anions such as perchlorate and perrhenate have been deterrnined as arsonium salts. Tetraphenyiarsonium permanganate is the only known insoluble salt of this anion. 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

Theory Molybdenum (VI) is mostly converted to molybdenum (V) when an acidic solution of the former is treated with tin (II) chloride preferably in the presence a little Fe2+ ion. The resulting molybdenum (V) form a red complex with thiocyanate ion as follows ... 

The non-aqueous medium favors the formation of the red-violet complex. The yellow complex is poorly soluble in a wide variety of organic solvents. Molybdenum, uranium, and iron interfere with the determination since these elements form colored thiocyanate complexes. Disadvantages of this method are the long time required and the formation of the yellow thiocyanate complex which cannot be excluded. 

The element is extractable from strong hydrochloric acid solutions into 4-methylpentan-2-one. This approach may be applied to the analysis of plant material, if the ash is extracted with the strong hydrochloric acid required.35 Kim et al.36 masked iron(m) by reduction to iron(n) with tin(n) chloride before extracting molybdenum as its thiocyanate complex with Aliquat 336 into chloroform. The latter was evaporated, and the residue extracted with 4-methylpentan-2-one prior to determination of molybdenum by AAS. The procedure was applied to soils, sediments, and natural waters. In fertilizer analysis, the thiocyanate complex of molybdenum has been extracted, after reduction of iron with tin(n) chloride, into 3-methylbutan-l-ol, and the latter extract analysed directly.37 In another thiocyanate-based procedure, total molybdenum from soils and geological materials was extracted into 4-methyl-pentan-2-one.38... 

Molybdenum gives a similar reaction. If, however, a thiocyanate is added, the red complex ion [Mo(SCN)6]3 is formed, and upon the addition of concentrated hydrochloric acid the red colour disappears and the blue colour due to tungsten remains. 

Thiocyanate complexes are formed by molybdenum in the III, IV, and V oxidation states, the last being of the oxo type, for example, [MoO(NCS)5]2. The [Mo(NCS)6]3 ion has been shown conclusively to have N-bonded thiocyanate ions, and this appears likely to be the case also in all other molybdenum thiocyanato species. 

Infrared and electronic spectra confirm the nature of the hexa-N-thiocyanatometallate anions of chromium(III) (293, 639, 647, 656, 663, 665) and molybdenum(III) (293, 489, 647, 663, 664), and molyb-denum(II) (534) a variety of salts have been reported containing these anions (see the foregoing and also Refs. 30 and 528). Infrared spectra and magnetic measurements have been used to characterize [M(NCS)e] (M = Mo, W) and [W(NCS)g] (386). A large number of mixed-ligand thiocyanate complexes of chromium(III) have been reported, and many of these are of the type M[Cr(CNS)4L2], similar to the original Reinecke s salt where L = NH3. 

Thiocyanates of Molybdenum.—Thiocyanates containing molybdenum as a cation are not known, but compounds similar to the complex cyanides, in which the metal figures as the central atom of a complex anion, have been prepared. 

Thiiranes can be prepared directly from alkenes using specialized reagents. Thiourea with a tin catalyst gives the thiirane, for example. " Interestingly, internal alkynes were converted to 1,2-dichorothiiranes by reaction with S2CI2 (sulfur monochloride).It is noted that epoxides are converted to thiiranes with ammonium thiocyanate and a cerium complex. " A trans-thiiration reaction occurs with a molybdenum catalyst, in which an alkene reacts with styrene thiirane to give the new thiirane. 

Trace amounts of molybdenum have been separated from natural waters by retention of the anionic Mo-thiocyanate complex on the anion-exchanger Dowex 1. Mixtures of 2 A/ HCIO4 and 1 M HCl are used for the elution [24]. The Mo-thiocyanate complex has also been concentrated on the anion exchanger Amberlite XAD [25,26], with the use of acetone as the eluent [25]. Molybdenum has been retained on anion-exchange columns from an HCl medium prior to being determined by the thiocyanate method [27-30]. Mo has been sorbed selectively (pH 3.5) on the anion exchanger Sephadex G-25, from which it can be eluted withEDTA [31]. 

Tungsten (and molybdenum) have been sorbed on Dowex 1 (Cl ) resin from sea-water acidified to pH -1 a 0.5 M solution of NaOH and NaCl has been used as eluent. The sorption of the thiocyanate complex on an anion-exchange column has been used for separation of W from rare earth elements [33]. 

Several other compounds that, like thiocyanate and dithiol, have sulphur as the ligand atom, are used in spectrophotometric methods for Mo. Some examples are thioglycolic acid (mercaptoacetic acid, thioacetic acid) [70,71], dithiocarbamates [72-74], rubeanic acid [75], and aminobenzenethiol [76]. Molybdenum has been determined also as the thiosulphate complex [77]. 

To determine W admixtures in molybdenum, the molybdenum thiocyanate complex is first extracted in the presence of thioglycolic acid, then the tungsten thiocyanate complex is extracted after addition of TiCla as reducing agent 128]. ... 

Tungsten, molybdenum, and vanadium interfere in the determination of niobium. In contrast to the corresponding tungsten complex, the niobium-thiocyanate complex is decomposed by oxalic acid. Fe(ni), U, Ti, and Ta do not interfere if they are present in no greater than hundred-fold amounts relative to niobium. Phosphate and fluoride interfere, but the latter can be masked with aluminium ions [37]. 

Numerous indirect amplification methods have been devised for the determination of phosphate. The molybdenum in an extract of molybdophosphoric acid (Mo P = 12 1) has been determined with thiocyanate [63], phenylfluorone [64], dithiol (e = 1.7-10 ), Sulphonitrophenol S (e = 4.610 ) [65], or 2,2 -diquinoxalyl [66]. The indirect method that involves the Fe(ll)-ferrozine complex [67] is unusually sensitive (e = 9.7-10 ). In another method involving the complex of Ce(III) and Arsenazo III, the phosphate gives a sparingly soluble CeP04 and liberates an equivalent quantity of Arsenazo III [68]. 

Many methods for sulphide and H2S are based on the reducing properties of S(-II). Hydrogen sulphide reduces molybdate in acid medium to molybdenum blue, and the molybdophosphate to phosphomolybdenum blue [52]. Iron(III) reduced by H2S in the presence of 1,10-phenanthroline gives the orange Fe(phen)3 complex [2,53], Hydrogen sulphide may be determined after conversion into thiocyanate by the reaction with Fe(III) [54]. Sulphide has been determined also by a colour redox reaction with nitroprusside [55-57], In another sensitive reaction the sulphide ions decompose the Ag complex with Cadion 2B and Triton X-100 (e = 2.5-10 ) [58], In another indirect method sulphide releases the chloranilate ion from the Hg(Il) chloranilate [59]. Sulphide has also been determined by a method based on its reaction with bromate, followed by bromination of 2 ,7 -dichlorofluorescein by the bromine released [60]. 

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