T H I S i s the first symposium involving the chemistry o f molybdenum. Molybdenum has belonged almost exclusively to the metallurgists. It still does in spite of a chemical history extending back nearly t w o centuries to Scheele’s discovery in 1778. Berzelius devoted a great d e a l o f his effort a n d talent t o investigating the chemistry o f molybdenum, a n d his “Lehrbuch der Chemie,” published in 1826, contains a substantial section devoted to researches on the subject, both o f his own a n d o f others. Even several decades before that, Lavoisier h a d suggested new names for the great number of known molybdenum compounds to fit them into his newly systematized chemical nomenclature. But metallurgical uses still consume most o f the world’s output of molybdenum. The best estimates place the present consumption o f molybdenum in ferrous alloys a t about 90% o f the total. A small, but important, output o f metallic molybdenum and high molybdenum alloys used principally in electrical applications accounts for another 5%. That leaves no more than 5% to cover all chemical uses. Even 5% represents a n increase in the amount of molybdenum going into chemical uses following the end o f stimulation of steel production in W o r l d W a r II. Element 42 thus suffers from comparison with COURTESY CLIMAX MOLYBDENUM CO.= the extraordinary value o f the metal in metallurgy. But the element possesses exciting posFlotation units separate molybdenum disulfide from ground rock sibilities through its remarkable chemical reactivity a n d its highly variable chemical character. Perhaps the-reason that these are not better known a n d dextrous, bisexual, a n d polygamous.” Linus Pauling in his more generally utilized i s the chaotic state o f the chemical “General Chemistry’’ (1 948) devoted only eighteen words literature o f molybdenum. The comprehensive treatisesto the chemistry o f molybdenum: “The chemistry o f molybGmelin, Abegg, Mellor, Friend-report everything pubdenum i s complicated. It forms compounds with oxidation lished about the chemistry o f the element, true and false, numbers o f +6, +5, 4-4, f3, a n d +2.” probable a n d fantastic. Pokorny published a brief monoIt is still difficult to present a satisfying picture o f the g r a p h on the chemistry o f molybdenum in 1927, but a t that whole chemistry o f molybdenum. It is our hope that this time the development o f molybdenum pigments h a d not yet very inadequacy will spur chemists everywhere to seek solubecome important. The late beloved Edgar Fahs Smith was tions to the many problems that remain to b e solved. fond o f describing molybdenum to his classes as “ambiARTHUR LINZ, Chairman
the element possesses exciting possibilities
B
AFFLIKG chemical puzzles characterize molybdenum, because it has several highly variable properties and because much of the research t o determine its characteristics was done in the early days of chemistry. Scheele, Lavoisier, and Berzelius, all names t o conjure with even while these men lived, conducted early studies of molybdenum compounds and published much about them. STow we realize more than ever before that it was impossible for the buret-and-balance methods used by these eminent chemists and by others for more than a century after them t o resolve the chemical problems of molybdenum. It is not surprising that the literature of molybdenum chemistry should be filled with uncertainties and that many of the important problems of molybdenum behavior should remain unsolved; it was practically impossible even t o define the problems exactly by the means available t o the older chemists. Modern chemists readily discern complexities in molybdenum chemistry and contradictions in the record that were not apparent August 1955
t o the original investigators. Even today many of these anomalies and uncertainties remain unsolved, and furthermore some cannot even be stated clearly as problems that can be Bolved! The variable valence of molybdenum is its basic complexity. This is particularly confusing because t h e observed valences are different for compounds with different elements and because there are gaps in each series. For instance, compounds are known in which the valence or oxidation state of the molybdenum atoms assumes each of the values in the series, + 2 , +3, +4, +5, and +6, and in which the atoms have coordination numbers of 4, 6, and 8, hut in no series of related compounds are all these values represented. I n addition to these normal states, molybdenum assumes a nonvalent, zero state in the hexacarbonyl, Mo(C0)G. Disproportionation commonly occurs t o transform compounds of molybdenum of a single valence t o mixtures containing several valence states; coordination numberschange readily under slightly modified conditions. Molybdate ions in solution readily aggregate
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ENGINEERING, DESIGN, AND EQUIPMENT or disaggregate, depending on pH, t o yield mixtures of polyions by polymerization-condensation reactions. These compounds are so sensitive t o changes of concentration and p H that they have baffled efforts t o define them accurately. Certain extraneous ions introduce other complications by shifting the character of these reactions and leading t o a variety of complex heteropolymolybdates. Only two oxides of molybdenum are definitely proved t o exist, MoOz, and MOO,, although the existence of other corresponding compounds strongly suggests t h a t there must be binary oxides of the di-, tri- and pentavalent metal. Even the most refined modern techniques, t h a t are free from the limitations of the usual methods of chemical analysis, fail to confirm fully the existence of these other highly probable binary oxides. The oxides missing from the binary series exist, if a t all, as hydrated compounds or in ternary compounds where another element is substituted for part of the oxygen. For instance, addition of ammonia t o a n aqueous solution of a compound of trivalent molybdenum yields a precipitate described as &Io( OH),. CJpon drying this compound loses water t o form what is presumably MoOOH, and when strongly heated, this compound decomposes and disproportionates t o yield MoOz and metallic &/Io. T h e pentahydroxide undergoes a similar series of changes ultimately yielding a mixture of the di- and trioxides. Divalent molybdenum exists, if a t all, only in ternary complexes t h a t do not yield simple hydroxide compounds but t h a t remain complex until they undergo disproportionation when heated t o a temperature high enough t o decompose them. If these complications among the oxides are not enough, then consider t h a t values given for t h e specific gravity of Moot, a highly stable compound, prior t o a decade ago showed a variation of more than 40% ( 2 ) . Then note t h a t all published calculations of thermodynamic properties of molybdic oxide before 1951 were based on values of its vapor pressure that the investigator who determined them did not believe ( I ) . The first was corrected b y x-ray measurements by Magneli ( 5 ) and the second by new determinations made b y Ueno ( 7 ) . While these new values seem far more reliable than the old it is possible t h a t even these may have to be corrected again soon. Besides the simple oxides, MOO, MozO3, and Mo206, analytical values had suggested a number of other complex oxides such as Mo6012and Mo308. The problems connected with molybdenum blue introduced still other complications from the notion built on analytical data t h a t this compound( s?) is actually a molybdenum ~ a lof t molybdic acid. The most careful analytical work could not alone resolve t h e problem, and although more than 100 years have passed since Berzelius worked on molybdenum blue, confirmation is still not certain. Best present information (6) suggests that the several molybdenum blues are highly hydrated compounds containing molybdenum atoms in a t least two different valence states. Recent researchers ( 3 ) have applied the most modern methods in an attempt t o unravel the complexity of the red complex of reduced molybdenum with thiocyanate, but the identity of this compound, like molybdenum blue, still appears to remain unsolved. Fortunately, the conditions of its formation can be controlled t o give a dependable color for the colorimetric determination of molybdenum, a convenient and reliable method. Characteristic of molybdenum compounds too is their ready reaction with water. This may take the form of chemical changes in the compounds themselves with the introduction of hydroxyl or oxygen in the compound, or as often occurs, water may form coordination compounds with the molybdenum-containing molecule, or both may happen a t once. This inclusion of water (or its elements) in practically every molybdenum compound that is formed in aqueous solution greatly complicates the chemistry of the molybdates as well as that of many molybdenuln complexes-molybdenum blue, the ryanides, and the thiocyanates, aniong others. 1494
Molybdenum’s variable valence toward oxygen is only l e v y confusing than toward the several halogens. The stable fluoride, for instance, is MoF6, and this is t h e only binary compound of fluorine and molybdenum whose existence is proved. Fluorine forms a number of fluoromolybdates by replacing part of the oxygen in the normal molybdates. T h e stable chloride of molybdenum is the pentavalent compound, although chlorides of the lower valencee are reported. Doubt is cast on molybdenum bromides and iodides that have been reported as the tetrabromide and the diiodide. If these compounds d o in fact exist, they easily decompose. The importance of molybdenum pentachloride commercially stems from the fact that it yields molybdenum hemcarbonyl when it is reduced by zinc dust in ether solution under carbon monoxide a t high pressure ( 4 ) . The molybdates are characterized by a remarkable series of polymerization-condensation reactions in solution. The literature is filled with descriptions of these polymolybdates. The degree of polymerization of molybdates depends closely on the hydrogen ion concentration of the solution and the course of the reaction is strongly influenced b y the presence of a second acid (required t o lower the pH to effect the higher polymerization) because this acid may form the central group in a heteropolymerization. A great many systematic researchers have sought t o unravel these overlapping problems of the polymolybdates and the polyheteromolybdates. A great many extremely able researchers have worked in this field and their results elucidate many facets of the large problem, b u t not all of the questions are yet answered. Modern x-ray techniques have sup lied a good picture of the heteropoly acid formed by molybd% around a phosphoric central group and this has cleared u p much imcertainty here. Similarly atomic configurations measured for ammonium molybdate--“85% molybdic acid”-answer another group of questions about the isopolymers. I n spite of these important determined points, no satisfactory final answers are yet available to many other questions. For instance, we know very little indeed about the characteristics of these ions in solution, where their behavior is important, and we can only guess what existed in solution by examining what came out of it, that really interests us ver little. I n all of these compounds water performs a vital function and the variable water content of the crystallized iso- and heteropoly compounds further complicates these problems. The variable coordination value of molybdenum atoms seems particularly active t o vary the water content of crystals of these compounds and thus t o introduce n e n piinales t o an already beclouded situation. These brief staiements suggest some of the problems t h a t await solution in the chemistry of molybdenum. Their complexity, whether it eprings from the innate intricacy of the subject or from the great power of authority t o misdirect our attention, challenges the researrhers of today, just as it did our chemical forebears. Rut the rising generation has a far brightcr prospect of solving them.
D. H. KILLEFE’KH 168 Westchester Ave., Tuckahoe 7, N . Y . LITERATURE CITED
(1) Feiser, J., Met. E T E 28, , 297 (1941). (2) Gmelins “Handbuch der anorganischen Chemie,” Systeln 53, p. 92, Verlag Chemie, G.m.b.H., Berlin, Germany, 1935. (3) Hiskey, C. F., and Rleloche, V. W., J . Am. Chem. Soc., 62, 1563 (1940). (4) Xilleffer, D. H., and Linz, A . , Molybdenum Compounds, p. 61, Interscience, New York, 1952. (5) Magneli, A., A r k . Kemi, Min. Grol., 24A, No. 2 (1946). (6) Schirmer, F. R . , Jr.. Audrieth, L. F., Gross, 8. T., 3IaClellm, D. S.,and Seppi, L.J., J . Am. Chem. SOC.,64, 2543 (1942). (7) Ueno, K., J. Chem. SOC.Japan. 62, 990 (1941). K E C E ~ V Efor D review January 19,1955.
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ACCEPTED May 2, lY5.5.
Vol. 47, No. 8
