By Ed Perley
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Some years ago I did a research project to meet the requirements for my Masters degree in chemistry. The subject of my masters thesis was a discussion of Molybdenum(V) solutions in hydrobromic acid. A fair amount of study had been done on these solutions, but conclusions made by diferent workers were contradictory in many ways. My objective was to get a clearer view of what chemical structures, usually referred as species, are present at different acid concentrations.
Since I am not writing this in a chemical journal, where articles must be scrutenized and approved by experts, I am not going to say much about the conclusions I made. My thesis is in the Briggs Library at South Dakota State University, Brookins, South Dakota, if anyone is reallly interested. I am going to use this page to discribe some of the things I observed in the course of my work.
A list of reference is given at the end of this page.
Most of the time I spent in my research was to answer this question. One would think this would be easy, since it is easy to precipitate oxobromomolybdate(V) compounds from concentrated hydrobromic acid solution by adding a suitable cation. Unfortunately, different cations precipitate a total of four different oxomolybdate(V) structures. These structures are shown below with the chemical formulas of the oxobromomolybdate(V) compounds I prepared. Most of these compounds were precipitated from concentrated(>12M) hydrobromic acid solutions. QuinolineH)2 MoObr5 and (dipyridylH)2 MoObr5 were separated from 6M hydrobromic acid solutions.
Type I Compounds: (MoOBr5)2-
Type II Compounds: (MoOBr4 - H2O) 1-
(CH3)4N MoOBr4 - H2O
(CH3)3phenN MoOBr4 - H2O
Type III Compound: (MoOBr4 1-)
Type IV Compounds: (MoOBr4) 1-
Most oxomolybdate(V) compounds show a single distinct band at 970 to 990 cm-1 that has been attributed to a terminal molybdenum - oxygen stretching vibration. Type IV compounds are unusual in that they only show a weak band there. Instead, there is a very broad, intense band at 879 cm-1. I had not found any reference to kind of spectrum in the the literature. These two compounds are quite obiously different from the other oxotetrabromomolybdates(V) I studied.
An X ray analysis of one of these, the pyridinium salt, suggested that each terminal oxygen is associated in some way with the vacant fifth position on another oxobromomolybdate ion. Unfortunately, the data was not good enough to allow a definite conclusion to be made about the structure. This was probably because of degradation of the crystal resulting from exposure to humidity.
Electronic Absorption Spectrum of
All of these compounds in hydrobromic acid solution show the same distinctive electronic absorption spectrum. In concentrated acid there are two distinct peaks at 413 nm and 376 nm and two weaker bands at 472 nm and 698 nm . There is strong, less distinct, absorption in the ultraviolet.
It was necessary to obtain electronic spectra of the solid compounds. One method I used was diffuse reflectance spectroscopy. Another, more sucessful, procedure was electronic absorption spectra of very small quantities of compounds pressed into potassium bromide pellets. With these, I was able to obtain spectra that were considerably more sharp and distinct than those shown by diffuse reflectance. I also found a solvent, that small quantities of most of these compounds could be partially dissolved in. It is stock, off the shelf chloroform. The resulting electronic absorption spectra were for the most part consistant with the solid spectra.
Other solvents that other workers had said were suitable, were tested. Spectra in acetonitrile were the same for all of the compounds, and the cations of the Type I compounds precipitated out their bromide salts. Solutions in DMSO were unstable with much different spectra.
Electronic Absorption Spectrum of (CH3)4N MoOBr4 - H2O
Electronic Absorption Spectrum of (NH4)2 MoOBr5
Type I compounds showed an electronic absorption spectrum that was considerably different from the spectrum of the hydrobromic acid solutions. Types II, amd III showed spectra that were most similar to that shown by concentrated hydrobromic acid solutions. The chloroform solutions of Type IV showed electron absorption spectra similar to those of Types II and III. But the spectra of the solid compounds were intermediate in appearance between Types I and Types I and II.
It appeared that all but one of the compounds had something associated at least weakly, in the fifth position, opposite the terminal oxygen. The type III compound was the lone exception. I suggested that the very bulky cation blocked the fifth position in this compound.
I came to the conclusion from this and other experimental data, and the literature, that the Molybdenum(V) species in concentrated hydrobromic acid is most like the the Type II oxobromomolybdate(V) anion.
Preparation of (NH4)2MoOBr5 and [pyrhMoOBr4]n
Kbr Pellet Spectra: How to do it
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