Molybdenum coordination number

When the calcined catalyst was exposed to air, so becoming hydrated, the Mo=0 stretching vibration in the infrared shifted to lower wavenumber (1000 to 950 cm" ) [94]. The shift to lower wavenumber was attributed to an increase of molybdenum coordination number from four to six as a result of binding of water molecules. Fig. 7. 23. 

Molybdenum (High Coordination Numbers) (see also Tables 5.12-5.14)... 

Much of what has been said so far in this chapter applies equally well to complexes of second- and third-row transition metals. However, there are some general differences that result from the fact that atoms and ions of the second- and third-row metals are larger in size than those of first-row metals. For example, because of their larger size (when in the same oxidation state as a first-row ion), ions of metals in the second and third rows form many more complexes in which they have a coordination number greater than 6. Whereas chromium usually has a coordination number of 6, molybdenum forms [Mo(CN)8]4 and other complexes in which the coordination number is 8. Other complexes of second- and third-row metals exhibit coordination numbers of 7 and 9. 

The data analysis in Table 9.3 summarizes the crystallographic information of the Co-Mo-S phase active for hydrodesulfurization. The Co-S distance in Co-Mo-S is 0.22 nm, with a high sulfur coordination of 6.2 1.3. Each cobalt has on average 1.7 0.35 molybdenum neighbors at a distance of 0.28 nm. Based on these distances and coordination numbers one can test structure models for the CoMoS phase. The data are in full agreement with a structure in which cobalt is on the edge of a MoS2 particle, in the same plane as molybdenum. 

An increase in the coordination number of molybdenum takes place in the second protonation step, which has a dramatic effect on the value of K2. Instead of the typical decrease of 3 to 5 log units from the first to the second protonation constant, K2 has in this case about the same value as Kx. In fact, these unusual values for the protonation constants compared to those of other oxyanions, along with the thermodynamic parameters AH° and AS0, were the basis on which the change in coordination number in the second protonation step was first proposed (54). Previously the small difference between the first and second pK value was interpreted in terms of an anomalously high first protonation constant, assumed to be caused by an increase in the coordination number in the first step (2, 3, 54-57). 

In dilute solutions ( 10-5 M) the equilibria (2) are governed by log K values of 3.87 and 3.70 respectively (3M NaC104, 25 °C). While the Mo04 anion is certainly tetrahedral, the anomalously small value of log has been taken to indicate an expansion of coordination number upon protonation. In view of the prevalence of octahedral cis M0O2 units in molybdenum(VI) chemistry, formulas such as [Mo02(H20)(OH)3] and [Mo02(H20)2(OH)2]... 

Molybdenum is a metal of the second transition series, one of the few heavy elements known to be essential to life. Its most stable oxidation state, Mo(VI), has 4d orbitals available for coordination with anionic ligands. Coordination numbers of 4 and 6 are preferred, but molybdenum can accommodate up to eight ligands. Most of the complexes are formed from the oxycation Mo(VI)022+. If two molecules of water are coordinated with this ion, the protons are so acidic that they dissociate completely to give Mo(VI)042, the molybdate ion. Other oxidation states vary from Mo(III) to Mo(V). 

Among the metals which have achieved prominence only recently, molybdenum must certainly occupy first place. It has attracted interest chiefly because of its role in biological processes and also because it shows a great variety of oxidation states and coordination numbers. Steifel s extensive review of molybdenum complexes covers their chemistry up to 1977.3 Because of their use in nuclear medicine, the complexes of technetium have received recent interest.4... 

It is interesting to speculate on the chemical properties of molybdenum which make it suitable for its biological function. Obvious features in the chemistry of molybdenum are (a) a range of oxidation states which can be stabilized in aqueous solution by the common ligands of biology (b) the formation of oxo compounds and the sulfur analogue (c) the ability to participate in atom-transfer reactions and (d) the possibility of higher coordination numbers. 

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