Monodentate ligands, molybdenum

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

In seven-coordinate molybdenum peroxo complexes of the type Mo(0)(02)2L2, there is the problem of dissociation of the complex in the case of monodentate ligands like pyridine, HMPA, DMF or H2O. Bidentate hgands like 2,2 -bipyridine, on the other hand, tend to give insoluble complexes. A solution to this problem was presented by Thiel in 1997 and 1998 . His group utilized the Mo-diperoxo complex 160 bearing a pyrazolylpyridine ligand. 

Molybdenum trioxide, intercalation into, 12, 823 Molybdocenes, as anticancer agents, 1, 892 MOMNs, see Metal-organometallic coordination networks Monisocyanides, with silver(I) complexes, 2, 223 Monitoring methods, kinetic studies, 1, 513 Mono(acetylacetonate) complexes, with Ru and Os halfsandwich rf-arenes, 6, 523 tj2-Monoalkene monodentate ligands, with platinum divalent derivatives, 8, 617 tetravalent derivatives, 8, 625 theoretical studies, 8, 625 zerovalent derivatives, 8, 612... 

Few monomeric d4 bisalkyne complexes with only monodentate ligands in the coordination sphere have been reported. The only molybdenum(II) complex in this category is Mo(PhC=CR)2(CO)(NCMe)I2 (R = Ph, Me), which was included in a reaction scheme illustrating products accessible from cleavage of the iodide bridges in dimeric [Mo(PhC=CR)(/r-I)-(I)(CO)(CNMe)]2 reagents (51). Efforts to convert Mo(RC=CR)(CO)-L2X2 complexes to bisalkyne derivatives were not successful (46). 

Fe2(CO)9, and Fe3(CO)12, respectively. Similar disproportionations occurred with Ni(CO)4 and Co2(CO)8 which gave anionic species such as [Ni2(CO)6]2, [Ni3(CO)8]2, [Co(CO)4] , etc., upon treatment with ammonia or other amines. In contrast to the carbonyls of iron, nickel and cobalt, those of chromium, molybdenum and tungsten reacted with pyridine and 1,2-ethylenediamine to afford substitution products of the general composition M(CO)6 (py) (n = 1, 2, and 3) and M(CO)4(en) with the metal remaining in the oxidation state zero [25], Mainly as the result of this work, Hieber became convinced that the metal carbonyls should be regarded as true coordination compounds, and the coordinated CO should not be considered a radical but a monodentate ligand like NH3, pyridine, etc. He held this view despite the criticism by several of his contemporaries [3, 19] and was very pleased to see that in most textbooks published after 1940 this view had been accepted. 

Green, M. L. H., Braithwaite, E. R., Haber, J., Eds. rs Molybdenum an Outline of its Chemistry and Uses, Elsevier Amsterdam 1994, Chapter 2. For an early system to provide a formal organization of coordination compounds with monodentate ligands, see King, R. B. Adv. Chem. Ser. 1967, 62, 203-220. 

An important recent development in the area of the monodentate amido metal chemistry has come from the group of Cummins who introduced ligands of the type —N(R)Ar (for example, R = Bu , 1-adamantyl (I-Ad), 2-adamantyl (2-Ad), —C(CD3)2(CH3) Ar = Ph, C6H3Me2-3,5, C6H4F-4).These allowed the isolation of an Mo N(R)Ar 3 species having three-coordinate molybdenum as well as three-coordinate derivatives of several other metals. Apart from [M N(SiMe3)2 3] (M = Y," La), the molybdenum compounds are the only examples of structurally characterized open shell, three-coordinate homoleptic amides... 

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