Compounds of Chromium, Molybdenum and Tungsten

Table 23.2 Oxidation states and stereochemistries of compounds of chromium, molybdenum and tungsten...

Fischer first prepared carbyne complexes by the action of boron halides on carbene compounds of chromium, molybdenum, and tungsten. 

Fourteen new tricarbonylchromium complexes of benzodiazaboroles, phenyl-boranes and related compounds have been prepared and characterired. In all compounds n.m.r. data indicate that the Cr(CO)8 moiety is bonded to the benzo and phenyl rings. New types of co-ordination compounds of chromium, molybdenum, and tungsten with phosphino-aminoboranes of general formula (COsMPRSBRS, (CO)4M(PRia)aBR , and (CO)3M(PRS)B have been synthesized. Suitably substituted carbonyl compounds of the same metals reportedly react with l-phenyl-4,5-dihydroborepin (35) to yield stable complexes in which the boron atom apparently participates in metal-ligand bonding. "... 

Trithiolene complexes are also known, for instance those of chromium, molybdenum, and tungsten [592f], having an octahedral geometry. Structures of the complexes above, as well as of a series of other thiolene (thiol) compounds, have been studied using various physical-chemical techniques (x-ray diffraction, NMR, EPR, and UV spectroscopy). Considerable attention has been devoted to their reducing and photochemical properties [1, vol.2]. 

The uranyl ion has a totally symmetric groundstate28 with Q = 0+. Hence, it is not open to the same kind of controversy as was quite vigorous around 195517,241 about whether uranium belongs to the 5 f group or is a higher homolog of chromium, molybdenum and tungsten, and hence is a 6 d element. It turned out that all non-metallic compounds of the... 

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. 

Oxofunctionalization with dimethyldioxirane can be additionally applied for the synthesis of a vast number of chromium-, molybdenum-, and tungsten-silanols (lOa-lOh), -silanediols (11a, 11b), and -silanetriols (12a, 12b). These compounds are usually not available via the hydrolysis route due to the electronrichness of the metal fragments, for which the Mc3P- and CsMcs-ligand are responsible, creating a silicon unsusceptible towards nucleophilic attack. 

The hexacarbonyls of chromium, molybdenum, and tungsten are mononuclear the internal bonding parameters are presented in Table 3. The coordination geometry is that of a regular octahedron of idealized Oh syuunetry. The crystal packing is due to weak van der Waals interactions see van der Waals Forces), which explains the volatility of these compounds (the vapor pressures of Cr(CO)6, Mo(CO)6, and W(CO)6 at 30 °C are 0.28, 0.27, and 0.06 mmHg, respectively), and their solubility in organic solvents. 

Methyl-3,5-diphenyl-l//-thiopyran 1-oxide forms red air-stable complexes with carbonyls of chromium, molybdenum and tungsten. (78CB1709). X-Ray diffraction measurements on these compounds confirm the non-planarity of the thiabenzene oxide nucleus and reinforce the evidence for high inversion barriers at sulfur. These complexes are found in isomeric forms with either the sulfur-oxygen bond axial to the half-chair conformation adopted by the ring, or the S- methyl group axial the complexes cannot be interconverted. 

The six unpaired electrons of the atoms of chromium, molybdenum and tungsten may form three double bonds with three carbonyl groups. Three unfilled p orbitals remain, which may be filled by the donation of an electron pair from each of three carbonyl groups. The structure of these compounds will therefore be represented by the valence-bond structure XXII ... 

The chemistry of the dicarbonyl(7 -cyclopentadienyl)nitrosyl and the chloro-(i7 -cyclopentadienyl)dinitrosyl complexes of chromium, molybdenum, and tungsten [i.e., fT -CsHs)M(CO)j(NO)] and [(i7 -CsHs)M(NO)2Cl] has not been studied extensively, partly because of the various difficulties associated with their preparation. The procedures described below are of general applicability to all three metals and lead to the desired compounds in high yields. The carbonyl nitrosyl complexes are the synthetic precursors of the chloro nitrosyl complexes and so their preparation is described first. 

The use of liquid ammonia well above its normal boiling point as a solvent and reactant has been fruitful. The metal hexacarbonyls themselves have afforded the complexes [M(CO)3(NH3)3] (M = Cr, Mo, W) and [Cr(CO)s NH3] 81). The tetracarbonyl(dipyridyl)metal complexes of chromium, molybdenum, and tungsten and their (o-phen) analogs react with liquid ammonia to give very air-sensitive compounds, such as [M(CO)3(dipy) (NH3)], which finally yield mainly [M(CO)3(NH3)3] 81). The [M(CO)4... 

Substituted-benzene-metal tricarbonyls of chromium, molybdenum, and tungsten do not have any intense band in the 900 to 980 cm range, while the parent compounds absorb at 902 and 978 cm, respectively 131). The IR spectra of hexamethyl-, hexaethyl-, and iefa-diisopropylbenzene(Mo, W) tricarbonyls show weak to medium intensity bands, probably due to some... 

I.3.4.2.6. Compounds with Unusual Double Bonds 1,3-Dipolar cycloaddition of l-chloro-2-phenyl-2-trimetkylsilyl-l-phosphaethene with nitrile oxides, followed by elimination of Me SiCl, results in 3,5-diphenyl-l,4,2-oxaphosphazole 190 (356). Chromium, molybdenum, and tungsten pentacarbonyls of 3,5-diphenyl-).3-phosphinins react with nitrile oxides to give the corresponding 1,3-dipolar cycloadducts, at the P = C bond, see 191 (Ar = Ph, Mes) (357). 

Although all of the above elements catalyze hydrogenation, only platinum, palladium, rhodium, ruthenium and nickel are currently used. In addition some other elements and compounds were found useful for catalytic hydrogenation copper (to a very limited extent), oxides of copper and zinc combined with chromium oxide, rhenium heptoxide, heptasulfide and heptaselen-ide, and sulfides of cobalt, molybdenum and tungsten. 

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