Homoleptic complexes tungsten

Hexaalkoxides are only known for the two elements tungsten and uranium. In both cases, physical and spectroscopic data are consistent with monomeric, octahedral complexes. Tungsten hexaalkoxides are pale yellow or colorless substances that readily sublime and exhibit the expected H NMR and mass spectra.189 The totally symmetric v(W—O) stretch occurs between 530 and 600 cm-1 in the IR spectrum, depending on the alkyl substituents. In contrast, the homoleptic phenoxide W(OPh)6 is intense red in color due to the presence of a strong oxygen-to-metal charge transfer band in this d° complex which absorbs in the visible part of the spectrum.36... 

The homoleptic complex tris(2,4-di- r/-butyl-lA3,3A3-diphosphete) Mo 5 was obtained by reacting fac-(CO)3(MeCN)3Mo] with P-alkyne 44a <2002AGE4047>. The compound exhibits a unique temperature-dependent, reversible P-G bond formation between two of the heterocycles in the solid state. A related close contact between a P-atom of one Jt-coordinated lA3,3A3-diphosphete ligand and a C-atom of another one at the same metal center has also been observed for the tungsten analogue of 5 <2000TH1>. 

Neutral and cationic homoleptic see Homoleptic Compound) tungsten-isocyanide complexes W(CNR)6 and [W(CNR)7] + (R = alkyl, aryl) are known, and W(CO)6-n(CNR) (n=l-3) complexes may be prepared from W(CO)6 and excess isocyanide in the presence of catalytic amounts of C0CI2 or PdO. Isocyanides are isoelectronic with CO and also insert into the W-R bonds of alkyl complexes see Alkyl Complexei). For instance, the alkyl-nitrosyl complexes Cp"W(NO)(X)(CH2BuO (X = NHBuC OBu ) react with CNBu to afford -iminoacyl complexes, and the isocyanide complexes, Cp W(CO)2(Me)(CNR) (10, R = alkyl), rearrange to afford either jj -iminoacyl Cp W(CO)2( -MeCNR) (11) or Li azaallyl Cp"W(CO)2(/ 3-CH2CHNR) (12) derivatives, depending upon the reaction conditions (equation 4). ... 

By way of example, we can focus on the cyano complexes. Palladium and platinum can form homoleptic complexes with two, four, or six cyanides, depending on the metal oxidation state zero, two, and four, respectively. Rhenium forms heptacyano complexes in its - -3 and -t-4 oxidation states but forms an octacyano complex in its -t-5 state. Molybdenum and tungsten also appear commonly in octacyano complexes for +4 and - -5 oxidation states, and heptacyano analogs for the - -2 and - -3 cases, with only one occurrence of [Mo(CN)6]" anions in two oxidation states (n = 3, 4). Similar situations can be found with phosphine or carbonyl complexes. For instance, the group... 

In the case of the alkoxides and phenoxides of molybdenum(III) the interaction between metal atoms becomes dominant, leading to diamagnetic compounds of stoichiometry Mo2(OR)6 or Mo4(OR )i2(R/ = the less bulky Me or Et).78 These compounds are readily sublimable, (70-90 °C/ 10-3 Torr for Mo OPr1 ) and retain either their dimeric or tetrameric nature both in solution and the gas phase, as determined by cryoscopic measurements and mass spectrometry, respectively.78 An analogous tungsten(III) complex W2(OBu )6 can be obtained, but for other alkoxides the simple homoleptic compounds cannot be isolated.79,168... 

The striking advance since 1973 has been the synthesis of homoleptic isocyanides for vanadium and almost all of the post-group-VIA metals up to group IB (see Table I), and this has been helped in some respects by synthetic rationale. For example, the discovery of the simple reductive or nonreduc-tive cleavage of multiple metal-metal bonds in suitable dinuclear complexes of molybdenum and tungsten has provided a high yield route to W(CNPh)6 and [M(CNR)7]2+ (M = Mo, W) and related complexes (5-14). During these studies the series [M(CNR)7]2+ was completed with the isola-... 

A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. Two of the older methods, namely, the reaction of the hexacarbonyls with halogens in the presence of an isocyanide (775,116) or reactions of the salt Ag4Mo(CN)8 with isocyanides (74), have given six- and seven-coordinate products. Recently, however, the discovery of the reductive or nonreductive cleavage of multiple metal - metal bonds in dinuclear group VIA compounds by isocyanides has provided a facile route to the synthesis of a variety of homoleptic and related isocyanide complexes of these metals in reasonable yields. 

The nature of the complexes, synthesized from metal vapor and t-BuC=P, is highly dependent upon the nature of the metal. Thus, with iron, chromium, or vanadium atoms, pentaphosphametallocenes are obtained.51 In contrast, the phosphaalkyne and molybdenum or tungsten atoms afford homoleptic tris[i74-l,3-diphosphete] metal(O) complexes [M(tj4-P2C2- -Bu2)3] (M = Mo, W).51... 

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