Osmium vi

Osmium(vi).— [0s04] has been shown to react with monoalkenes (R) to give five-co-ordinate mono- and di-esters of the saturated diolato ligand (02R)2-, namely [0s204(02R)2] and [OsO(02R)2].104 These complexes were assigned structures (21) and (22). respectively, on the basis of i.r. and molecular weight measurements. 

In contrast, treatment of [0s04] with alkenes (R) or acetylenes in the presence of tertiary nitrogen bases (L = pyridine or isoquinoline) yielded the octahedral osmyl species (23) and (24). From measurements of the ionic products of the sparingly 

The mixed-ligand nitrido-complex (NOsO(bigH)2](OH)3 (bigH = C9N5H7) has been prepared (equation 25).107 Its unusually low frequency v(Os=N) stretch was 

Ph4As[OsNCl4] 0.036 anion has a square-base pyramidal shape a 

Sn—Os—Sn geometry. The Os—Sn distance of 2.712 A indicates little double bond character 

---

Interaction between osmium(IV) and osmium(VI) and derivatives of dimerkaptotiopiron (DT) has been studied by amperemetric, potentiometric and spectral photometric methods in different mediums. It has been found out that in reactions of methyldimerkaptotiopiron (R) with Os(IV) and Os(VI) complex formation prevails. It has a step-like nature, being revealed by such change of ratio of Os R in the course of titration as Os(VI) R - from 1 1 to 1 4 in acid medium and from 2 1 to 1 4 in weak alkaline medium Os(IV) R - from 1 1 to 1 5 in medium of H SO and from 1 1 to 1 6 - in HCl medium. 

Consequent potentiometric titration of osmium(IV) and laithenium (IV) in their mixtures has been canied out in broad range of concentrations from 1 mkg to 200 mkg in samples of 20 ml. It has been shown the possibility of amperemetric determination of osmium(VI) in binary and triple systems with silver(I), platinum(IV), palladium(II), gold(III), founded on formation of corresponding compounds with dimerkaptotiopiron, having a different solubility. The deteriuination of Os(VI) is possible under tenfold - hundredfold excess of above mentioned metals. 

The extraction of osmium(VI) and osmium(IV) compounds with dimerkaptotiopiron by polar solvents has been resear ched. 50 -100 - multiple concentrating has been carried out in acid solutions regardless from chemical form of Os(VI) and Os(IV) with the following determination of each of them from one sample by amperemetric titration of reextracts by means of DT. A methodic of atom absor ption determination of osmium has been suggested. 

The osmium(VI) complexes 0s02X2(PR3)2 are not generally obtainable with the smaller alkyl and alkyl(aryl)phosphines, which tend to be good... 

The olive-green osmium(VI) octaethylporphyrin complex 0s02(0EP) (IR v(0s—0) 825 cm-1) is representative of a number of osmyP porphyrins [185] they can readily be transformed into a number of osmium porphyrins in lower oxidation states (Figure 1.73). 

Another type of osmium(VI) compound involving multiple bonds can be viewed as a derivative of OsN3+. The nitrides have attracted interest as they are often photoluminescent... 

All the OsNX4 complexes are distorted square pyramids (with N-Os-X angles of 103.7 to 104.5°) [188]. The stability of an osmium(VI) to iodine bond is unusual and is presumably owing to the extensive Os=N 7r-bonding reducing the positive charge on the metal and stabilizing it to reduction. 

The change in formal oxidation state from osmium(VI) to osmium(II) is noteworthy [192],... 

Natiirliche Aminosauren werden in der Regel ohne merkliche Racemisierung zu Amino-alkoholen reduziert7. Zur Reduktion von Osmium(VI)-estern der a-Amino-sauren mit komplexen Hydriden s. Lit.8. 

The osmium-catalyzed dihydroxylation reaction, that is, the addition of osmium tetr-oxide to alkenes producing a vicinal diol, is one of the most selective and reliable of organic transformations. Work by Sharpless, Fokin, and coworkers has revealed that electron-deficient alkenes can be converted to the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side [199]. One of the most useful additives in this context has proved to be citric acid (2 equivalents), which, in combination with 4-methylmorpholine N-oxide (NMO) as a reoxidant for osmium(VI) and potassium osmate [K20s02(0H)4] (0.2 mol%) as a stable, non-volatile substitute for osmium tetroxide, allows the conversion of many olefinic substrates to their corresponding diols at ambient temperatures. In specific cases, such as with extremely electron-deficient alkenes (Scheme 6.96), the reaction has to be carried out under microwave irradiation at 120 °C, to produce in the illustrated case an 81% isolated yield of the pure diol [199]. 

Osmium(VIII) tetraoxide (0s04) is an effective reagent for the cis hydroxylation of olefins under stoichiometric conditions as well as in a variety of catalytic variants.213 Under both catalytic and stoichiometric conditions, the critical step is the formation of an osmium(VI) cycloadduct, the formation of which is dramatically accelerated in the presence of amine bases such as pyridine,214 i.e.,... 

The osmium(VI) arylimide Os(TTP)(NAr)2 (TTP = tetra(p-tolyl)porphyrin Ar = />-N02QH4) also has short Os=NAr distances (1.820-1.822 A) [186], One of the rare examples of a compound with a cis-0s02 group is made ... 

The first attempt to effect the asymmetric cw-dihydroxylation of olefins with osmium tetroxide was reported in 1980 by Hentges and Sharpless.54 Taking into consideration that the rate of osmium(VI) ester formation can be accelerated by nucleophilic ligands such as pyridine, Hentges and Sharpless used 1-2-(2-menthyl)-pyridine as a chiral ligand. However, the diols obtained in this way were of low enantiomeric excess (3-18% ee only). The low ee was attributed to the instability of the osmium tetroxide chiral pyridine complexes. As a result, the naturally occurring cinchona alkaloids quinine and quinidine were derived to dihydroquinine and dihydroquinidine acetate and were selected as chiral... 

Osmium(VI) forms irons-0s02(malt)2 (98), while dioxouranium(VI) forms irons-U02(malt)2 (98) and many hydroxypyridinonate complexes, with bidentate and with tetradentate (177) ligands - trans-UO2L2 and UO2L, respectively. Several actinide elements form complexes with hexa- and octa-dentate hydrox5rpyridinonates (see Section IV.C.7 later). 

The vast majority of ruthenium(VI) and osmium(VI) complexes are those stabilized by metal-ligand multiple bonds. These are mainly imido, nitrido, and 0x0 species. 

The complex (PPh4)[Os NC(CCl3)NCCl(CCl3) Cl5] may be regarded as an imido complex of osmium(VI), made by the reaction of OS2CI10 with trichloroacetonitrile and has been characterized by X-ray structural studies. The Os—N distance of 1.97 A is slightly shorter than that expected for a single bond. 

---