Osmyl complexes

The most important members of this class are the osmium nitrido, and the osmyl complexes. The reddish-purple K2[OsNCl5] mentioned above is the result of reducing the osmiamate. The anion has a distorted octahedral structure with a formal triple bond Os=N (161pm) and a pronounced /ram-influence (pp. 1163-4), i.e. the Os-Cl distance trans to Os-N is much longer than the Os-Cl distances cis to Os-N (261 and 236 pm respectively). The anion [OsNCls] also shows a rram-effect in that the Cl opposite the N is more labile than the others, leading, for instance, to the formation of [Os NCl4] , which has a square-pyramidal structure with the N occupying the apical position. 

I osmyl complexes (a) 3-cenire rr bond formed hy overlap of ligand p, and nilar tv>nd is produced hy p, and d,. ovcrlapi, and Ihi MO diagram Isee lexij,... 

These CID results suggest that 02 is acting as a weak ligand in the Cr and Ru complexes but as a strong ligand with Os, maybe even forming an osmyl complex, [0=0s=0(bpy)]2+. 

Many ruthenyl and osmyl complexes with O donor ligands are known. The ruthenium compounds are usually prepared from [RuOJ, which can be generated by the oxidation of RuCl3 xH20 or [RUO2] with [104] . The osmium compounds are usually prepared from [OSO4] or K2[0s(0)2(0H)4]. 

Other osmyl complexes with O donor ligands include (PPh4)2[0s(0)2(0SiMe3)4], prepared from (PPh4)2[0s(0)2(Cl)4] and NaOSiMe3 and tra i s(0)2(trop)2] (trop = tropolonato C7H502 ), prepared from K2[0s(0)2(0H)4] and the ligand. ... 

Osmyl complexes of phthalocyanine have been claimed. Reaction of 0s04 and 1,2-dicyanobenzene at 180°C gives the dark blue Os02-Pc C6H4(CN)2, which is paramagnetic (/u,eff = 1.4 BM at 20°C) so this could well be an osmium(IV) or even an osmium(III) species (231). Another osmyl species claimed is 0s02Pc-3PhNH2, made from... 

Many osmyl complexes with group VI ligands have been reported. The purple diamagnetic potassium osmate K2[0s(0H)4(0)2] is the best known and is a useful starting material for the preparation of other osmyl or osmium complexes. It is best prepared from the reaction of 0s04 with excess KOH. The X-ray crystal structure of K2[0s(0H)4(0)2] shows that the complex has the trans-dioxo unit, with a d(0s=0) of 1.77 A and a 0=0s=0 angle of 180° (238,239). The acid dissociation constants of H2[0s02(0H)4] have been determined. 

The osmyl complex ra j -[0s02(en)2]2+ is discussed on p, 583 (see also Table 21). The only other species in this category is the deep red crystalline l[Os(en)3I2j4 , obtained by treatment of [Os(en)(en — H)2]Br2 with HI.79 Although analytical data fit the formulation reasonably well it seems unlikely that this really is an eight-coordinate osmium(VI) species an alternative possibility is [Os(en)(en — H)2](I3)2. 

In the area of osmium oxo chemistry there are three main topics of interest the tetroxide itself, the osmyl complexes which contain the irons 0=0svi=0 unit, and the cyclic oxo ester species which contain the OsVI moiety. This is perhaps the most appropriate point at which to comment briefly on the reasons for the stability of these three systems. 

Almost all osmyl complexes are octahedral (the few exceptions are treated on p. 582) and the great majority have linear or almost linear 0=0s=0 systems (Table 20) most are mononuclear but a few are binuclear with a pp -dioxo bridge. 

We classify here mononuclear osmyl complexes fran.s-[0s02X4] " by the nature of the ligand X binuclear osmyl complexes are considered on p. 595. 

Table 21 Vibrational and Other Data for Osmyl Complexes...

Osmyl complexes with nitrogen donor ligands.The ammine [0s02(NH3)4]C12 was first made by Fremy in 1844474 and is still sometimes called Fremy s salt . It is most easily made by reaction of trans-K2 [0s02 (OH)4] with ammonium chloride490 and is pale yellow and only sparingly soluble in water. On heating it decomposes to hydroxoammine complexes.491... 

The oxo esters Os02(02R)py2 are considered on p. 584 but since they are osmyl complexes we list X-ray data for four of these in Table 20 there are also IR data for them.490,497... 

Osmyl complexes with miscellaneous donors.The cyano complex K2[0s02(CN)4] is made from the action of aqueous KCN solution on OsO4.S05 There are a number of osmyl complexes with sulfur donors. Reaction of Os04 with sodium sulfite in alkali gives Na2[0s02(S03)4] 506 Raman and IR spectra suggest that the sulfite is S rather than O bonded. 7 There are complexes with thiourea, e.g. 

In the case of 0s204(04CsH12)py4-4H,0 a crystal X-ray study shows that, as expected, a binuclear osmyl complex is formed with a bridging tetrolato ring (Figure 31).522... 

The conformation of the central ring is essentially that of a skewed chair-boat form. The osmyl units (Os=0 = 1.72 A) are substantially bent (OOsO angle = 163.5(7)°) from the ester group (Os —O (ester) = 1.95 A) towards the nitrogen atoms (Os—N = 2.19 A),522 as in other osmyl complexes (p. 583). 

It is a consequence of the unusual structure of these complexes in both the solid (dimer) and solution (monomer) states that the IR and Raman spectra show v(0s=0) frequencies to be higher than in osmyl complexes.519,545 In particular the IR band assigned to vas(OsOz) lies near 890 cm 1 in all these oxo esters containing Os L units rather than the 830 cm-1 absorption typical of those containing OsL2 or Os(L) units such behaviour is more typical461 of cis rather than of trans dioxo complexes. 

As might be expected for chelating 0,0 donor ligands it is the intermediate trivalent state of osmium which is stabilized by /J-diketonates (as is the case for catechols, p. 597, and tropolonato complexes, p. 597). In the presence of rc-acceptor chelates such as bipy, phen and terpy the II state is favoured, while with halides the IV state is preferred. The apparent absence of an osmyl complex, e.g. 0s02acac2, is surprising. 

The K2[0s02(N02)4] reported in the early literature,204 made by reaction of 0s04 with nitric oxide in an aqueous solution of KN02, was later reformulated by the author as an osmyl complex, K2[Os02(N03)(NO2)3]503 and then, after second thoughts, as a different osmyl species, K2[Os-02(N03)2(N02)2].21... 

---