Osmium, mononuclear complexes

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

The analogous osmium polymers have also been studied in great detail. The synthetic procedures required for these metallopolymers are the same as those described above for ruthenium however, the reaction times are longer. The similarity between the analogous mononuclear and polymeric species is further illustrated by the fact that the corresponding osmium polymers have considerably lower redox potentials and are also photostable, as expected on the basis of the behavior observed for osmium polypyridyl complexes. 

Although the above discussion is centered on the synthesis of polymeric osmium and ruthenium complexes, the methods employed are also very successful in the preparation of mononuclear complexes. In this context, the preparation of ruthenium or osmium complexes which are suitable for the formation of self-assembled monolayers (see Section 4.3 above) can be prepared by using the same approach. Starting from the precursor [M(bpy)2Cl2], one chloride atom can be replaced to yield complexes of the type [M(bpy)2Cl L]+, where L is the surface active ligand. In the presence of water, species of the type [M(bpy)2(L)2]2+ are obtained. 

The arrangement of this large section on osmium oxo complexes follows the pattern of other sections in this chapter. We consider mononuclear complexes first, by increasing oxidation state and, within each oxidation state, we consider those complexes with the largest number of oxo ligands first. The same procedure is then followed for binuclear species. For convenience, however, mononuclear and polynuclear oxo esters are considered together. 

On strongly basic oxides such as MgO, osmium cluster complexes are converted irrespective of their nuclearity to mononuclear Os carbonyls bound to Mg " on MgO at higher temperatures (above 200°C). Under CO -I- H2 at 473 K, a mixture of H30s4(C0)i2 and OsioC(CO)24 is regenerated (64), which is active for the methanation reaction (Fig. 18) ... 

A series of osmium carbonyl complexes have been prepared by the reaction of OsO-with CO or decomposition of Os3(CO)i2 [226] and a mononuclear homoleptic osmium carbonyl complex, Os(CO)5, is also known. It is a volatile, colorless liquid and is the most robust M(CO)5 type complex of the iron triad against both oxidation and heat but it gradually loses CO to form Os3(CO)i2. Multinuclear osmium carbonyl clusters such as Os3(CO),2, Os5(CO)i6, Os5(CO)i9, Os6(CO)ig, 087(00)2, and OsgfCO).. have also been reported [227]. In this section, several carbonyl complexes based on Os3(CO)j2are described. 

The first osmium Tp complex was prepared by Singleton et al in 1990 via the reaction of the -carboxylato dimer 0s2(jU-02CMe)2(C0)6 with KTp in refluxing methanol to give Tp20s2(C0)4 Os-Os). Treatment of this dinuclear compound with Br2 affords the mononuclear Os(II) complex TpOsBr(CO)2 via oxidative cleavage of the Os-Os bond (Scheme 35). 

Osmium(IV) complexes containing terminal 0s=0 units are rather rare, and it is more usual for a bridging mode to be adopted as in [OszOClio]" - One example of a mononuclear species is shown in scheme 23.89, in which the Os(IV) dioxo derivative is obtained by activation of molecular O2 via an Os(VI) intermediate. The square planar geometry of the Os(IV) product is unusual for a configuration. The precursor in scheme 23.89 is prepared by treating OsCls xHzO with P Pr3, and has a distorted 6-coordinate coordination sphere. 

The formation of either mononuclear or binuclear ruthenium and osmium carbonyl complexes bearing bis(2-pyridyl)amine (Hdpa) or the deprotonated ligand (dpa) can be driven by choice of solvent and reaction temperature. For example, [Ru3(GO)i2] reacts with Hdpa in HGl solution at 200 °G to give mainly m,m-[Ru(hdpa) (GO)2Gl2], whereas in toluene solution in the absence of HGl at 200 °G, Ru(dpa)(GO)2 2 predominates. In related chemistry, [Ru(GO)2Gl2] reacts with bis(pyrazol-l-yl)methane (BPM), bis(3,5-dimethylpyrazol-l-yl)... 

Mononuclear Carbenes and Carbynes.—The osmium-carbyne complex (1) has been structurally characterized, and the Os=C distance found to be 1.77(2) A and the Os-C -C angle 164(2)°. The osmium-carbon triple bond in (1) interacts in an acetylene-like manner with AgCl to give the dimetallocyclopropene derivative (2), in which the Os-Q distance has increased to 1.839(5) A. 

Organometallic Compounds. Osmium forms numerous mononuclear and polynuclear organometaUic complexes, primarily iu lower oxidation states. There are many complexes of carbon monoxide, such as [Os(CO)3] [16406-49-8], [Os(CO) H2] [22372-70-9], [Os3(CO)2 H2] [56398-24-4],... 

Osmium forms a large number of hydridophosphine complexes, principally mononuclear. The three main families are OsH6(PR3)2, OsH4(PR3)3 and OsH2(PR3)4. 

Reactions between salts of [m Jo-7-CBioHi3] and [Fc3(CO)i2] afford the mononuclear anionic iron compound [2,2,2-(CO)3-c/o5o-2,l-FeCBioHn], typically isolated as its [N(PPh3)2] salt (11) (Chart 4). No anionic triiron complex analogous to 5 and 7 is formed in this reaction. The anionic mononuclear iron, ruthenium and osmium complexes and the previously mentioned neutral mononuclear ruthenium dicarbollide complex 4, obtained from [Ru3(CO)i2] and /Jo-7,8-C2BgHi3, are iso-lobal with the cyclopentadienide species [Mn(CO)3(ri-C5H5)] and [Fe(CO)3 (il-CsHs)]. ... 

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