Homoleptic complexes oxidation states

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

This means that, for instance, assuming that the metal centre does not change its oxidation state, homoleptic bisquinoidal metal(II)-complexes, which are one of the most common classes, should theoretically exhibit the sequence illustrated in Scheme 3. 

The classical homoleptic cyanido complex of Tc , the seven-coordinate, yellow-orange complex [Tc(CN)7]" (397), is formed from [Tcle] and an aqueous KCN solution after 24 h of heating under reflux. Careful exclusion of O2 is required, otherwise appreciable amounts of cyano complexes in higher oxidation states are formed. The iodide acts as a reducing agent, since the... 

Lanthanide-based catalysts, despite finding a lot of application in homogeneous catalysis, can be rather problematic due to the lability of some ligand types and the versatility of their coordination chemistry in the -1-3 oxidation state this makes the controlled synthesis of single-site Ln complexes a quite ambitious goal [92]. McLain and coworkers first demonstrated the high potential of a homoleptic yttrium complex Y(OCH2CH2NMe2)3 as ROP catalyst for the preparation of PLA from rac-lactide and that it promotes a rapid and controlled polymerization... 

Manganese forms a large number of mononuclear complexes in oxidation states II, III, and IV with acyclic and other common ligands. The complexes primarily are six-coordinate species with octahedral or pseudo-octahedral geometries. Some exceptions to this categorization are the tetraoxo anions in oxidation states V, VI, and VII and the manganocenes. Table 1 contains electrochemical data for representative redox couples in which the Mn coordination environment remains invariant with oxidation state. Many entries are for homoleptic complexes, but some species with mixed... 

Such J-mctals as Cu(I) [but not Cu(II)], form a variety of compounds with ethenes, for example [Cu(C2H4)(H20)2]C104 (from Cu, Cu2+, and C2H4) or Cu(C2H4)(bipy)+. It is necessary to mention that, of all the metals involved in biological systems, only copper reacts with ethylene [74b]. Such homoleptic alkene complexes can be useful intermediates for the synthesis of other complexes. The olefin complexes of the metals in high formal oxidation states are electron deficient and therefore inert toward electrophilic reagents. By contrast, the olefin complexes of the metals in low formal oxidation states are attacked by electrophiles such as protons at the electron-rich metal-carbon a-bonds [74c]. 

The single homoleptic bis(dithiolene) example based on Ag and shown in Fig. 4 is quite remarkable. The structure demonstrates the longest average M—S bond length (2.563 A) of all the homoleptic bis(dithiolene) complexes. The structural unit has a —3 charge relating to a formal oxidation state of Ag(I) . The geometry is distorted tetrahedral with A = 85.2°. 

For a molecule with such a low basicity (only protonated in super-acidic media), CO is a surprisingly versatile ligand. The homoleptic (or binary) carbonyls (i.e. in which CO is the only ligand) already span oxidation states from -IV to +III, and higher oxidation states are known for heteroleptic complexes (mixed ligand sets) which have good 71-donor co-ligands, e.g. RuIV(CO)(SR)4. 

Isocyanides (RNC) are better a donors and poorer xr acceptors than CO, as indicated by the observation that typical homoleptic isonitrile complexes of many metals are in higher oxidation states than the typical carbonyl complexes of the same metal. Some metal isocyanide complexes are given in Table 7-4. It should be pointed out that there are no carbonyl analogues of those compounds in the higher oxidation... 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

While inorganic complexes of osmium in oxidation states +4 through +8 have been known for many years, the study of high-valent alkyl and aryl complexes of osmium is much more recent. The organometallic complexes include homoleptic see Homoleptic Compound) alkyls and aryls, oxo alkyls and aryls, nitrido/imido alkyls and aryls, and cyclopentadienyl see Cyclopentadienyl) alkyls and aryls. The majority of these are complexes of osmium(VI). 

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