Trans-Dioxo

Monooxo and cis- and trans-6ioxo complexes of ruthenium(VI) and osmium(VI) are known, with the trans-6ioxo species being most common. In general, these complexes are all diamagnetic and are characterized by vibrational spectroscopy and/or X-ray crystallography. One intense metal-0x0 stretch is usually observed for monooxo and trans-dioxo complexes while two metal-oxo stretches, j/s(M(0)2) and J/as(M(0)2), are found for the cw-dioxo species in the IR spectra. The structural and vibrational data are listed in Table 4. 

Rhenium forms a number of mono-oxo, CTs-dioxo, and trans-dioxo complexes in oxidation states IV through VII. Reversible, proton-independent, one-electron potentials for several systems are represented in Table 1 [13-22, 35, 36]. As is the case forTc, a large change in E° for a given redox step can be produced by changing the ancillary ligands. 

Generally, two strong tt bases are required to stabilize Os(VI), hence its chemistry is dominated by octahedral complexes with the trans-dioxo moiety. However, a complex containing three doubly depro-tonated 2-aminobenzenethiol ligands, [Os(abt)3], has been characterized (24). In addition, a growing number of five-coordinate complexes... 

X6]2- with L-L. The trans-dioxo complexes were characterized by IR, H NMR, and electronic absorption spectroscopies (209). 

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 majority of U(VI) coordination chemistry has been explored with the trans-dioxo uranyl cation, U02+2. The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatility as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned in the introduction have been complexed with U(VI) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples include U02X2Lw (X = halide, OR, N03, RC02, L = NH3, primary, secondary, and tertiary amines, py, n = 2-4), U02(N03)2Lk (L = en, diaminobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J2 xj H20 (x = 2-5). 

Ru(0)2(NH3)4]2+ and related ruthenium(VI) complexes incorporating the trans dioxo moiety have been reported.65... 

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. 

The M(CN)j ions in aqueous solution are photochemically converted, through several intermediates, into isolable species long believed to be [M(CN)4(OH)4]4 but more recently shown to be 6-coordinate, trans-dioxo complexes [M02(CN)4]4. A cyano complex reported for Mo111 is KtMo(CN)7-2H20, which readily oxidizes... 

EXAFS studies of PuIV in HN03 solutions have shown that the complexes (Et NV [Pu(N03)6], Pu(N03) +, and Pu(N03)4 have structures similar to the solid state structures of Th and Np nitrates with fi2-N03 ligands.66 Similar studies of alkaline Npvn solutions prepared by bubbling 03 through a 2.5 M NaOH solution of Np02(0H)2 show the presence of a trans dioxo ion,67 formally NpO, which must be coordinated by OH and possibly H20 ligands. 

A significant amount of structural information has been gathered on neptunium complexes. For the main, they resemble corresponding U and Pu complexes. However, an EXAFS study of alkaline solutions of Np " found evidence for a trans dioxo ion of the type P4p02(OH)4(OH2)]i-. 

Actinide complexes rarely follow the conventional rules, for example, 18-electron rule, typically found in inorganic and coordination chemistry of the transition metals. A prime example of the difference in actinide chemistry is the pervasiveness of the linear dioxo rmit, which is immatched in transition metal chemistry. For the actinides, the trans dioxo structure is maintained through different metal ions, oxidation states, and valence electron cormts, for example,... 

A 7r-donor ligand possesses a high-lying filled orbital that can serve to interact with and destabilize a member of the nonbonding metal d set. As an iUnstrative example we present the trans and cis dioxo problem. Fignre 11(a) constmcts the n orbitals for a trans dioxo ML4 complex. Two of the three members of the t2g set can interact with two of the fonr SALCs of oxygen p AOs. Thus, one metal d and two 0x0 orbitals are left nonbonding. The orientation... 

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