Monoanionic ligands

The compounds Per2Au(i-mnt)2 and Per2Au(i-mns)2 have been reported as very thin needle crystals obtained by electrocrystallisation procedures similar to those used to obtain the Per2M(mnt)2 compounds (see Section 5.1.1). For these ligands, monoanionic complexes with other metals such as Pt or Pd are not stable. Their structure was not solved but the physical... 

In connection with the anion binding ligands, it should be added that tetraprotonated [18]aneN6 -4H+ in aqueous solutions interacts with the monoanions Cl- and NOf having log of 1.8 and 2.3 53). Interestingly, the anion binding was found... 

Due to their stability and their easy formation, many examples of transition metal complexes containing benzoyl-substituted thiourea ligands have been described [59-62]. Most of them concern Ft species in which the thiourea ligands behave as monoanions and are bounded to the metal centre through the S and O atoms, forming a six-member ring system (Scheme 10). 

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. 

Many of the problems and misconceptions occurring for dithiolene compounds are related to the fact that the ligands are redox-active and can be oxidized to monoanionic radicals. Typical examples for this phenomenon are the mono and diradical complexes [Fe ( "bdt )( "bdt)(PMe3)] (9) and [Fe ( "bdf)2(PMe3)]-" (10) for which bdt and bdt are tcrt-butyl-dithiolene and its one-electron oxidized form. Originally, these and other bdt derivatives had been described as... 

By far the most studied pyrazole-containing ligand is the symmetrical monoanionic tripod hydridotrispyrazolylborate, Tp (3). Substituents introduced at each of the 3-, 4- or 5-positions (3,5-Me2Tp108, 3-z Pr,4-BrTp109) have led to a number of sterically encumbered threefold symmetric complexes and some remarkable Co coordination chemistry has ensued. Hydride substitution at the B atom is also possible by organic groups such as /z-bromophcnyl, which enables further functionalization of the ligand.110... 

Mixed O, N donor molecules are truly extensive and structurally diverse, and only a few selected examples will be presented. In line with other 2-substituted pyridine analogs reported in this chapter, it is worthwhile noting the chemistry of 2-pyridone (or 2-hydroxypyridine, Hopy), which can form O-bonded monodentate complexes such as Co(Hopy)4(N03)2, but as the monoanion is an effective chelate ligand, forming Co(opy)2 and Co(bpy)(opy)2 compounds.454 An unusual solid state melt reaction with Co(OAc)2 yields the dodecanuclear cluster Co12 (OH)6(OAc)6(opy )12.455... 

The terdentate cyclometalated complexes [Ir(L)(L )]2+ and [Ir(L )2]1, L = 2,6-bis(7 -methyl-4 -phenyl-2 -quinoyl)pyridine (233), L = monoanion of L (234), luminesce at 77 K in MeOH/EtOH (lmax = 592 nm, r =20 ps) and at room temperature in deoxygenated acetonitrile (imax 620 nm, r = 325 ns).405 Both compounds undergo four reversible, ligand-centered, one-electron reduction processes. 

While the structural and electronic properties of the dianions are consistent with the indicated Nin-enedithiolate description, two and four formulations are conceivable for the monoanion and the neutral complex, respectively. For the latter, one is a ligand diradical whose spins are antiferromagnetically coupled, consistent with diamagnetism. Numerous members of the series have been isolated and/or generated in solution by chemical or electrochemical reactions. Because redox potentials are markedly dependent on the nature of the R substituent, certain members of a given series have not been isolated in substance. 

Complex [Ni(468)] is formed with Ni11 salts and the amine-thiolate ligand, or by reduction of the corresponding Schiff base complex with NaBH4-[Ni(468)] is easily deprotonated to form the dianionic complex, which is subsequently oxidized to the monoanion. On the other hand, the dianionic complex reacts with CH3I to form the fourfold methylated octahedral complex (469).1305 Since without preceding deprotonation only the S-donors are methylated, the complex with twofold amine methyla-tion must be synthesized via protection and subsequent deprotection of the S-donors.1306... 

Oxidaton of heme goes through the biliverdin species. Octaethylbiliverdin can exist in coordinated form as the fully reduced trianion (OEB)3-, as the two-electron-oxidized monoanion (OEBox), or as the one-electron-oxidized radical (OEB-)2-. Nickel forms complexes with all three moieties, [Nin(OEB)]ra with n I 1, 0, and -1 (689).1787 The most highly oxidized species [Ni(OEBox)]I3 could be crystallized. The structure shows a helical coordination of the linear tetrapyrrole ligand around nickel with Ni—N distances of 1.867 A and 1.879 A. 

---