Transition mononuclear

The dioxygen ligand in mononuclear group VIII transition metal complexes. J. S. Valentine, Chem. Rev., 1973, 73, 235-245 (101). 

The simplest transition metal carbonyls are mononuclear of the type M(CO)x, in other words those with only one metal atom. They are hydrophobic but soluble to some extent in nonpolar liquids, such as n-butane or propane. The dinuclear carbonyls are more complex but have the same general characteristics as the mononuclear carbonyls. The carbonyls, which are or could be used in CVD, are listed in Table 3.4 with some of their properties. 

This is only the beginning of a process which ultimately results in the formation of solid state hydroxides or oxides. Actually, the solution species present in neutral or alkaline solutions of transition-metal ions are relatively poorly characterized. The formation of numerous hydroxy- and oxy-bridged polynuclear species makes their investigation very difficult. However, it is clear that there is a near-continuous transition from mononuclear solution species, through polynuclear solution species to colloidal and solid state materials. By the way, the first example of a purely inorganic compound to exhibit chirality was the olated species 9.11. 

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

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. 

For di- and polynuclear gold(I) complexes vith two Au(I) centers held in close proximity, a lower energy 5denergy from its mononuclear counterpart is a spectroscopic signature [2, 7, 44]. Excitation to the 5da —> 6pG transition gives rise to a [5da, 6pa] excited state having a formal... 

Further examples of emissive cyclometallated gold(III) complexes are [Au(L)Cl] (L = tridentate carbanion of 4 -(4-methoxyphenyl)-6 -phenyl-2,2 -bipyridine) [53], as well as mono- and binuclear bis-cyclometallated gold(III) complexes, namely [Au (C N C )L ]" (C N C = tridentate dicarbanion of 2,6-diphenylpyridine L = depro-tonated 2-mercaptopyridine (2-pyS ), n = 0 L = PPh3 or 1-methylimidazole, n = 1) and [Au2(C N C )2(P P)](C104)2 (P P = dppm, dppe) respectively [54]. The crystal structures of the binuclear derivatives show intramolecular interplanar separations of 3.4 A between the [Au(C N C)] moieties, implying the presence of weak n-n interactions. The mononuclear complexes show absorption with vibronic structure at 380-405 nm (e > 10 cm ), attributed to metal-perturbed intraligand transition. 

The active sites in our investigations are constituted by mononuclear surface transition-metal complexes (corresponding to low TMI loadings). Empirical models of such sites... 

Figure 2.6. Generic mechanism of NO decomposition over mononuclear intrazeolite transition metal ions.

Dehydrative condensation of transition metal hydroxo complexes is a versatile synthetic method, when the conjugated acid of the incoming ligand (H-A) is a protic compound. There are reports of the synthesis of two types of square-planar hydroxopalladium complexes, mononuclear [TplPr2 x(L)Pd-OH] (X = H, Br) and dinuclear complexes [TplPr2Pd(H20)( -0H)2 (H20)PdTplPr2] (TplPr2 = hydrotris(3,5-diisopropylpyrazolyl)borate), and some dehydrative condensations of them.278... 

Transition metal carbyne complexes are still relatively uncommon as only a few synthetic approaches to these compounds has proved generally applicable. In addition to making the initial characterization (723), the Fischer group has made the largest contribution to carbyne complex chemistry, with some 200 mononuclear complexes of Group 6 and 7 metals having been prepared. 

Amino acid is one of the most important biological ligands. Researches on the coordination of metal-amino acid complexes will help us better understand the complicated behavior of the active site in a metal enzyme. Up to now many Ln-amino acid complexes [50] and 1 1 or 1 2 transition metal-amino acid complexes [51] with the structural motifs of mononuclear entity or chain have been synthesized. Recently, a series of polynuclear lanthanide clusters with amino acid as a ligand were reported (most of them display a Ln404-cubane structural motif) [52]. It is also well known that amino acids are useful ligands for the construction of polynuclear copper clusters [53-56], Several studies on polynuclear transition metal clusters with amino acids as ligands, such as [C03] [57,58], [Co2Pt2] [59], [Zn6] [60], and [Fe ] [61] were also reported. 

The ability of thioether macrocyclic complexes (and especially those of [9]aneS3) to support multi-redox behaviour at the coordinated metal centre is particularly notable. This allows a series of reversible stepwise one-electron oxidation and/or reduction processes, and stabilization of highly unusual transition metal oxidation states e.g. mononuclear [Pd([9]aneS3)2]2+/3+/4+,149 [Au([9]aneS3)2]+/2+/3+,150 [Ni([9]aneS3)2]2+/3+,151 and [Rh([9]aneS3)2]+/2+/3+.152 It appears to be the ability of the crown thioethers to readily adjust their... 

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