Donor atoms in ligands

An area of current development is the nomenclature of organometallic compounds. Organometallic compounds of Main Group elements can, to a first approximation, be considered to be derivatives of hydrides, and the methods of substitutive nomenclature can be applied. Even then, the accessibility of different oxidation states, as with phosphorus(iii) and phosphorus(v), introduces complications. Transition metal organometallic compounds are even more difficult to treat, and the development of a unified, self-consistent and accepted and applied nomenclature is not easy. Witness the different ways (k, t and italicised symbols) for denoting donor atoms in ligands. 

A great number of Schiff-base complexes of oxovana-dium(IV) have been studied by X-ray diffraction. Many of these bases contain 0,0,N,N-donor atoms in ligands formed from reaction between substituted salicylaldehyde and aliphatic or aromatic amines, " but examples with S-donor ligands have also been reported. The distorted square... 

Most donor atoms in ligands are members of the main group (p block) of the Periodic Table. Of these, common simple ligands you are likely to meet will be the following ... 

The CK" ion can act either as a monodentate or bidentate ligand. Because of the similarity of electron density at C and N it is not usually possible to decide from X-ray data whether C or N is the donor atom in monodentate complexes, but in those cases where the matter has been established by neutron diffraction C is always found to be the donor atom (as with CO). Very frequently CK acts as a bridging ligand - CN- as in AgCN, and AuCN (both of which are infinite linear chain polymers), and in Prussian-blue type compounds (p. 1094). The same tendency for a coordinated M CN group to form a further donor-aceeptor bond using the lone-pair of electrons on the N atom is illustrated by the mononuclear BF3 complexes... 

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

The methanol can be removed by heating gently in vacuo. Similar compounds can be made with other carboxylate groups, either by using this method or by heating the acetate with excess carboxylic acid. Treatment of the anhydrous carboxylate with various neutral ligands (L) or anionic donors (X-) forms Rh2(OCOR)4L2 and [Rh2(OCOR)4X2]2-, respectively. The colour of the adduct depends on the donor atom in L (or X) ... 

Various thioether complexes have been synthesized for example, 6-coordination is found in [Ag(18S6)]+ and [Ag(9S3)2]+ but in [Ag(16S6)]+, tetrahedral coordination occurs, with two unused donor atoms in the ligand [59],... 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

A chelating ligand contains two or more donor atoms in a structure that allows the ligand to wrap around the metal. Examples featured in Chapter 18 are the bidentate ligand ethylenediamine (en, chemical formula H2 NCH2 CH2 NH2) and the hexadentate ethylenediaminetetraacetate (EDTA). 

Each heme unit in myoglobin and hemoglobin contains one ion bound to four nitrogen donor atoms in a square planar arrangement. This leaves the metal with two axial coordination sites to bind other ligands. One of these sites is bound to a histidine side chain that holds the heme in the pocket of the protein. The other axial position is where reversible binding of molecular oxygen takes place. 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

X-ray structural studies of the diamagnetic anion (406) confirm that the Ir(-I) center is in a distorted coordination geometry intermediate between square planar and tetrahedral, with the P donor atoms in a cis position. The metal-ligand bond distances do not show significant changes among (404), (405), and (406). The Ir1/0 and Ir0/(-1) redox couples are measured at easily accessible potentials and are solvent dependent. 

Phosphorus is the most common donor atom in Ni° chemistry. Likewise, As- and Sb-based ligands are well suited for stabilizing low-valent Ni°. Ni° phosphine chemistry is extensive, and the general synthetic approaches have been summarized previously. 

Potentiometric and H-NMR methods were combined in the studies of the interaction of [Pd(en)(H20)2]2+ and [Pd(pic)(H20)2]2+ (pic = 2-picolylamine) with monodentate ligands containing nitrogen or sulfur donor atoms. The ligands represent the side chain residues of nucleic acids and proteins.178... 

Schiff bases provide useful mixed donor sets. The carbonyl function of the most frequently used ligands is derived from either 1,3-dicarbonyl compounds or salicylaldehyde. Favourable combinations involve O-, N- and S-donor atoms. A range of technetium and rhenium complexes exist with bi-, tri-, tetra- and pentadentate ligands. The geometry of these complexes depends on the number and type of coordinating atoms as well as on the chain length between the donor atoms in the SchifF-base ligands. 

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