Coordination compounds donor atoms

Coordinated nitrogen donor atoms can be involved in chelate-forming template reactions by virtue of nucleophilic addition to carbonyl compounds. An early and rather specific example does not allow the possibility of elimination following the addition step (equation 46).171 More recent work on ruthenium(III) and osmium(III) results in the formation of a-diimine chelate rings... 

This class of compounds is conveniently investigated by CP/MAS NMR spectroscopy ( P, /= 1/2, 100% nat. ab.). The general considerations on the advantages of using solid state NMR described above for may be extended to P NMR. Furthermore, additional information is usually provided by the determination of the coordination shift [52] (i.e the difference between the solid state chemical shifts of free and coordinated P donor atoms). 

The variation of colour in silver compounds including sulphamides has been correlated with structure. The colour changes go from colourless to red, depending on the number of silver(I) atoms coordinating the donor atoms of the bases used318. Ag(I) sulphamides show different colours depending on the stoichiometry, disilver sulphamides are colourless, trisilver sulphamides are deep red. 

Compound Coordination Number Donor Atoms Pb Chemical Shift (ppm) References... 

The X-ray structure analysis of (salen )SiF2 (8) clearly demonstrates the octahedral coordination of the silicon atom (Fig. 1) [4], There are a number of crystal stractures of bis-chelate compounds with hexacoordinate silicon. Most of these had essentially a tetrahedral arrangement around silicon with the coordinated nitrogen donor atoms capping the tetrahedra at relatively large distances (N-Si between 2.5 and 3.0 A) [5]. 

Attempts to substitute acetylacetone by a-diketones in the reactions considered above were unsuccessftd. However, if the number of CH2 units between the phosphorus atoms is increased by one, the non-coordinated phosphorus donor atoms can be involved in ring-closure reactions by virtue of nucleophilic addition to a-dicarbonyl compounds. This pathway has been illustrated by the synthesis of... 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

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... 

A coordination compound, or complex, is formed when a Lewis base (ligand) is attached to a Lewis acid (acceptor) by means of a lone-pair of electrons. Where the ligand is composed of a number of atoms, the one which is directly attached to the acceptor is called the donor atom . This type of bonding has already been discussed (p. 198) and is exemplified by the addition compounds formed by the trihalides of the elements of Group 13 (p. 237) it is also the basis of much of the chemistry of the... 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

Like Ag, Au also readily forms linear 2-coordinate complexes such as [AuX2] (X = Cl, Br, I) " and also the technologically important [Au(CN)2] . But it is much more susceptible to oxidation and to disproportionation into Au and Au which renders all its binary compounds, except AuCN, unstable to water. It is also more clearly a class b or soft metal with a preference for the heavier donor atoms P, As and S. Stable, linear complexes are obtained when tertiary phosphines reduce Au in ethanol. 

The most usual type of coordination in compounds of Hg with other donor atoms is a distorted octahedron with 2 bonds much shorter than the other 4. In the extreme, this results in linear 2-coordination in which case the bonds are largely covalent. Hg(CN)2 is actually composed of discrete linear molecules (C-bonded CN ), whereas crystalline Hg(SCN)2 is built up of distorted octahedral units, all SCN groups being bridging ... 

The interactions between different organotin(IV) compounds and 6-thiopur-ine, an antitumor metabolite, are more complex. At 0 °C, a polymeric mixture is formed, with N, 5 or N, N donor atoms coordinated. At T>0°C, only N, N -bound product is observed. 

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