N- and S-Donor Ligands

Organometallic Niobium ) and Tantalum ) Compounds. Reports this year have concerned two main areas of interest the preparation of alkyl-niobium and tantalum halides and their reactions with donor ligands, and insertion reactions into methyl-metal bonds. The first examples of monomethyl niobium and tantalum compounds have been prepared by the low-temperature methylation of the appropriate halides 

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Mixed N and S donor ligands (Figure 13) combine hard and soft ligand characteristics, while typically the M-N distance... 

Fig. 4. Plot of enthalpies of complex formation in the gas phase of CpNi complexes (Cp = cyclopentadienyl) vs those for the corresponding Mn complexes. The ligands are segregated into correlations for soft (O) (N and S donors) and hard (oxygen donors) ( ). Energies are in kcal mol-1. Redrawn after Ref. (14). 

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. 

The Zn + ion interacts favorably with N-, O- and S-donor ligands and shows a strong preference for tetrahedral over octahedral coordination in enzymes. In the case of model complexes, Zn + can form four-, five- and six-coordinate complexes, dictated by the... 

The chloro complex CrCl(TPP) is a non-electrolyte in DMSO and neutral N, O and S donor ligands will bind to give six-coordinate species.1255 There is little difference in the stability... 

The coordination chemistry of silver has historically been centred on the reaction of silver(I) ions with N-donor ligands and halides. However, an extensive chemistry now exists for P- and S-donor ligands, whilst for O-donor ligands only weak complexes are generally formed and they have been studied in much less detail. Based on the reactivity and stability of its coordination complexes, the silver(I) ion has been characterized as a class B or soft acid, for which the following stability order is observed N P>As>Sb 0 S Se Te FComparative studies between ligands with these donor atoms allowed the relative stability of silver bonds to be determined as P > S > N > O. 

The solvent influences on the complex formation and stability have been reviewed by Golub et al.194 Several monomeric complexes of Hg(SCN)2 with N, O, P, As and S donor ligands are known with terminal Hg—SCN bonds.224-2 Some thiocyanate-bridged dimeric complexes of mercury(II) are also known.225,226 Recently the isolation of a mercury(II) thiocyanate complex with hexamethylenetetramine with exclusively N-bonded SCN groups has been published (Figure 9).233 The compounds (CH2)6N4-Hg(SCN)2 and (CH2)6N4-2Hg(SCN)2 exhibit covalently bonded Hg—s.394,595... 

Related ligands such as (51) have also been reported,184 and mixed N and S donor atoms have been used to isolate lower oxidation states, e.g. cobalt(I).185... 

Bis(cyclopentadienyl) niobium complexes halides, 5, 90 N-donor ligands, 5, 94 O- and S-donor ligands, 5, 93 P-donor ligands, 5, 94... 

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