Ligands in transition metal complexes

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

Pyridine 1-oxide, like pyridine, can act as a ligand in transition metal complexes, but unfortunately good stability constants are not known. However, Shupack and Orchin have found that the C===C stretching frequency of the ethylene ligand in trans-ethylene pyridine 1-oxide dichloroplatinum(II) varies linearly with the pA and hence with the C7-value (ct+ or a, respectively) of substituents in the pyridine oxide. The data for the above reaction series are included in Table V. 

B-Heterocycles as ligands in transition metal complexes 98AG(E)1786. 

Thiolates as Ligands in Transition Metal Complexes (J. R. Dilworth)... 

Abstract The theoretical and experimental research on carbodiphosphoranes C(PR3)2 and related compounds CL2, both as free molecules and as ligands in transition metal complexes, is reviewed. Carbodiphosphoranes are examples of divalent carbon(O) compounds CL2 which have peculiar donor properties that are due to the fact that the central carbon atom has two lone electron pairs. The bonding situation is best described in terms of L C L donor acceptor interactions which distinguishes CL2 compounds (carbones) from divalent carbon(ll) compounds (carbenes) through the number of lone electron pairs. The stmctures and stabilities of transition metal complexes with ligands CL2 can be understood and predictions can be made considering the double donor ability of the carbone compounds. 

Organic isocyanides (C=N-R) are versatile ligands in transition metal complex chemistry. As compared with their pseudo-isoelectronic cousin, C=0, they are stronger o-donors [1], As a result, isocyanides form more stable complexes with metals in relatively high oxidation states (e.g., +2 and +3) than CO. In contrast, they have a lower ir-accepting ability than CO and therefore form less stable complexes with metals in low oxidation states (e.g., -1 and -2). Nevertheless, they form a broad range of metal complexes, and various aspects of their syntheses, structures and bonding have been reviewed [1-7]. 

MORE ABOUT C LIGANDS IN TRANSITION METAL COMPLEXES... 

Various neutral and, especially, anionic boron species are well known as ligands in transition metal complexes. In the majority of cases, however, one or more H, N or other basic atom of the boron compounds is coordinated to the transition metal (e.g. Chapters 13.8, 13.6 and 19). In the last 20 years, however, several complexes containing boron-transition metal bonds have been prepared with ligands such as sBX, -BX2 or BX3 (Table 14). 

The As—N bond is labile and this has been widely exploited in synthetic arsenic chemistry. Some idea of the versatility168 can be seen from Schemes 1 and 2. Refluxing secondary amines with tris(dimethylamino)arsine effects transamination (equation 6). These tris(dialkyl-amino)arsines undergo the general reactions in Scheme 1, enabling ready access to a wide variety of compounds, many of them finding use as ligands in transition metal complexes (see Chapter 14 of this work). 

Of commercial interest are benzo- and other fused aromatic 1,2,3-diazaborine derivatives which have exhibited good antibacterial activity against a variety of microorganisms (155—157). The reaction of pyrazole or C-substituted pyrazoles with boranes yields the pyrazabole system, a class of exceptionally stable compounds. More than 70 species in this system have been reported and the subject comprehensively reviewed (158). These compounds have been used as ligands in transition-metal complexes (159). 

The pentaphenylborole dianion was incorporated as a ligand in transition metal complexes with platinum and cobalt as well as with iron, nickel and manganese. An interesting formation of such a complex from 1-phenyl-4,5-dihydroborepin (53) was performed in boiling mesity-lene in the presence of iron pentacarbonyl. Six-membered borinate complexes were also found in the reaction mixture (77AG43). 

Pyridines are also well known as ligands in transition metal complexes, and if the equilibrium constants for the formation of such complexes can be related to base strength, it is expected that such constants would follow the Hammett equation. The problem has been reviewed,140 and a parameter S, formulated which is a measure of the contribution of the additional stabilization produced by bond formation to the stabilization constants of complexes expressed in terms of a.141 The Hammett equation has also been applied to pyridine 1 1 complexation with Zn(II), Cd(II), and Hg(II) a,/3,y,<5-tetraphenylporphins,142 143 the a values being taken as measures of cation polarizing ability. Variation of the enthalpy of complexation for adducts of bis(2,4-pentanediono)-Cu(II) with pyridines plotted against a, however, exhibited a curved relationship.144... 

A. Carbon Dioxide as a Ligand in Transition Metal Complexes. 121... 

Similar effects have also been observed for solids and seem to become a very important source of information, because for a given ion the satellites are found to depend in intensity, position and shape on the type of ligand in transition metal complexes 24,13s,im, 174,175) Thus in many cases the study of shake-up peaks has provided additional evidence for a specific oxydation state 17> and will certainly increase our understanding of crystal and ligand field effects. 

Phosphorus magnetic shielding tensors, general phosphido ligands in transition metal complexes, 1, 472 Phosphorus(III) mercury compounds, preparation, 2, 425-426 Phosphorus—metal complexes, characteristics, 2, 34 Phosphorus—nitrogen-containing rings, chromium carbonyl link, 5, 225... 

Bipyridines have been intensely used as ligands in transition metal complexes[6]. Within a broader investigation of the redox and photophysical behavior of Ru and Os complexed macrocycles[4,8], the Ru(II) and Os(II) complexes shown in Schemes 4 and 5 were prepared. 

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