CO and Related Ligands

Most reports have been concerned with identification of particular intramolecular exchanges. For example, the nmr spectrum of [Os3(CO)n(Bu NC)] at -60°C shows that axially and equatorially substituted isomers are present. Between - 60°C and - 30°C localized axial-equatorial exchange at the Os(CO)4 groups of the axial isomer can be observed, but at 50°C there is a rapid exchange 

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Figure 3.19 Electrophilic attack at coordinated CO and related ligands...

CO and related ligands stabilize very low oxidation states of transition elements, often zero (see below). 7i-acceptor interactions remove electron density from a metal atom and make possible a lower oxidation state than is commonly found with ligands such as water and ammonia. 

In the conventional cases (e.g., with RCN, 02N, SCN , R3N) the N atom is mainly a donor of a electrons and no further discussion is required. The cases of N2 and NO are discussed in the next Chapter along with CO and related ligands. The N atom (or nitride ion) and a few other special cases will be discussed below. 

It is believed [1135,1136] that the decomposition of metal complexes of salicyaldoxime and related ligands is not initiated by scission of the coordination bond M—L, but by cleavage of another bond (L—L) in the chelate ring which has been weakened on M—L bond formation. Decomposition temperatures and values of E, measured by several non-isothermal methods were obtained for the compounds M(L—L)2 where M = Cu(II), Ni(II) or Co(II) and (L—L) = salicylaldoxime. There was parallel behaviour between the thermal stability of the solid and of the complex in solution, i.e. Co < Ni < Cu. A similar parallel did not occur when (L—L) = 2-indolecarboxylic acid, and reasons for the difference are discussed... 

NIR region (see Chapter 9.13), complexes of 1,2-dithiolene (DT) and related ligands have attracted considerable attention for their (largely cubic) NLO properties. The complex (156) (a.k.a. BDN) is a highly photochemically stable, saturable absorber and has hence found extensive applications in laser Q-switching. The cubic NLO properties of (156) have been studied by DFWM148,403-407 and more recently, Z-scan.408 Time-resolved DFWM has been applied to square planar Co, Ni, Cu, or Pt complexes of 1,2-benzenedithiolate (BDT) or 1,2-aminobenzenethiolate ligands by Lindle and co-workers.409,410... 

Winter, Underhill, and co-workers have published extensively on the cubic NLO properties of complexes of DT and related ligands,411 22 particularly those containing formally Ni11 centers. For example, time-resolved 1,064 nm DFWM was used to obtain resonantly enhanced values for group 10 complexes such as (157).411 15 The smaller of (157) compared with (156) is largely due to resonance effects since the absorption maximum of (157) is somewhat removed from the laser fundamental. However, figures of merit derived from measurements of 2 and linear and two-photon absorption (TPA) coefficients show that low optical losses render complexes such as (157) superior to (156)413 for potential all-optical switching applications.411 14... 

For a comprehensive summary of the electronic structure of Co(II) complexes with Schiff bases and related ligands, the reader s attention is directed to a recent review by Daul et al.218). 

Complexes. Pyridine and related ligands. Recent Mossbauer studies suggested that [FeCl2(py)4] has a cis-octahedral structure however. X-ray studies have shown it to be isomorphous with trans-[MCl2(py)4] (M = Co or Ni). Magnetic and spectroscopic properties of [Fe(py)2(phen)(NCS)2] have been reported. An n.m.r. study of [FeL(PF )2] [L = (42)] shows the presence of both cis- and trans-isomers in solution.The methyl-substituted pyridyl-thiazoles (43) form the complexes [FeL2-X2],nH20 (X = Cl, Br, I. NO3,... 

Imidazole, pyrazole, and related ligands. The structures of [Co(imidazole) ]-... 

The cobalt(II) complexes with R-dtp and related ligands are readily oxidized by air and other oxidizing agents to yield the corresponding cobalt III) tm-chelates. Electronic spectra and Co NMR spectra data 334,33s) hayg shown that the spectrochemical series of sulfur ligands is ... 

The net electron acceptance of carbenes (Table 2) and related ligands [38] is weaker than that of CO and is very sensitive to the nature of the groups attached to the carbene carbon. Their Pi values spread over a remarkably wide range [14, 28, 31, 38], from vinylidenes C=CHR Pi from —0 to —0.6 V) [14, 38] to anionic oxycarbenes C(0 )Y Pi from —1.1 to — 1.7 V) [38], in the following the order vinylidenes C=CHR > allenylidenes C=C=CR2 > bithiophene-carbenes >... 

Both Co(tame)2+ and CoCl(tame)(tame-H)2+ have been isolated, with the latter431 having one arm of the ligand not coordinated (49). The isomeric possibilities for both these complexes have been discussed in relation to the crystal structures.581,731 One interesting feature of the divalent metal ion chemistry of tamm (33) and related ligands is that while crystalline solids of the type NiL2+ can be isolated,732 there is little evidence that such species are present in solution.589... 

Transfer of a hydride to another ligand on substitution is also possible (equation 59). Alternatively, substitution reactions can occur without loss of the H ligand. This is common where only one H is present (equation 60). The presence of the coordinated hydrogen can even encourage such reactions by its high trans effect and perhaps even by transfer to a CO (or related ligand, where one is present) to give a transient formyl and an open site (equation 61). 

Hiraki et al.104 have prepared a yellow carbonyl solution in methoxyethanol. Addition of 2-phenylpyridine and related ligands led to the formation of orthometallated monochelates, [Ru(CO)2C1(AB)]2. Addition of further ligands then yielded mononuclear complexes. 

Another set of metal-metal bonded complexes that have received some detailed study102,117-119) are the d5-ds [(rjs-CsH5)M(CO)3]2 (M = Mo, W) complexes. Early work indicated that photolysis in the presence of PPI13 (and related ligands) could yield simple ligand substitution products, reaction (25).U7) This is to be contrasted with the observation of net disproportionation as indicated in reaction (26).118) And... 

Photoelectron spectra have been reported for [RhH(PF3)4] and some related tri-fluorophosphine species.120 Comparison with related carbonyl complexes suggests similar 7t-acceptor properties for CO and PF3 ligands. Studies have shown that the activity of the hydrogenation catalyst [RhH(CO)2(PPh3)3], which decreases with time, can be regenerated using weak u.v.-visible irradiation.121... 

As far as we are aware the first example of a ruthenium(II)-arene compound to be evaluated for anticancer activity was described by Tocher and co-workers in 1992 [18]. The field remained essentially dormant until the explosion in interest led largely by our group and that of Sadler, who has extensively studied ruthenium(II)-arene complexes with ethylenediamine and related ligands, and describes these compounds in the Chapter X in this book. It should be noted that other groups have also contributed to the field as will become clear in the course of this Chapter. 

The reactions of [WI2(CO)(NCMe)(7]2-RC2R)2] with a series of dithiocar-bamates and related ligands have been described.268 The molecular structure of one of the complexes, [WI(CO)(S2CNC4H8)(T)2-MeC2Me)2], has been determined crystallographically (Fig. 18), and variable-temperature H NMR studies also show the complex to be fluxional in solution.268... 

Similar surface structures should exist in large clusters, beginning with a two-layer 55 atom cuboctahedron. The question arises, whether or not the best coverage of a metal surface can be realized by many small ligands or by fewer bulky ligands, such as phosphines. In contrast to CO, phosphines and related ligands can easily be dosed, in order to prevent formation of smaller clusters. 

In the biologically relevant, usually monodentate, ademne " " (238) and related ligands (239-241), it is often difficult in deciding if binding takes place at the N-1 or N-7 sites, and it is possible that the binding position for adenine may be metal ion specific Ni°, Cu , Co°, Cd at N-7 Zn at both Hg and at N-1 (see Nucleic Acid-Metal Ion Interactions). 

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