Simple Ligands

Simple Ligands.—Exchange. N N.m.r. studies of exchange of pyridine with CoClaPyi, in pyridine solution, indicate a rate constant of 6 x 10 s  

Exchange of acetylacetone with Al(acac)3 takes place by a mechanism involving intermediates containing monodentate acac in non-aqueous as in aqueous solution. The kinetics of acetate exchange with trans-[Co(OAc)2 enal in acetic acid have been reported.  

The electronic structure of these small molecules could serve as the basis of a full article in another context, and our short summary of trends in MOs (or negative ionization potentials) lacks the depth the topic deserves. However, an understanding of these preliminary considerations is requisite for understanding the trends in structure and bonding observed in main group element-transition metal compounds. 

Additional information on electronic structure comes from a NMR study of a closely related series of compounds of the type (Cp )2Hf(X)(H), where X is H, Me, OH, NH2, NHMe, or NMe2. Bercaw and co-workers observed that in the H-NMR spectrum the metal-bonded H atom becomes more shielded as the tt donor ability of X increases (51). The hydride chemical shifts corresponding to X are 8 15.6, 13.1, 10.2, 9.3, 9.1, and 11.5, respectively. Clearly, the tt interaction of the main group ligand with the metal is important in compounds of this type. 

Compounds 11 and 14 react with moisture to lead to the hydroxide (Cp )2Hf(OH)(H) (13) (50), presumably through the dihydroxyl 

Although there are many isoelectronic series in the category of dinuclear complexes, we discuss only one set of examples taken from Herrmann s 

These examples demonstrate that the bonding in these ostensibly similar compounds can be significantly varied by changing either the metal or main group atom cores. 

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Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

Ligands may be conveniently classified on the basis of the number of points of attachment to the metal ion. Thus simple ligands, such as halide ions or the molecules HzO or NH3, are monodentate, i.e. the ligand is bound to the metal ion at only one point by the donation of a lone pair of electrons to the metal. 

This simple ligand exchange reaction is less likely for [Ni ( 11)3. As Figure 18-12 shows, when one... 

This interpretation, however, faces a new problem If the low frequency bands are not due to bridging species, what is the explanation of the distinct downward shift of the vco bands upon CO addition and also of their strong intensity Authors of quoted works [48,53,54,77,99] have probably solved this contradiction. The surface process depicted is not a simple ligand insertion into a pre-existing coordinative vacancy, but more Ukely a Hgand displacement reaction of the type reported in Eq. 2 ... 

The formation constants of simple ligands such as F , Cl , and OH with the O-BISTREN complex (92) with two Cu(II) ions in it can be compared with the situation where these ligands bind to the free Cu2+ ion. 

Trimethylphosphate, despite being neutral, is known to form the simple complex [Co(NH3)5(OP(OMe)3)]3+ this reacts with nucleophiles X" S2C)32, I and SCN in that order of reactivity to produce [Co(NH3)5(02P(OMe)2)]3+ and MeX up to 150-fold faster than in the absence of coordination.1008 Both hydroxide and water are ineffective in this reaction, with simple ligand hydrolysis occurring more rapidly than any reaction. 

The above results justify a simple ligand field model for the bis(ligand) metal species which is based on the assumption of pseudoaxial symmetry (40). This model allowed a consistent reinterpretation of an early ESR study of Co(C5H5BPh)2 (13) (42) the reinterpretation was later confirmed by additional and more sophisticated ESR work on 13 (43,44). 

The co-condensation reactions described above have led to the formation of interesting new compounds and sometimes very unexpected products. The nature of the products formed for example in the osmium atom experiments indicate high degrees of specificity can be achieved. However, the detailed mechanisms of the co-condensation reactions are not known. It seems most likely that in all cases the initial products formed at the co-condensation temperature are simple ligand-addition products and that the insertion of the metal into the carbon-hydrogen bond occurs at some point during the warming up process. In support of this hypothesis we note the virtual absence of any... 

Presumably these transformations begin with one or more simple ligand replacement steps but these early intermediates are too unstable or reactive to be detected because of the action of the 7r-acceptor ligands in draining off electrons from the M-M tt and 6 bonds. 

Determinations in Table 15 cover some twenty-five years, and a number of studies suffered from poor crystals, so the initial level of comparison is at the level of a few picometers. In a number of cases, disorder between E and E vitiated detailed discussion, for example of the potentially interesting and well-represented series of ArgEE molecules. For simple ligands, the E—E bond lengths fall into quite tight ranges, slightly shorter than in the elements, and the E—E values interpolate. 

What makes beryllium so special is the fact that in aqueous solution its cation, Be2+, is the only cation that is both habitually 4-coordinate and forms a range of complexes with simple ligands. This means that an understanding of the chemistry of this cation should... 

Ni2+ was very popular in the early days of the investigation of mechanisms of complex formation, since the time-scale for its reactions with simple ligands was so convenient for the then recently developed stopped-flow technique. However, interest has now moved on to other first-row cations, especially to Cu2+. A review of the kinetics and mechanisms of formation of tetraazamacrocyclic complexes concentrates on Ni2+ and Cu2+, and their reactions with cyclam and similar ligands (267). The tetra(4 -sulfonatophenyl)porphyrin complexes of Ni2+ and of Cu2+ react immeasurably slowly with cyanide, but their IV-methyl derivatives do react, albeit extremely slowly. The relevant time scales are hours for removal of Ni2+, months for the removal of Cu2+, by 10-4 M cyanide at pH 7.4 (268). 

A new aminocarboxylate chelator of potential therapeutic value, 77(2-hydroxybenzy -Al -benzylethylenediamine-A Al -diacetate, reacts as LH4 and LH3 with Fe(OH)2q by dissociative activation with rate constants of 770 and 13 300 M s-1, respectively. These rate constants are similar to those for reaction of Fe(OH)2q with edta and with nta. These formation reactions are, however, considerably faster than with simple ligands of identical charge thanks to the zwitterionic properties of ami-nocarboxyl ates (334). 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

As discussed in Chapter 5 of this volume [104], chemical speciation can be defined as the physicochemical distribution of a chemical among all of its possible forms. In environmental systems, ligands range from simple ligands... 

In the simplest case, it can be assumed that the water exchange reaction with a complexing site of a heterogeneous ligand will be governed by the Eigen-Wilkens mechanism as for complexes with simple ligands. In this case, equations (26) and (29) can be combined to yield ... 

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