Borderline metals complexation

For space reasons, we will deal mainly with the electrochemical behavior of large dendritic compounds. Therefore, the electrochemical properties of a number of borderline compounds [14] between metal complexes and dendrimers have not been included in this review. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

The question then arises what is a heavy atom Br and I can be considered as heavy atoms in organic molecules, but Cl is a borderline case. Most metals qualify as heavy atoms and the photophysical properties of metal complexes are related to this increased spin-orbit coupling. Some noble gas heavy atoms like Xe show important external heavy atom effects. 

Thioacetamide, MeC(=S)NH2 (taa), is isoelectronic with thiourea which it resembles by acting as a unidentate S-donor ligand. Thiobenzamide, PhC(=S)NH2 (tba), behaves similarly. Both thioamides form stable complexes with (b) class and borderline metals. Thioacetamide forms the tetrahedral complexes [MX2(taa)2] (M = Fe, Co, Zn X = Cl, Br, I, NCS).47,153-157 The v(M—S) frequency is rather low, ranging from 255 to 230cm, .14s The Mossbauer spectra of FeX2(taa)2 PC = Cl, Br) are consistent with a tetrahedral configuration.158... 

The / -mercapto thioesters (69 R = Et, Pr1) and the o-mercaptobenzoic acid thioesters (68 R = Et, Pr1, Pe ) form complexes with Cu1, Ni11 and Co111.255,258 Since several (b) class and borderline metals form more stable complexes with the S-esters (68) than with the O-esters (67), it is likely that the S-esters behave as S,S-bidentates. 

Although this view is oversimplified and borderline metal carbene complexes have been isolated, this approach is convenient for discussing the activity of metal carbene species in the ring-opening metathesis polymerisation of cycloolefins. Calculations have predicted [81,82] and recent results have shown [83] that, in some systems, metal alkylidene reactivity is competitive with metal carbene reactivity, i.e. olefin metathesis is competitive with olefin cyclopropanation. 

In spray pyrolysis, a solution of metal complexes is sprayed directly or via aerosol formation onto the substrate. Complicated reactions between precursors and molecules of the solution occur finally before forming the deposit. It is difficult to separate the work on spray pyrolysis from CVD, because the borderline between aerosol CVD and spray pyrolysis is not well defined and, therefore, the latter was included in this report. 

In addition to these two classes there are borderline elements which can show both characters. Since their character depends on the donor atoms, it may be interesting to study the stability of the complexes of the borderline elements and look for factors which effect the strength of metal-donor bonds. From this point of view the stability constants of some metal complexes formed with the ligands containing O, S, or Se as the donor atom were studied and their affinities towards bivalent metals were compared (2),... 

The widely applied Nieboer and Richardson (1980) tabulation of these metal ions classes is summarized in Table 1.1. The general trend in bond stability of the class (a) metal ions with various ligand donor atoms is O > N > S and that for class (b) metal ions is S>N>0 (Nieboer and Richardson 1980). Borderline metal ions are more complex, having binding tendencies intermediate between class (a) and (b) metals. Interactions between the hard class (a) metal ions and ligands tend to be ionic in... 

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