Chatt systems

In contrast to the original Chatt systems such as 98-Mo or 98-W, the addition of H2SO4 in methanol to iran5-[W(N2)2(depf)2] (107-W) yields 129% of NH3. This presumably proceeds via the diazenido species tram-[W(NNH2)(OTf)(depf)2][OTf] (110-W) that is generated by the addition of 2 equiv. of TfOH to 107-W and produces NH3 in 99% yield after... 

The alternative mechanistic scenario for the protonation and reduction of end-on terminally coordinated N2 through the Schrock cycle is represented by the Chatt cycle which has been developed many years earlier (5). This system is based on Mo(0) and W(0) dinitrogen complexes with phosphine coligands (Fig. 3). As expected, the intermediates of the dinitrogen reduction scheme are very similar to those of the Schrock cycle. Moreover, a cyclic generation of NH3 from N2 has been demonstrated on the basis of this system, however, with very small yields (3,4a). In order to obtain general insight into the mechanism of the Chatt cycle we have studied most of the intermediates of Fig. 3 with... 

Further reduction and protonation of the imido group leads to the formation of a Mo(I) ammine complex (8b) in a mechanism similar to the fluoro system in the classic Chatt cycle. The Mo(I) ammine complex 8b is first reduced to the corresponding Mo(0) complex 8c at which stage the ammine ligand is exchanged with... 

The electronic structure of the surface chemical bond is discussed in depth in the present chapter for a number of example systems taken from the five categories of bonding types (i) atomic radical, (ii) diatomics with unsaturated -systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), (iv) lone pair interactions, and (v) saturated hydrocarbons (physisorption). 

In terms of the Dewar-Chatt model of bonding, for v metal complexation one double bond is effectively removed from the fullerene conjugation system due to extensive interaction between metal d orbitals and the fullerene HOMO and LUMO (7). The remaining 29 double bonds then behave almost identically to uncomplexed C60 with their IR, Raman, UV-vis, and 13C NMR spectra showing only slight perturbations, mainly as a result of diminution of symmetry effects. Nevertheless, it is important to state that the fullerene metal interaction is not confined purely to the former s HOMO and LUMO, and that other molecular orbitals are energetically suitable for interaction 89,90). The spectroscopic evidence cited for the preceding statement is as follows ... 

The names and symbols for elements 112-118 are under review. The temporary system recommended by J. Chatt, Pure Appl. Chem., 51, 381-384 (1979) is used above. The names of elements 101-109 were agreed in 1997 (See Pure Appl. Chem., 69, 2471-2473 (1997)), for element 110 in 2003 (see Pure Appl. Chem., 75, 1613-1615 (2003)) and for element 111 in 2004 (see Pure Appl. Chem., 76, 2101-2103 (2004)). 

The concept of soft and hard acids and bases (7), which is in effect an extension of the Chatt-Ahrland classification (2) of A and B metals, is applied in the 1963 paper by Pearson (7) particularly to equilibria involving mainly inorganic systems. This paper follows an earlier discussion by Edwards and Pearson (3) of the Swain-Edwards equation (4) (1) for nucleophilic reactivity. 

Finally, as Ahrland, Chatt and Davies (4) extended their observations to non-ionic systems, we must also look at such equilibria as MRn -f- DRn 5s - MRn... 

The matrix isolation experiments using epr, ir, uv-visible and other spectroscopic techniques on transition metal-olefin complexes [8,49] have naturally attracted the attention of theoretical chemists and calculations on the Ni-C2H4 system were reported in one of the first theoretical-experimental papers mentioned in the introduction [16]. These results were later supplemented with a larger (double-zeta) basis set [3Q] and also [31] extended for a Ni(C2H4)2 system. The main conclusions are that a net charge transfer of almost 1/5 of an electron from the metal to the ethylene is evident and that a donation and back donation mechanism consistent with a classical Dewar-Chatt-Duncanson model exists. The Ni-ethylene binding energy is 12.8 kcal/mol. 

Another system that has been studied theoretically is CU-C2H4 where, in contrast [34], it was suggested that a weak charge transfer from the olefin to the metal (0.164e) without the participation of the carbons and the unpaired electron remaining in the 4s — 4p hybridized orbital exists. This is indeed very far from the Dewar-Chatt-Duncanson model [52-53]. We shall now report our results on Pd-C2H4 which are much more in coincidence with those of Ozin et ai [13] and Siegbahn and coworkers [50-51]. 

Diborane-ammonia Systems. Consider the formal analogy between the chloroammines of platinum and the ammonia adducts of diborane. It is useful to consider the hydrogen atoms attached to boron in the ammonia addition compounds of diborane as negative hydridic ligands, comparable to the chloride ions found in coordination chemistry. Although this concept may have been considered heresy some years ago, it was made easier to accept when Chatt (2) and his colleagues demonstrated the reaction ... 

The existence of isolated ions of high charge such as Ti in chemical systems is energetically unfavorable. Ncvenheless, complexes exist with these elements in high format oxidation states. Ahrland, S. Chatt, J. Davies, N. R. Quart. Rev. Cliein. Soc. 1958, 12, 265-276. See also Schwarzenbach, G. Experienlia Suppl. 1956, 5, 162. 

Since Chatt and Davidson discovered C—H bond activation by reduction of RuCl2(dmpe)2 with a sodium/naphthalene system, the iron analogues have been widely investigated. Actually reductions of FeCl2(diphos)2 or FeH2(diphos)2 have played the central role in this area and FeCl2(diphos)2 can be prepared by the reaction of purified FeCl2 with diphosphine. 

As expected, there is generally a large dependence of the modes for bound H2 on both the metal center and coligands. Vibrational analysis of a M-T 2-H2 system is further complicated by the three-center, two-electron bonding. The bonding is of the Dewar-Chatt-Duncanson type present in metal-olefin complexes, where there is a strong M —> H2 G component, FBD, to the bonding in addition to electron donation, Ed, to the empty metal d-orbital from the H2 electron pair (Scheme 5.18). 

Cotton in 1960 writes "The geometry of the olefin-metal system. ..is in accord with the type of bonding discussed by Chatt and Duncanson following the proposal by Dewar. In Cotton s review the work of Taufen et al. is given due prominence.. 

Olefin Coordination Compounds. Part 1. Discussion of Proposed Structures. The System Ethylene-Trimethylborine , J. Chatt, J. Chem. Soc., 1949,3340. 

---