Carbonyl compounds, electron ligand interaction

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

The position of the equilibrium between imine and carbonyl may be perturbed by interaction with a metal ion. We saw in Chapter 2 how back-donation of electrons from suitable orbitals of a metal ion may stabilise an imine by occupancy of the jc level. It is possible to form very simple imines which cannot usually be obtained as the free ligands by conducting the condensation of amine and carbonyl compounds in the presence of a metal ion. Reactions which result in the formation of imines are considered in this chapter even in cases where there is no evidence for prior co-ordination of the amine nucleophile to a metal centre. Although low yields of the free ligand may be obtained from the metal-free reaction, the ease of isolation of the metal complex, combined with the higher yields, make the metal-directed procedure the method of choice in many cases. An example is presented in Fig. 5-47. In the absence of a metal ion, only low yields of the diimine are obtained from the reaction of diacetyl with methylamine. When the reaction is conducted in the presence of iron(n) salts, the iron(n) complex of the diimine (5.23) is obtained in good yield. 

Numerous dynamic and kinetic studies have been performed using O NMR. Line shape analysis is not straightforward because the line width itself is temperature dependent. Nevertheless, it has been shown that dynamic O NMR is a convenient alternative to NMR with, for example, methyl carbonyl compounds. A substantial number of publications have been concerned primarily with studies of the hydration (solvation) of ions in solution and the determination of rates and activation parameters for ligand substitution (a rapid development of high-pressure NMR has occurred in the last twenty years). There is an extensive literature of NMR spectroscopy used for the study of hyperfine interactions between the unpaired electrons in paramagnetic molecules and ions and the nucleus. The... 

Knowing all these facts, especially the difficult access to fluorophosphines and the poor donating abilities of phosphorus trifluoride (5, 6), we decided to use another approach, which readily led to a number of coordination compounds with fluorophosphine ligands—namely, the fluorination of chlorophosphines already coordinated to the transition metal, where the 3s electrons of phosphorus are blocked by the complex formation. There was no reaction between elemental nickel and phosphorus trifluoride, even under extreme conditions, whereas the exchange of carbon monoxide in nickel carbonyl upon interaction with phosphorus trifluoride proceeded very slowly and even after 100 hours interaction did not lead to a well defined product (5,6). 

Amides and related compounds have been thoroughly investigated as ligands for alkali metals. Studies in solution have been made using various physicochemical techniques these, and MO calculations, show that the interaction of amides with M+ is via the carbonyl function.56,57 This leads to a delocalization of the amide N electron pair and so it is possible to follow the decrease in C—O, and the increase in C—N, bond orders using IR spectroscopy. X-Ray crystal structures of amide complexes have shown both types of complexation interaction to be present (Table 2). 

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