Polymetallic systems

In the case of polyionic systems the picture becomes still more complicated. There are no examples of prediction of xs values. However, we have empirically learned that zs of the entire system is shorter than in the absence of magnetic coupling. High resolution NMR experiments are, in general, successful with magnetically coupled polyionic systems. 

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B. Polymetallic Systems Valence Delocalization Considerations on the Reduction Potentird Solution Structure... 

Contact shifts give information on the electronic structure of the iron atoms, particularly on the valence distribution and on the magnetic coupling within polymetallic systems. The magnetic coupling scheme, which is considered later, fully accounts for the variety of observed hyperfine shifts and the temperature dependence. Thus, through the analysis of the hyperfine shifts, NMR provides detailed information on the metal site(s) of iron-sulfur proteins, and, thanks to the progress in NMR spectroscopy, also the solution structure 23, 24 ). 

Polymetallic systems are magnetically coupled. The magnetic coupling in the simple Heisenberg model can be described by the following Hamiltonian (38-40) ... 

Table I reports the observed NMR linewidths for the H/3 protons of the coordinating cysteines in a series of iron-sulfur proteins with increasing nuclearity of the cluster, and in different oxidation states. We have attempted to rationalize the linewidths on the basis of the equations describing the Solomon and Curie contributions to the nuclear transverse relaxation rate [Eqs. (1) and (2)]. When dealing with polymetallic systems, the S value of the ground state has been used in the equations. When the ground state had S = 0, reference was made to the S of the first excited state and the results were scaled for the partial population of the state. In addition, in polymetallic systems it is also important to account for the fact that the orbitals of each iron atom contribute differently to the populated levels. For each level, the enhancement of nuclear relaxation induced by each iron is proportional to the square of the contribution of its orbitals (54). In practice, one has to calculate the following coefficient for each iron atom ...

Polymetalated systems of this type without phenyl substitution at the lithiated carbon centre are only accessible when solutions of LiCioHg (144a) or LiDBB (145) in THF instead of a suspension of metallic lithium in THF are reacted with bis(phenylthiomethyl)silanes of type 155. In our group, variously substituted bis(lithiomethyl)silanes 117a, 156b-e and 101 were synthesized by reductive cleavage of the carbon-sulphur bond with LiCioHg... 

Several polymetallic systems experience magnetic coupling, either ferromagnetic or antiferromagnetic. Such magnetic coupling may affect the hyperfine shift and electron relaxation. Beneficial effects are always expected and observed on nuclear relaxation. A series of examples are discussed. 

THE INDUCED MAGNETIC MOMENT PER METAL ION IN POLYMETALLIC SYSTEMS, THE HYPERFINE CONTACT SHIFT, AND THE NUCLEAR RELAXATION RATES... 

The induced magnetic moment per metal ion in polymetallic systems 207... 

In polymetallic systems, the larger the number of coupled ions, the larger the spreading of the S levels. As a consequence, even relatively small / values give rise to large separations of the S levels and therefore to depopulation of the highest levels. The general theory is still the same. Analytical solutions, as in dimeric systems, are seldom possible. Often, numerical solutions are possible. 

IV. LUMINESCENT ORGANOMETALLIC POLYMETALLIC SYSTEMS AND COORDINATION POLYMERS CONTAINING BRIDGING ISOCYANIDES... 

Heteronuclear gold cluster compounds have, therefore, been proposed as potential precursors for the synthesis of selective catalysts and also as models of the modifications to the substrate that arise at a molecular level. Braunstein has reviewed in more detail the catalytic applications of gold-containing polymetallic systems (212). 

This proposal ignited a worldwide effort by transition metal dimer and cluster chemists to show that polymetallic systems could be effective homogeneous catalysts. Pittman and... 

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