Concerning the outer sphere relaxation theory, the sensitivity of a T(2) MRI contrast agent, expressed by the transverse relaxivity r(2), depends on the diffusion length of water molecules relative to the particle size. For T(2)-weighted spin-echo imaging, theoretical concepts reveal three regimes regarding the r(2) relaxivity depending on the nanocrystal size: the motional averaging regime (MAR), the static dephasing regime (SDR), and the echo-limiting regime (ELR). The r(2) maximum corresponds to the SDR, which represents a small size regime. To verify the theoretical concepts and to adjust the SDR, tailor-made T(2) contrast agents were synthesized by controlled self-assembly of superparamagnetic iron oxide nanocrystals (SPIOs) into raspberry-like nanoclusters with diameters of 30-200 nm using a PEG-based ligand. The results highlight an opportunity to optimize the relaxivity of T(2) contrast agents by tuning the cluster size of SPIO nanocrystals.

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phinomethyl)pyridine) have been prepared and characterized by 1 H and 13 C NMR spectroscopy, and the molecular structure of the ethylene complex has been determined by X-ray diffraction analysis. Nucleophilic attack at the coordinated double bond takes place more readily than for known neutral and monocationic species, and a variety of protic nucleophiles NuH (MeOH, H 2 O, aromatic amines) give the corresponding -functionalized σ-alkyl complexes [Pt(PNP)(CHRCHR′Nu)]BF 4 also in the absence of auxiliary bases. In the case of the styrene derivatives an unusual decomposition pathway gives the stable alkenyl complex [Pt(PNP)(CHdCHPh)]BF 4 . A competitive equilibrium process between substitution and addition was revealed by reacting ethylene with the isolated aniline complex [Pt(PNP)-(PhNH 2 )](BF 4 ) 2 . The nucleophilic addition product was found to be thermodynamically favored over the substitution product more than in the case of the analogous palladium complexes.

Among a large variety of fine-tuning parameters for homogeneous catalysts the net charge of transition-metal complexes appear to be an interesting factor that considerably affects activation of substrates and catalytic activity in general. The electrophilicity of coordinated alkenes in transition-metal complexes can be strongly enhanced by increasing the positive net charge, resulting in strong carbocationic properties. Theoretical and experimental studies have shown that the alkene in cationic complexes is kinetically and thermodynamically more activated towards nucleophilic addition than in neutral complexes. The concept of increasing the positive complex charge is thought to be useful for the development of new catalysts for reactions in which alkenes or other unsaturated substrates are involved.

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