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Oxide-free cobalt nanoparticles (NPs) of 2.5−3 nm diameter have been synthesized in the presence of di-isobutyl aluminum hydride (DiBAH), a potential alumina source, from two different precursors:  Co(η³C8H13)(η⁴C8H12) and Co[N(SiMe3)2]2. The NPs display very different structural and magnetic features, which are related to the difference in chemical environment at the NPs surface.

Very small nanometric oxide particles (CoO and Co3O4, around 2 nm in diameter) well dispersed inside a polymer matrix were prepared by solid-state oxidation of a metallic colloid precursor (1.6 nm Co particles). WAXS (wide-angle X-ray scattering) and HREM (high-resolution electronic microscopy) were used to observe very precisely the structural changes occurring during the oxidation process. With cobalt particles a few nanometers large, air oxidation at room temperature results in a surface passivation. Consequently the particles show a composite structure with a metallic core surrounded by an oxide surface layer. Preliminary magnetic measurements reveal that unidirectional exchange anisotropy between the metallic ferromagnetic core and the antiferromagnetic oxide layer occurs below 130 K.

The reaction of Pt(dba)2 with CO (1 atm) in toluene affords a brown precipitate which can be isolated and redissolved in CH2Cl2 to give a colloidal solution of fcc platinum particles with a large size dispersity (10−20 Å, I). Redissolution of I in THF leads to stable 12 Å fcc platinum particles (II) which can be isolated and characterized by high-resolution electron microscopy (HREM) and wide-angle X-ray scattering (WAXS). The original reaction in THF leads to the direct formation of II. Addition of 0.2 equiv of PPh3 to II yields a new colloid, III, of the same size but of icosahedral structure. III was characterized by HREM and spectroscopic techniques. In particular, ¹³C and ³¹P NMR spectroscopy demonstrates the absence of a Knight shift for these particles. Addition of excess triphenylphosphine leads to another species, IV, displaying broader size distribution with a maximum at 17 Å and an fcc structure. The reaction of I with more than 0.2 equiv of PPh3 affords mixtures containing colloids, clusters (predominantly Pt5(CO)6(PPh3)4), and mononuclear complexes. Both the formation of the colloids and their transformation into molecular species is rapid at room temperature. It is suggested that such processes may be more frequent in organometallic chemistry than previously thought.