Dedicated to Professor Hans Bock on the occasion of his 60th birthdayIn both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three-or four-membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe,)' (E = S, Se, Te) and E'R(SiMe3)2 (R = Ph, Me, Et; E' = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations: a variety of reactions in the ligand sphere are also possible.

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A surprisingly easy access to the preparation of transition‐metal clusters is the reaction of MCl2 (M  Co, Ni) with PhP(SiMe3)2 and PPh3. In this way [Co4(μ3‐PPh)4(PPh3)4] and [Ni8(X)4(μ4‐PPh)6(PPh3)4] (XCl and CO) can be isolated. The paramagnetic complex with X  Cl contains 116 valence electrons and can be considered as a mixed‐valence cluster of Ni0 and Ni1+.

k 0 0~1 -1~ 1' 1 Fig. 2. J3-Keto esters I and 2 2 l a l b Fig. 3. Stereoisomers of 1. The bonds to the H-atoms on C2a, CS, C6, C8, and C9 are shown.nary measurement of the hetero NOE effect, the multiplet lines of the C5 protons and then of the C9 protons were, as described above, successively irradiated several times : the two spectra were (in each case at the stage of the free induction decay) subtracted from each other. The I3C-NMR difference spectrum (Fig. 4a) shows a positive signal for C4a and, thus, close proximity to the protons on C5, and a negative signal for C9a, and thus, a close proximity to the protons on C9. Since the quaternary C-atoms C4a and C9a can be identified from their chemical shifts, the position of the lactone bridge relative to the methine group C6 is established and, thus, also the structure of the main component 1. c4a a) c7 C1' I C1I C? 230 220 210 200 1 so Fig. 4. Hetero-NOE difference spectra (6 values). a) Difference of the "C-NMR spectra in the region of C4a and C9a after irradiation of the protons on CS and C9; b), c) difference of the "C-NMR spectra in the carbonyl region after irradiation of the proton on C8 (b) and of the proton on C6 (c). 706 0 VCH VerlagsyesellschaJi mhH. 0-6940 Wernherm, I985 0570-083For clarification of the stereochemistry in the eightmembered ring, the methine proton on C6 and one of the methyiene protons on C8 were irradiated separately in a second hetero-NOE experiment. On the basis of the 'Hcoupling constants for their coupling with the neighboring methylene protons it could be assumed that they were the protons which lie relatively close to the lactone bridge in the case of l a but not so in the case of l b (Fig. 3).Hence, in the presence of l a an NOE effect on C11 had to be expected from at least one of these protons. Indeed, the difference spectrum (Fig. 4c) shows, in the case of the irradiation of the proton on C6, a signal of C 11 as well as the expected signals of C7 and C I ' (ester carbonyl-C). In the second difference spectrum (Fig. 4b), after irradiation of the one proton of the methylene group C8, only the expected signal of C7 appears. This confirms the finding of the preliminary NOE experiment and, due to the spatial proximity of the proton of the methine group C6 to C11 (lactone carbonyl-C), proves the presence of structure l a .Thus, as a result of increased selectivity, the hetero-NOE experiment can be successfully employed even when strongly overlapping proton spectra are obtained.We recently described a simple method for the preparation of PPh-bridged clusters of Co and Ni. Reaction of MC12 ( M = C o , Ni) with PhP(SiMe,)2 in the presence of Ph3P leads to the formation of SiMe,CI and [Co&,-PPh)4(PPh3)4] or 1.['.21 1 contains four coordinatively unsaturated Ni atoms which are shielded in a crown-like fashion by Ph groups of the PPh-and PPhJigands. As a result the cluster contains a channel of about 400-500 pm in diameter, and only small molecules, e.g. CO, can bind to the four Ni atoms with coordination vacancies.In attempts to prepa...