Magnetic quenching in carbonylated Ni clusters

We have seen that ligand-free, gas-phase Ni clusters have magnetic ground states (Section 4.6.3). This is completely different from carbonylated Ni clusters of the same size. Accurate magnetic measurements on these systems have unambiguously 

There are a few important consequences of this interaction, besides the magnetic quenching. While free Ni clusters have virtually no gap at the Fermi level, a typical sign of developing metallic character, carbonylated Ni clusters do have a gap of ca 1-2 eV, typical of semiconducting materials (Fig. 11). 

Another noteworthy consequence of the metal-ligand interaction in polynuclear cluster complexes is the reduction of the metal-metal bond strength in a ligated cluster compared to the bare metal core without ligands. To illustrate this effect we consider again Ni clusters and in particular a series of Nig clusters stabilized by terminal CO and 4-X ligands where X = PR, S, GeR, Te, As, etc. Several clusters 

The Ni-Ni distances, determined by X-ray diffraction, are ca. 2.61-2.67 A, i.e. considerably longer than the Ni-Ni distance in the bulk metal, 2.49 A. The geometric structure of Nig(/i4-PH)6(CO)8 has been optimized at the LSDA level the resulting optimum Ni-Ni distance is 2.65 A, in close agreement with experiment. When the same distance is recomputed after stepwise removal of the ligands, a progressive reduction, hence an increase of the metal-metal bond strength is found 

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