The reaction of [NEt][Ni(CO)] in thf with 0.5 equiv of PCl affords the monophosphide [NiP(CO)] that in turn further reacts with PCl resulting in the tetra-phosphide carbonyl cluster [HNiP(CO)]. Alternatively, the latter can be obtained from the reaction of [NEt][Ni(CO)] in thf with 0.8-0.9 equiv of PCl. The [HNiP(CO)] penta-anion is reversibly protonated by strong acids leading to the [HNiP(CO)] tetra-anion, whereas deprotonation affords the [NiP(CO)] hexa-anion. The latter is reduced with Na/naphthalene yielding the [NiP(CO)] hepta-anion. In order to shed light on the polyhydride nature and redox behavior of these clusters, electrochemical and spectroelectrochemical studies were carried out on [NiP(CO)], [HNiP(CO)], and [HNiP(CO)]. The reversible formation of the stable [NiP(CO)] tetra-anion is demonstrated through the spectroelectrochemical investigation of [NiP(CO)]. The redox changes of [HNiP(CO)] show features of chemical reversibility and the vibrational spectra in the ν region of the nine redox states of the cluster [HNiP(CO)] (n = 3-11) are reported. The spectroelectrochemical investigation of [HNiP(CO)] revealed the presence of three chemically reversible reduction processes, and the IR spectra of [HNiP(CO)] (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNiP(CO)] and [HNiP(CO)] support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by H NMR spectroscopy failed, as previously found for related large metal carbonyl clusters. Thus, the presence and number of hydride ligands have been based on the observed protonation/deprotonation reactions and the spectroelectrochemical experiments. The molecular structures of the new clusters have been determined by single-crystal X-ray analysis. These represent the first examples of structurally characterized molecular nickel carbonyl nanoclusters containing interstitial phosphide atoms.

The reaction of [Ni6(CO)12]2– as a [NBu4]+ salt in CH2Cl2 with 0.8 equiv of PCl3 afforded [Ni14P2(CO)22]2–. In contrast, the reactions of [Ni6(CO)12]2– as a [NEt4]+ salt with 0.4–0.5 equiv of POCl3 afforded [Ni22–x P2(CO)29–x ]4– (x = 0.84) or [Ni39P3(CO)44]6– by using CH3CN and thf as a solvent, respectively. Moreover, by using 0.7–0.9 mol of POCl3 per mole of [NEt4]2[Ni6(CO)12] both in CH3CN and thf, [Ni23–x P2(CO)30–x ]4– (x = 0.82) was obtained together with [Ni22P6(CO)30]2– as a side product. [Ni23–x P2(CO)30–x ]4– (x = 0.82) and [Ni22P6(CO)30]2– were separated owing to their different solubility in organic solvents. All the new molecular nickel phosphide carbonyl nanoclusters were structurally characterized through single crystal X-ray diffraction (SC-XRD) as [NBu4]2[Ni14P2(CO)22] (two different polymorphs, P21/n and C2/c), [NEt4]4[Ni23–x P2(CO)30–x ]·CH3COCH3·solv (x = 0.82), [NEt4]2[Ni22P6(CO)30]·2thf, [NEt4]4[Ni22–x P2(CO)29–x ]·2CH3COCH3( x = 0.84) and [NEt4]6[Ni39P3(CO)44]·C6H14·solv. The metal cores’ sizes of these clusters range from 0.59 to 1.10 nm, and their overall dimensions including the CO ligands are 1.16–1.63 nm. In this respect, they are comparable to ultrasmall metal nanoparticles, molecular nanoclusters, or atomically precise metal nanoparticles. The environment of the P atoms within these molecular Ni–P–CO nanoclusters displays a rich diversity, that is, Ni5P pentagonal pyramid, Ni7P monocapped trigonal prism, Ni8P bicapped trigonal prism, Ni9P monocapped square antiprism, Ni10P sphenocorona, Ni10P bicapped square antiprism, and Ni12P icosahedron.

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