Molecular Nickel Phosphide Carbonyl Nanoclusters: Synthesis, Structure, and Electrochemistry of [NiP(CO)] and [HNiP(CO)] (n = 4 and 5).

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.