The reaction of [NEt4](2)[Ni-6(CO)(12)] in thf with 0.5 equiv of PCl3 affords the monophosphide [Ni11P(CO)(18)](3-) that in turn further reacts with PCl3 resulting in the tetra-phosphide carbonyl cluster [HNi31P4(CO)(39)](5-). Alternatively, the latter can be obtained from the reaction of [NEt4](2)[Ni-6(CO)(12)] in thf with 0.8-0.9 equiv of PCl3. The [HNi31P4(CO)(39)](5-) penta-anion is reversibly protonated by strong acids leading to the [H2Ni31P4(CO)(39)](4-) tetra-anion, whereas deprotonation affords the [Ni31P4(CO)(39)](6-) hexa-anion. The latter is reduced with Na/naphthalene yielding the [Ni31P4(CO)(39)](7-) 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 [Ni11P(CO)(18)](3-), [HNi31P4(CO)(39)](5-), and [H2Ni31P4(CO)(39)](4-). The reversible formation of the stable [Ni11P(CO)(18)](4-) tetra-anion is demonstrated through the spectroelectrochemical investigation of [Ni11P(CO)(18)](3-). The redox changes of [HNi31P4(CO)(39)](5-) show features of chemical reversibility and the vibrational spectra in the upsilon(CO) region of the nine redox states of the cluster [HNi31P4(CO)(39)](n-) (n = 3-11) are reported. The spectroelectrochemical investigation of [H2Ni31P4(CO)(39)](4-) revealed the presence of three chemically reversible reduction processes, and the IR spectra of [H2Ni31P4(CO)(39)](n-) (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNi31P4(CO)(39)](5-) and [H2Ni31P4(CO)(39)](4-) support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by H-1 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.