The reaction between [Ru3(μ-H)(μ-NC5H4)(CO)10] and chlorodiphenylphosphine in refluxing chlorobenzene, followed by metathesis with bis(triphenylphosphoranylidene)ammonium chloride ([PPN]Cl), affords [PPN][Ru8(μ8-P)(CO)22] (1) in around 30% yield. 31P-NMR solution spectra are consistent with the presence of at least two isomers of the cluster anion, presumably due to differing carbonyl distributions; the chemical shifts for these configurations (600–800 ppm downfield of H3PO4) are consistent with a highly deshielded interstitial phosphorus atom. An X-ray structural study of one isomer of 1 reveals that the phosphorus atom occupies an interstitial square antiprismatic site defined by the eight ruthenium atoms, with two bridging carbonyl ligands on opposite faces spanning interplanar Ru–Ru bonds, and twenty terminal carbonyl ligands completing the ligand set. The solid state 31P-NMR spectrum of the crystallographically-identified isomer reveals a signal at 596.1 ppm assigned to the square antiprismatic interstitial phosphorus atom. Formal electron counting suggests that 1 has four electrons less than expected using Wade's rules. The reductive electrochemistry of 1 has been examined by cyclic voltammetry, and reveals the presence of two one-electron and one two-electron reduction waves, an uptake of four electrons in total, consistent with the cluster's theoretical electron deficiency in the resting state.