The chemical activation of white phosphorus by transitionmetal complexes is a versatile albeit often unpredictable route to compounds containing homoatomic phosphorus ligands with nuclearities ranging between 1 and 29. [1] In principle, many of the same transformations available for P 4 should also be possible employing other closely related cage compounds, such as the bare Group 15 Zintl anion [P 7 ] 3À , although the products resulting from the activation of these anionic cages have little in common with those typically observed for white phosphorus. In most instances, heptapnictide [E 7 ] 3À anions (E = P, As, Sb) react with metal reagents by maintaining their structure and nuclearity or undergoing the activation of a single EÀE bond, resulting in a geometric change of the starting nortricyclane-like cage (C 3v ) to a geometry reminiscent of norbornadiene (C 2v ). [2,3] More extensive [E 7 ] 3À activation reactions have been reported on a handful of occasions for the heavier Group 15 elements (E = As, Sb), yielding species with increased cluster nuclearities and often unprecedented geometries. [3][4][5][6][7][8][9][10] Such transformations are entirely without precedent for the [P 7 ] 3À cage, however, which is presumably due to the greater bond dissociation energy of the P À P single bond. [11] Herein we report the chemical activation of the heptaphosphide trianion, [P 7 ] 3À , by the transition-metal complex [Co(mes) 2 (PEt 2 Ph) 2 ] (mes = 2,4,6-trimethylphenyl) to yield the novel anionic species [Co(h 5 -P 5 ){h 2 -P 2 H(mes)}] 2À (1; Figure 1). To our knowledge, this reaction is the first example of a transformation involving the chemical activation of an [E 7 ] 3À cage where the nuclearity of the heptaphosphide starting material remains the same but where a significant alteration of the nortricyclane-like cage geometry has resulted. The chemical activation of neutral systems with related topologies has been reported previously. [12] Coordination to the cobalt metal center has allowed us to "trap" the known cyclopentaphosphide ligand [P 5 ] À alongside the diphosphene-like moiety P 2 H(mes). The isolobal relationship between phosphorus atoms and CÀH fragments has led to phosphorus being referred to as the carbon copy. [13] This relationship makes [Co(h 5 -P 5 ){h 2 -P 2 H(mes)}] 2À a model system for the unknown family of anionic 18-electron organometallic complexes [Co(h 5 -C 5 H 5 )(h 2 -H 2 C = CHR)] 2À , which are in turn closely related to cobalt(I) bis(alkene) complexes, such as [CpCo(cod)] (cod = 1,5-cyclooctadiene) and Jonass reagent. [14, 15] Reaction of an ethylenediamine (en) solution of K 3 P 7 with one molar equivalent of [Co(mes) 2 (PEt 2 Ph) 2 ] yielded the heteroatomic ion [Co(h 5 -P 5 ){h 2 -P 2 H(mes)}] 2À . Monitoring of the crude reaction mixture by 31 P NMR spectroscopy reveals the presence of other minor unidentified phosphorus-containing side-products in addition to [Co(h 5 -P 5 ){h 2 -P 2 H-(mes)}] 2À . The [Co(h 5 -P 5 ){h 2 -P 2 H(mes)}] 2À cluster anion can be isolated as a pu...