The diniobium octaphosphorus complex (P-8)[Nb(OC[(2)Ad]Mes)(3)](2) (1) (Ad = adamantylidene, Mes = 2,4,6-Me3C6H2) contains a reactive niobium phosphinidene moiety that can be exploited for metathetical scission of the Nb=P bond. When 1 is treated with aryl ketones, loss of ONb(OC[(2)Ad]Mes)(3)(OEt2) (2) is observed along with concomitant formation of the corresponding phosphaalkene (RC6H4)(2)C=PP7Nb(OC[(2)Ad]Mes)(3) (3- R). Complexes 3-R rearrange to incorporate the (RC6H4)(2)C=P unit into the phosphorus cage, thereby generating a saturated organo-phosphorus cluster complexed to the niobium tris-enolate platform, (RC6H4)(2)CP8Nb(OC[(2)Ad]Mes)(3) (4-R). The structure of one such rearranged cluster 4-H, as determined by X-ray crystallography, is briefly discussed. An Eyring analysis of the first-order rearrangement of 3-H to 4-H gives activation parameters of Delta H-double dagger = 16.7 kcal/mol and Delta S-double dagger = -20.4 eu. A Hammett analysis of the phosphaalkene rearrangement, 3-R to 4-R, with substitution varying at the para positions of the aryl rings, reveals a linear relationship between the a values and the rearrangement rate constants. A concerted, asynchronous mechanism for the least-motion rearrangement of 3-H to 4-H is presented. When 1 is treated with alkyl ketones, similar loss of 2 and formation of the corresponding phosphaalkene is observed; however, the phosphaalkene complexes have considerably greater stability and are readily isolated.