In this study, three isolobal complexes of the form [{LnM}(η3-P4R2)]+ ({LnM} = {CpMo(CO)2} (A), {Cp'''Ni} (B), {Cp'''Co}– (C), R = Ph, iPr; Cp''' = 1,2,4-tBu3C5H2) are reacted with nucleophilic carbenes (L). While C does not show any reactivity, the cationic complexes A and B undergo addition reactions. The respective products [CpMo(CO)2(η3-P4R2L)]+ (1a – d) and [Cp'''Ni(η1:1-P4R2L)]+ (3a – d) show different geometries for the L-P3PR2 ligands. Their reactivity towards EtO– results in either a simple addition (for 1) or a complex addition, ring-opening, rearrangement sequence (for 3). Moreover, [Cp'''Ni(η2-IDippPP(OEt)PPiPr2)] (4) could be methylated by the reaction with MeOTf, which affords an iso-tetraphosphine ligand, marking the first example of complete functionalization of a polyphosphorus ligand to a complexed phosphine. Mechanistic studies shed light upon the fundamental principles, which differentiate the influence of the isolobal {CpMo(CO)2}, {Cp'''Ni}, and {Cp'''Co}– transition metal units. Lastly, 3a–d were chosen as model substrates for further nucleophilic functionalization. In this regard, 3 reacts with [CN]− in a [3+1] fragmentation reaction affording the dimerized species [{Cp'''Ni}2(μ,η1:1:1:1-cyclo-P4(PR2)2)] (6a: R = Ph, 6b: R = iPr) together with IDippP-CN. In contrast, the reaction with [ECO]− (E = P, As) led to an extension of the pnictogen framework yielding [Cp'''Ni(η1:1-EP4Ph2IDipp)] (8a: E = P, 8b: E = As).