The reaction of the pentaphenylborole [(PhC)(4)BPh] (1) with sodium phogphaethynolate.1,4-dioxane (NaOCP(1,4-dioxane)(1.7)) afforded the novel sodium salt of phosphaboraheterocycle 2. It comprises anionic fused tetracyclic P/B-heterocycles that arise from multiple bond activation between the borole backbone and [OCP](-) anion. Density functional theory calculations indicate that the [OCP](-) anion prefers the form of phosphaethynolate O--C=P over phosphaketenide O= to interact with two molecules of 1, along, with various B-C, C-P, and C-C bond activations to form 2. The calculations were verified by experimental studies: (i) the reaction of 1 with NaOCP(1,4-dioxane)(1.7) and a Lewis base such as the N-heterocyclic carbene I-Ar [:C{N(Ar)CH}(2)] (Ar = 2,6-iPr(2)C(6)H(3)) and amidinato amidosilylene [{PhC(NtBu)(2)}(Me2N)Si:] afforded the Lewis base-pentaphenylborole adducts [(PhC)(4)B(Ph)(LB)] (LB = I-Ar, (3), :Si(NMe2){(NtBu)(2)CPh} (4)), respectively; (ii) the reaction of 1 with the carbodiimide ArN=C=NAr afforded the seven-memberecIB/N heterocycle [B(Ph) (cph)(4)C(=NAr)N(Ar)] (5). Compounds 2-5 were fully characterized by NMR spectroscopy and X-ray crystallography.