To assess the viability of 1,4diphosphabutadiyne (PCCP, 2 a) as a target for synthesis, we have investigated C 2 P 2 isomers 2, the end-on complexes [Cr(CO) 5 PCCP] (3) and [(CO) 5 Cr-(PCCP)Cr(CO) 5 ] (4), as well as their side-on analogues 6 and 7, respectively, using nonlocal density functional theory and a large, doubly polarized triple-z STO basis (BP86/TZ2P); C 2 N 2 isomers 1 were included for comparison. The PCCP molecule 2 a turns out to be a thermodynamically stable, linear CP dimer with a 298 K carbon ยฑ carbon bond dissociation enthalpy (BDE) of 152.2 kcal mol ร1 . The relatively high BDE and a C ร C bond length of 1.336 , together with the results of a careful bond analysis, show that the PCรCP bond is best conceived as having partial triple bond character (i.e., PCรCP 6 PรCCรP) similar to the NC ร CN bond. However, the relatively low HOMO ยฑ LUMO gap of 2.5 eV in the p system of 2 a [versus 5.6 eV in NCCN (1 a)] is indicative of a low kinetic stability: 2 a is likely to have a strong tendency toward polymerization. A conceivable strategy to protect and stabilize the evasive target molecule is coordination of the terminal P atoms to a transition metal fragment, for example, {Cr(CO) 5 }. The (first) CrรP bond dissociation enthalpy (for 298 K) in our model systems 3 and 4 amounts to 20 kcal mol ร1 ; side-on coordination (6 and 7) leads to an additional stabilization of about 1 ยฑ 2 kcal mol ร1 . In a preliminary investigation, the effect of double side-on coordination of the PC bond of 2 a to {Co 2 (CO) 6 } was also briefly explored and found to furnish considerable stabilization. Furthermore, the differences in geometry (e.g., linear versus nonlinear) and bonding in CP and CN dimers are discussed and related to the electronic structures of the monomers. The bonding in the linear CCรNN and CCรPP codimers is also analyzed. We also address the question why flash vacuum thermolysis (FVT) of norbornadienonazine, although it does contain the CNNC unit, yields nearly exclusively CNCN (1 b) and only a trace of CNNC (1 c).