1,4-Diphosphabutadiyne: A realistic target for synthesis? A theoretical investigation of C2P2, C2N2, [Cr(CO)(5)PCCP], and [(CO)(5)Cr(PCCP)Cr(CO)(5)]

To assess the viability of 1,4-diphosphabutadiyne (PCCP, 2a) as a target fo r synthesis, we have investigated C2P2 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 targe, doubly polarized triple-zeta STO basis (BP86/TZ2P); C2N2 isom ers 1 were included for comparison. The PCCP molecule 2a turns out to be a thermodvnamically stable, linear CP dimer with a 298 K carbon-carbon bond d issociation enthalpy (BDE) of 152.2 kcal mol(-1). The relatively high BDE a nd a C-C bond length of 1.336 Angstrom, together with the results of a care ful bond analysis, show that the PC-CP bond is best conceived as having par tial triple bond character (i.e., P=C-C=P<->P-C=C-P) similar to the NC-CN b ond. However. the relatively low HOMO-LUMO gap of 2.5 eV in the re system o f 2a [versus 5.6 eV in NCCN (la)] is indicative of a low kinetic stability: 2a is likely to have a strong tendency toward polymerization. A conceivabl e strategy to protect and stabilize the evasive target molecule is coordina tion of the terminal P atoms to a transition metal fragment, for example, { Cr(CO)(5)}. The (first) Cr-P bond dissociation enthalpy (for 298 K) in our model systems 3 and 4 amounts to 20 kcal mol(-1); side-on coordination (6 r ind 7) lends to an additional stabilization of about 1-2 kcal mol(-1). In a preliminary investigation, the effect of double side-on coordination of th e P=C bond of 2a to {Co-2(CO)(6)} was also briefly explored and found to fu rnish considerable stabilization. Furthermore, the differences in geometry (e.g. I linear versus nonlinear) and bonding in CP and CN dimers are discus sed and related to the electronic structures of the monomers. The bonding i n the linear CC-NN and CC-PP codimers is also analyzed. We also address the question why flash vacuum thermolysis (FVT) of norbornadienonazine, althou gh it does contain the CNNC unit, yields nearly exclusively CNCN (1b) and o nly a trace of CNNC (1c).