SINGLE AND DOUBLE PI-ALKYNE COORDINATION IN OXIDATION-REDUCTION EQUILIBRIA OF LOW-VALENT TRIMETHYLPHOSPHINECOBALT COMPLEXES - SPONTANEOUS FORMATION OF A MIXED-VALENCE DICOBALTATETRAHEDRANE CORE IN THE STRUCTURE OF CO2CL(MU-2-ME3E-CC-CC-EME3)(PME3)4 (E=SI, C)

Neutral dicobaltatetrahedrane complexes Co2Cl(mu2-Me3E-C=C-C=C-EMe3)(PMe3)4 (E = Si (1), C (2), Ge (3)) containing cobalt(0) and cobalt(I) have been isolated from oxidation-reduction equilibria. Compound 1 crystallizes in the triclinic space group P1BAR with a = 9.747(2), b = 12.140(4), c = 16.111(5) angstrom, alpha = 97.29(2), beta = 95.72(2), gamma = 103.70(2)-degrees, V = 1820.5 angstrom3, Z = 2. Final discrepancy indices are R = 0.064 and R(w) = 0.044 for 3110 independent data (F > 4sigma(F)). The structure has a Co2C2 tetrahedrane core formed by double pi-coordination of one alkyne function to Co-Co connected Co(PMe3)3 and CoCl(PMe3) fragments giving a 33-electron count for the tetrahedrane unit. ESR spectra (toluene solution at 298 K) suggest the unpaired electron to reside at the Co(PMe3)3 site in a metal orbital. Diphenylbutadiyne exclusively forms diamagnetic CoCl(eta2-PhC=CC=CPh)(PMe3)3 (4) while with Me3CC-CC-CCMe3 both types of complexes are observed. Under 1 bar carbon monoxide, 1 is transformed to mononuclear carbonyl(trimethylphosphine)cobalt compounds and diyne is liberated.