RESUMEN
Co atoms were reacted with ethene at 77 K and the paramagnetic products studied by electron spin resonance (ESR) at X- and K-bands. The ESR spectra of the major product at both frequencies showed eight cobalt multiplets (ICo=7/2) indicating a mono-cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g1=2.284, g2=2.0027, g3=2.1527; A1<-25 MHz, A2=-109 MHz, A3=-198 MHz. Proton and 13C (1% natural abundance) hyperfine couplings were lower than the line widths (<2 MHz) indicating less than 0.5 spin transfer to the ethene ligands. We assigned the spectrum to a Jahn-Teller-distorted planar trigonal mono-cobalt tris-ethene [Co(eta-C2H4)3] complex in C2v symmetry. The SOMO is either a 3dx2-y2 (2a1) orbital in a T-geometry or a 3dxy (b1) orbital in a Y-geometry but there is only a spin density, a2, of 0.30 in these d orbitals. The spin deficiency of 0.70 is attributed to two factors; spin transfer from the Co to ethene pi/pi* orbitals and a 4p orbital contribution, b2, to the SOMO. Calculations of a2 and b2 have been made at three levels of spin transfer, theta. At theta=0.00a2 is 0.23 and b2 is 0.78, at theta=0.25a2 is 0.25 and b2 is 0.52 and at theta=0.50a2 is 0.28 and b2 is 0.23. The other possible assignment to a mono-cobalt bis-ethene complex [Co(eta-C2H4)2] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to approximately 220 K indicating a barrier to decomposition of approximately 50 kJ Mol-1
RESUMEN
The structure of B8F12 has been shown by gas electron diffraction and computational methods (up to MP2/6-31+G*) to have the same highly asymmetric form observed in crystalline phases. The structure can be regarded as derived from a central B2 group, bridged by two BF2 groups to give a central B4 core that is folded, not planar, and with a very short bond [164.3 pm calculated, 164.2(19) pm experimental] along the fold line. There are also four terminal BF2 groups. One of the other four bonds in the core is consistently 20-30 pm longer than the others. This asymmetry has been attributed to many intra-molecular B...F interactions, particularly those between core boron atoms and fluorines of the terminal BF2 groups. Calculations for the chloro analogue lead to a structure similar to that for B8F12, but with the long core bond extended so that one of the bridging BCl2 groups may now be regarded as terminal. With bromine as the halogen the structure changes again, with one bromine atom taking up a bridging position. With iodine, this process continues further, and there are three bridging iodine atoms. However, in this case this is not the lowest energy structure, and instead a loosely associated dimer of B4I6 is preferred. In all these cases, and particularly with the heavier halogens, there are huge differences between the results obtained with different computational methods.
