Abstract
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 (I~Co~ = 7/2) indicating a mono‐cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g~1~ = 2.284, g~2~ = 2.0027, g~3~ = 2.1527; A~1~ < − 25 MHz, A~2~ = − 109 MHz, A~3~ = − 198 MHz. Proton and ^13^C (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(η‐C~2~H~4~)~3~] complex in C~2__v__~ symmetry. The SOMO is either a 3d~x__2−y__2~ (2__a~1~) orbital in a T‐geometry or a 3d~xy~ (b~1~) orbital in a Y‐geometry but there is only a spin density, a^2^, 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 π/π* orbitals and a 4p orbital contribution, b^2^, to the SOMO. Calculations of a^2^ and b^2^ have been made at three levels of spin transfer, θ. At θ = 0.00__a^2^ is 0.23 and b^2^ is 0.78, at θ = 0.25__a__^2^ is 0.25 and b^2^ is 0.52 and at θ = 0.50__a__^2^ is 0.28 and b^2^ is 0.23. The other possible assignment to a mono‐cobalt bis‐ethene complex [Co(η‐C~2~H~4~)~2~] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to ∼220 K indicating a barrier to decomposition of ∼50 kJ Mol^−1^ Copyright © 2006 John Wiley & Sons, Ltd.