Evidence from ESR studies for [Co(η‐C 2 H 4 ) 3 ] produced at 77 K in a rotating cryostat

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.