DITHIOLENES REVISITED .2. ELECTRON-SPIN-RESONANCE STUDY OF THE FROZEN-SOLUTION EQUILIBRIUM 2))(2)L]-REVERSIBLE-ARROW[CO(S2C2(CF3)(2))(2)L(2)] [L=P(OR)(3) OR PPH(OME)(2) - R=ALKYL](L)

The ESR spectra have been recorded for a series of cobalt dithiolene c omplexes [Co(S(2)C(2)R(2))(2)L] [R = CF3; L = P(OMe)(3), P(OEt)(3), P( OBu(n))(3) or PPh(OMe)(2)]. In toluene, tetrahydrofuran or CH2Cl2-1,2- C2H4Cl2 solutions containing a small excess of L, reversible spectral changes are observed between 165 and 130 K which correspond to the add ition of a second ligand to form a six-co-ordinate complex. For L = P( OMe)(3) in toluene, equilibrium constants were estimated from the spec tra which lead to Delta H degrees = -10 kJ mol(-1), Delta S degrees = -68 J K-1 mol(-1) for the five-/six-co-ordinate equilibrium. Ligand ad dition is not observed when CF3 is replaced by Ph, 4-MeC(6)H(4) or 4-M eOC(6)H(4) or L = PPh(3), PEt(3), P(OPri)(3), P(OCH2CF3)(3) or P(OPh)( 3). Complexes with L = (Ph(2)P)(2)CH2, (Ph(2)PCH(2))(2) or [(MeO)(2)PC H2](2) are five-co-ordinate with no evidence for chelation. Thus both steric and electronic effects are critical to the ability of the five- co-ordinate complex to add a sixth ligand. Unlike the five-co-ordinate complexes which are significantly distorted from ideal C-2 upsilon sy mmetry, the six-co-ordinate complexes are highly symmetrical with grea ter delocalization of the unpaired electron onto the phosphite ligands .