Cyanide Ligand Basicities in Cp‘M(L)2CN Complexes (M = Ru, Fe). Correlation between Heats of Protonation and νCN

Basicities of the cyanide ligands in a series of Cp‘M(L) 2 CN complexes were investigated by measuring their heats of protonation (−Δ H CNH ) by CF 3 SO 3 H in 1,2-dichloroethane solution at 25.0 °C to give Cp‘M(L) 2 (CNH) + CF 3 SO 3 -, in which the N−H + group is probably hydrogen-bonded to the CF 3 SO 3 - anion. Basicities (−Δ H CNH ) of the CpRu(PR 3 ) 2 CN complexes increase from 20.5 (PPh 3 ) to 22.4 (PMe 3 ) kcal/mol with increasing donor abilities of the phosphine ligands. Basicities of all the Cp‘Ru(PR 3 ) 2 CN complexes, where Cp‘ = Cp or Cp*, are linearly correlated with their νCN values; the nonphosphine complexes, CpRu(1,10-phen)CN and CpRu(COD)CN, do not follow the same correlation. For a large number of Cp‘M(L) 2 CN complexes (M = Ru, Fe, L 2 = mono- and bidentate phosphines, CO, 1,10-phen, and COD), their νCN values parallel νCN values of their protonated Cp‘M(L) 2 (CNH) + analogues. Also, 31 P NMR chemical shifts of the unprotonated Cp‘M(PR 3 ) 2 CN and protonated CpM(PR 3 ) 2 (CNH) + complexes are linearly related. Despite the high basicity of Ru in Cp*Ru(PMe 3 ) 2 Cl (30.2 kcal/mol), the CN - in Cp*Ru(PMe 3 ) 2 CN (25.0 kcal/mol) is the site of protonation; factors that determine whether protonation occurs at the Ru or the CN - are discussed.