Basicities of the cyanide ligands in a series of Cp'M(L)(2)CN complexes were investigated by measuring their heats of protonation (-Delta N-CNH) by CF3SO3H in 1,2-dichloroethane solution at 25.0 degrees C to give Cp'M(L)(2)(CNH)+CF3SO3-, in which the N-H+ group is probably hydrogen-bonded o the CF3SO3- anion. Basicities (-Delta H-CNH) Of the CpRu(PR3)(2)CN complexes increase from 20.5 (PPh3) to 22.4 (PMe3) kcal/mol with increasing donor abilities of the phosphine ligands. Basicities of all the Cp'Ru(PR3)(2)CN complexes, where Cp' Cp or Cp*, are linearly correlated with their nu CN 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 nu CN values parallel nu CN values of their protonated Cp'M(L)(2)(CNH)(+) analogues. Also, P-31 NMR chemical shifts of the unprotonated Cp'M(PR3)(2)CN and protonated CpM(PR3)(2)(CNH)(+) complexes are linearly related. Despite the high basicity of Ru in Cp*Ru(PMe3)(2)Cl (30.2 kcal/mol), the CN-in Cp*Ru(PMe3)(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.