A theoretical analysis of ambivalent and ambiphilic Lewis acid/bases with symmetry signatures

• DFT calculations have been completed which define the relative abilities of ligands to adopt non-linear and non-planar geometries.
• The trans-influences of linear and planar and bent and non-planar ambivalent and ambiphilic ligands are compared using DFT calculations.
• The energetics of valence tautomerism in nitrosyl SO2 and phophido-complexes are discussed.
• The energetics of SO2, AlCl3 and NH3 and CO adducts of zero oxidation state complexes of platinum phosphine complexes have been calculated and discussed.
• The role of the bite angle of the spectator ligand on the relative stabilities of the isomeric forms of SO2 complexes has been evaluated.

This review provides a theoretical underpinning of previously published definitions of ambidentate, ambivalent and ambiphilic ligands. The study encompasses ambivalent ligands such as NO, NR, N2R; ambiphilic molecules such as SO2, I2 and ambiphilic transition metal complexes, e.g. [Pt(PCy3)2]. These ambivalent molecules adopt alternative geometries which depend primarily on the number of electrons which they formally donate or accept. The theoretical analysis focuses initially on those complexes where the same ligand displays ambivalent properties within the same molecule in order to define the energetics of their interconversion. These square-pyramidal complexes provide a test-bed for generating data which throws light on the relative abilities of ambivalent ligands to adopt linear or bent geometries. The ligands were compared with NO and their relative abilities were placed in the following order PO > PH2 > N2H > SO2 > NO > NH2 > NS. The linear nitrosyl ligand does not exert a trans-influence and this property has been contrasted with the nitrido-ligand which shows a large trans-influence. The conversion of NO to a non-linear geometry results in a strong trans-influence and this has significant catalytic and biological importance. Calculations on octahedral palladium complexes have been used to order the trans-influences of ambivalent ligands when they adopt their alternative symmetry signatures. The relative trans-influences are NO > PH2 > NS > N2H > NH2. The interconversion of linear and bent dinitrosyls provides an interesting inorganic example of valence tautomerism and this is noted as a general characteristic of ambivalent and ambiphilic ligands. The soft energy surface associated with these interconversions leads to the experimentally verified fluxional process. The energetics of adduct formation by ambiphilic ligands has been studied using a series of SO2 complexes of palladium and platinum and the results contrasted with adducts of SO2 with main group Lewis acids and bases. The isomers {(PH3)2M(SO2) p}16 and {(PH3)2M(SO2) np}14 are calculated to have very similar energies and the relative stabilities of analogous isomers may be manipulated by varying the bite angle of the phosphine ligands in {[(PH2)2CnH2n]M(SO2)}.