Comparative study of tertiary phosphine and arsine coordination to the transition metals. Stabilization of high formal oxidation states by o-phenylene-based chelate ligands

Abstract

o-Phenylenebis(dimethylphosphine), o-c&[P(CH3)2]2 (diphos), resembles its much studied arsenic analogue (diars) in its ability to stabilize high formal oxidation states of the transition elements. The first trivalent copper and silver complexes of a tertiary phosphine or arsine, planar [IrlL2l3+ and square-pyramidal [MClL2l2+ (M = Cu, Ag; L = diphos, diars), have been isolated. diphos resembles diars in forming octahedral, spin-paired, tri- and tetravalent nickel and iron complexes, ~rans-[MC12L2]~+ (M = Si, Fe; n = 1, 2), but differs from it in forming octahedral manganese(II), -(HI), and -(IV) complexes, trans- [MnC12L2lfl+ (n = 0-2), which are the first well-characterized tertiary phosphine complexes of manganese in its normal and higher oxidation states. These and other diphos complexes of general formula [M*+XnL2]”-” (M = first-, second-, or third-row transition metal; X = CI, Br; m = 1-4; n = 0-2) have been characterized by room-temperature magnetic susceptibility measurements, electronic and far-ir spectra, conductivities, and cyclic voltammetry. diars generally forms more labile complexes than diphos but appears to stabilize five- and six-coordination in d8 complexes more readily than diphos. The ligand field splitting parameter DqxY of 2850 cm-* estimated for diphos on cobalt(II1) is higher than that of most neutral donors. Many of the six-coordinate d3-d7 systems undergo reversible redox reactions, the potentials of which are generally only slightly affected (<lo0 mV) by interchange of diphos with diars, or of chloride with bromide, implying that the two group 5 ligands are about equal in their ability to stabilize high oxidation states such as nickel(1V) and iron(1V) toward reduction. In the d8 series the redox potentials are usually irreversible, but the trends suggest that diphos stabilizes the d8 configuration more effectively than diars. Electron-transfer processes in complexes of diphos and diars are believed to be primarily metal based, so that the ligands are “innocent”. The pioneering studies of Nyholm and co-workers since 1950 have shown o-phenylenebis(dimethylarsine), O-C&~[AS(CH3)2]2 (diars), to be a most versatile ligand which is capable of stabilizing a range of common and uncommon oxidation states and stereochemistries for almost all of the transition elements.’ We2 recently noted that the analogous di(tertiary phosphine) O-C~H~[P(CH~)~]~(~~P~OS)~-~ stabilizes high