Spectroscopic studies of transition-metal ion complexes of diethylenetriaminepenta-acetic acid and diethylenetriaminepenta-methylphosphonic acid

Abstract

The structures of diethylenetriaminepenta-acetate (dtpa) and diethylenetriaminepentamethylphosphonate (dtpmp) complexes of transition-metal ions in aqueous solution have been investigated by n.m.r., e.s.r., and electronic absorption spectroscopy. It is shown that dtpa complexes are much less hydrated than corresponding ethylenediaminetetra-acetate (edta) complexes, consistent with the increase in stability constants. The most marked reduction in water binding on replacing edta by dtpa occurs with Fe³⁺ and Mn²⁺ and it is demonstrated using e.s.r. that there is a change in overall stereochemistry from a seven- to a predominantly six-co-ordinate structure for Fe³⁺. Other dtpa complexes adopt octahedral symmetry as do all the dtpmp complexes. Complexes of dtpmp with Co²⁺, Ni²⁺, and, to a lesser extent, Cu²⁺ have lower hydration numbers than the corresponding ethylenediaminetetramethylphosphonate (edtmp) counterparts, due to simultaneous binding of three nitrogen donors in dtpmp. The [Fe^III(dtpmp)]^7– complex has one bound water similar to [Fe^III(edtmp)]^5–, but the manganese(II) complex of dtpmp is more hydrated than the corresponding edtmp complex. It is concluded that phosphonate groups produce a less effective crystal field than do carboxyl groups.