Complexation equilibria and structures of dimethyltin(IV) complexes with N-methyliminodiacetate, pyridine-2,6-dicarboxylate, ethylenediamine-N,N′-diacetate and ethylenediamine-N,N,N′,N′-tetraacetate

The complexation equilibria of the dimethyltin(IV) ion with N-methyliminodiacetate (mida), pyridine-2,6-dicarboxylate (pdc), ethylenediamine-N,N′-diacetate (edda) and ethylenediamine-N,N,N′,N′-tetraacetate (edta) have been investigated in aqueous solution at 25.0 °C and I = 0.1 mol dm^-3 ((H, Na)NO₃) by potentiometric measurements. The overall stability constants β_pqr for [(SnMe₂)_pH_qL_r]^(2p+q-nr)+ (n = 2 for L = mida, pdc, edda; n = 4 for L = edta) defined as [[(SnMe₂)_pH_qL_r] ^(2p+q-nr)+] [SnMe₂²⁺]^-p[H⁺]^-q[L ^n-]^-r are evaluated as follows: log β_1-11 = 2.53(3), log β₁₀₁ = 9.625(4) and log β₁₁₂ = 20.73(4) for L = mida; log β₁₀₁ = 10.533(6), log β₁₁₁ = 12.65(5) and log β₁₁₂ = 17.69(3) for L = pdc; log β₁₀₁ = 12.412(8), log β₁₁₁ = 15.75(3), log β₁₁₂ = 24.12(4) and log β₁₂₂ = 30.87(4) for L = edda; log β_2-21 = 4.75(1), log β_2-11 = 10.44(2) and log β₂₀₁ = 15.41(2) for L = edta. The crystal structures of [SnMe₂(mida) (H₂O)]·H₂O, [SnMe₂(pdc) (H₂O)] · H₂O, [SnMe₂(edda)] and [(SnMe₂)₂(edta) (H₂O)₂] · H₂O have been determined by X-ray crystal structure analyses. The mida and pdc complexes have a dimeric structure which consists of distorted pentagonal-bipyramidal tin units with two methyl groups in the axial position. The edda complex has a distorted octahedral monomer structure with two methyl groups in the cis position. The edta complex consists of a polymer structure with spanning edta ligands in which each tin unit has a geometry quite analogous to those of the mida and pdc complexes. The preference in geometry of the dimethyltin(IV) complexes is discussed in terms of the effects of the bound ligands. The dimethyltin(IV) complexes formed in aqueous solution have been characterized on the basis of the molecular structures in the crystals and the values of the stability constants and pK_a.