Some mixed ligand halo complexes of mercury(II). Experimental support for Basolo’s principle of crystallisation

By means of characterisation of four mixed ligand halo complexes of Hg(II), it is shown that matching of the sizes of the counter ions is crucial for crystallizing ionic complexes. [Display omitted] •Ligand L is the 1:2 condensate of 1,2-diaminoethane and 1-methyl-2-imidazolecarboxaldehyde.•L affords neutral Hg2Cl4L (1), Hg2Br4L·dimethylformamide (2) and Hg2I4L (3).•While 2 is a one dimensional polymer, 1 and 3 are not.•In reactions with R4NX, only 2 gives an ionic complex (4) that too when R=n-Bu and X=Br−.•Basolo’s principle of crystallisation of ionic complexes is vindicated. Neutral Hg2Cl4L (1), Hg2Br4L·dimethylformamide (2) and Hg2I4L (3) are synthesized where L is the 1:2 condensate of 1,2-diaminoethane and 1-methyl-2-imidazolecarboxaldehyde. X-ray crystal structures show that 1 and 3 are dinuclear with a distorted tetrahedral N2X2 (X=Cl or I) coordination sphere for Hg. Complex 2 is a one-dimensional polymer with bromide bridges. The metal has a square pyramidal (N2Br3) environment in all three complexes. Complexes 1–3 are reacted with R4NX (R=Me, Et or n-Butyl; X=Cl−, Br− or I−). While 1 and 3 show no reactions, 2 gives a crystalline complex (4) only when R=n-Bu and X=Br−. Complex 4 is ionic containing two independent n-Bu4N+ cations and two independent halves of centrosymmetric Hg2Br6L2− anions. The observed crystallisation of the anion Hg2Br6L2− only with the cation n-Bu4N+ and not with Me4N+ or Et4N+ is a consequence of Basolo’s principle of crystallisation of ionic salts which insists on the matching of the sizes of the counter ions. Though the size of the cation n-Bu4N+ matches with the hypothetical chloride and iodide analogues of 4, they are not formed. This is because while 2 has a tendency to increase the coordination number of Hg (as it polymerises), 1 and 3 do not show any such tendency.