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

Abstract Neutral Hg 2 Cl 4 L ( 1 ), Hg 2 Br 4 L·dimethylformamide ( 2 ) and Hg 2 I 4 L ( 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 N 2 X 2 (X = Cl or I) coordination sphere for Hg. Complex 2 is a one-dimensional polymer with bromide bridges. The metal has a square pyramidal (N 2 Br 3 ) environment in all three complexes. Complexes 1 – 3 are reacted with R 4 NX (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-Bu 4 N + cations and two independent halves of centrosymmetric Hg 2 Br 6 L 2− anions. The observed crystallisation of the anion Hg 2 Br 6 L 2− only with the cation n-Bu 4 N + and not with Me 4 N + or Et 4 N + 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-Bu 4 N + 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.