Evidence on ligand strengths from the crystal structures of [InCl3(dmf)3], [InCl3(dma)3], [{InCl3(PhCHO)3}2]·PhCHO and [InCl3(PhCOMe)2] from dimethylformamide (dmf), dimethylacetamide (dma), benzaldehyde and acetophenone

X-Ray crystal structure determinations have been made for the fac octahedral complexes [InCl3(dmf)3]1, [InCl3(dma)3]2 and [{InCl3(PhCHO)3}2]·PhCHO 3, from dimethylformamide (dmf), dimethyl-acetamide and benzaldehyde and the trigonal-bipyramidal complex [InCl3(PhCOMe)2]4 from acetophenone. The average In–O bond length for 3 is ca. 0.07 A greater than for 1 or 2. In 4, In–O is 0.07 A longer than that known for [InCl3(tmu)2] from tetramethylurea (tmu). An extension of the comparison to structures involving other ligands shows there to be a general correspondence between In–O bond lengths and ligand donor values. By contrast with the weak co-ordination of aldehydes and ketones, co-ordination of an amide is enhanced through polarisation. In the crystal of 3, individual [InCl3(PhCHO)3] molecules are paired, almost centrosymmetrically, with intermeshing of their PhCHO ligands.