Preparation, crystal structures and spectroscopic properties of chloro(pentane-2,4-dionato){1,1,1- tris(dimethylphosphinomethyl)ethane}chromium(III), -cobalt(III) and -rhodium(III) hexafluorophosphate: Comparison of the M-P, M-Cl and M-O (M = Cr, Co and Rh) bond lengths among the three M^III complexes

Three metal(III)-tripodal tridentate phosphine complexes of the formula [MCl(acac)(tdmme)]PF₆, where M = Cr, Co or Rh; acac = pentane-2,4-dionate; tdmme = 1,1,1 -tris(dimethylphosphinomethyl)ethane, were prepared and their molecular and crystal structures and spectroscopic properties examined. All the complexes crystallized in an isomorphous space group P2₁2₁2₁, which is indicative of spontaneous resolution of the enantiomeric pair. The Cr-P bond lengths in [CrCl(acac)(tdmme)]PF₆ (average 2.437 Å) are remarkably longer than the corresponding Co-P (average 2.201 Å) and Rh-P (average 2.252 Å) ones. The longer and therefore the weaker Cr-P bonds may explain the relatively weak trans influence of the -PMe₂ group in [CrCl(acac)(tdmme)]PF₆; among the three M^III complexes, the elongation of M-O bond lengths in [MCl(acac)(tdmme)]PF₆ from those in [M(acac)₃] has been found to be Cr^III (0.016 Å) < Co^III (0.059 Å) < Rh^III (0.096 Å), indicating a much weaker trans influence of the -PMe₂ group in the Cr^III complex than in the CO^III and Rh^III complexes. The first d-d transition band of [CrCl(acac)(tdmme)]⁺ was observed at 18590 cm^-1. This is appreciably smaller than the corresponding band of the C^III complex at 20960 cm^-1, suggesting the ligand-field strength of tdmme is fairly weak in the Cr^III complex. The ligand-field strength Δ and the Racah's β parameters of [CrCl(acac)(tdmme)]⁺ were also discussed.