Comparison of solid-state and solution structures of (R3P)2CdX2, (Et3P)2Cd2X4 and (Bu3P)3Cd2X4 complexes

The cadmium (II) phosphine complexes (Et3P)2Cd2X4, (R3P)2CdX2 [R3P = Ph3,P, Bu3P, Et3P, 1-phenyldibenzophosphole (DBP), and 1-phenyl-3,4,-dimethylphosphole (DMPP)] and (Bu3P)3Cd2X4 (X = Cl, Br, I) have been prepared and their solution and solid state structures determined by a combination of elemental analyses, conductance, infrared and NMR spectroscopy. The structures of (Ph3P)2CdI2 (1) and (DBP)2CdI2 (2) have been determined from three-dimensional X-ray data collected by counter methods. Compound 1 crystallized in space group P21,/a with a = 18.312 (9), b = 10.285 (5), c = 19.311 (9) A, β = 115.53 (4)° and Z = 4. Compound 2 crystallized in space group P21,/n with a = 12.698 (3), b = 15.302 (4), c = 17.477 (4) A, β = 96.66 (2)° and Z = 4. The structures were refined by least-squares methods with RF = 0.041 and 0.048 for 4157 and 3393 unique reflections with I/σ(I) ⩾ 2.0 for 1 and 2, respectively. Both molecules deviate from ideal C2v symmetry and have very slightly different Cd-I (1; 2.724 (2), 2.731 (2); 2; 2.718 (1), 2.721 (1) A) and Cd-P (1, 2.631 (2), 2.653 (2); 2; 2.616 (3), 2.603 (3) A) bond distances. The Cd-P bond distance differences are sufficient to give rise to a second order ABX CP/MAS 31P{1H} NMR spectrum for 1 but for 2 the phosphorus nuclei of the two DBP ligands are chemical shift equivalent. The CP/MAS 113Cd{1H} NMR spectra of both compounds 1 and 2 show apparent first order triplets. The (Bu3P)3Cd2X4 complexes are shown by variable temperature 31P{1H} NMR and conductance measurements to exist in solution as equilibrium mixtures of (Bu3P)2Cd2X4 and (Bu3P)2CdX2. CP/MAS 31P{1H} and 113Cd{1H} NMR spectra suggest that these compounds exist as doubly halide bridged (R3P)2Cd(μ-X)2CdX2(R3P) species containing four- and five-coordinate cadmium in the solid state. Equilibrium and activation thermodynamics for the ligand exchange processes of representative complexes have been determined from variable temperature 31P{1H} NMR spectra in CDCl3/CH2Cl2 (1:1) solutions.