Experimental and computational investigations of arsenicIII and phosphorusIII complexes of bis(diphenylthiophosphinoyl)methanediide

The reactions of Li2[C(PPh2S)2] with EI3 (E = As, P) have been investigated in order to determine the nature of the coordination mode between the methanediide ligand and lighter pnictogens. Treatment of this reagent in toluene with AsI3 in a 1:1 M ratio produces the heteroleptic complex AsI[C(PPh2S)2] (2a), which was characterized by multinuclear NMR spectra and by a crystal structure determination. Attempts to isolate the phosphorus analogue PI[C(PPh2S)2] (2b) were unsuccessful. The dianionic methanediide ligand [C(PPh2S)2]2− in 2a is S,C,S-coordinated to the AsIII centre. The lengths of the weak As–S bonds differ markedly (2.852(2) and 2.443(2) Å) and the As–C bond distance 1.864(6) Å falls midway between single and double bond values. DFT calculations for AsI[C(PPh2S)2] reveal that the structure with a symmetrically coordinated ligand is lower in energy than the experimental structure by only 8.3 kJ mol−1 suggesting that the stabilization of the latter in the solid state is due to packing effects. The calculated bond orders for the M–C bond in MI[C(PPh2S)2] are 1.23 (M = As) and 1.31 (M = P). The π M–C interaction is predicted to be significantly stronger in the corresponding cations [M{C(PPh2S)2}]+ with bond orders of 1.68 (M = As) and 1.75 (M = P).