Unusual hetero-atomic RhSCNSb(Rh) co-ordination ring. Synthesis and X-ray structure of [Rh(N1,S2-2-thiopyrimidinato)2(S2(Rh),N1(Sb)-2-thiopyrimidinato){Sb(C6H5)3}] and long time sought structure of mer-[RhCl3{Sb(C6H5)3}3]

The reaction of RhCl3 center dot 3H(2)O with SbPh3 (Ph, C6H5) in ethanol produced mer-[RhCl3(SbPh3)(3)] (1), as the main product, together with trans-[RhCl2(Ph)(SbPh3)(3)](2) (R. Cini, G. Giorgi, L. Pasquini, Inorg. Chim. Acta 196 (1992) 7; A. Cavaglioni, R. Cini, J. Chem. Soc., Dalton Trans. (1997) 1149). Ways to improve the yield for 1 and 2 were a subject of further works by this laboratory during the past decade and the reactivity of the organometallic compound 2 was investigated towards a variety of bases. Long sought single crystals of 1 suitable for X-ray diffraction have been obtained from hot acetone solution. The metal centre has a pseudo-octahedral coordination environment with the three chloride anions and the three SbPh3 molecules in mer positions. The Rh-Cl bond trans to Sb is significantly weakened by the high trans influence by antimony itself. This gives the rationale for the formation of 2 from RhCl3 and SbPh3 via the preliminary formation of 1, followed by dissociation of the activated chloride anion, and the subsequent formation of the Rh-C(Ph) linkage. The reaction of 1 with 2-thio-1,3-pyrimidine (Htpym) in ethanol produces crystalline [Rh((NS2)-S-1-tpym)(2)(N-1(Sb),S-2(Rh)-tpym)(SbPh3)] (3). Single crystals suitable for X-ray diffraction studies as obtained from ethanol/diethyl-ether contain a complex molecule and a quarter of an ethanol molecule in the asymmetric unit. Two out of three tpym(-) anions chelate the metal centre via the N-1 and S-2 atoms, whereas the third tpym- anion behaves as mono-dentate through the S-2 atom only. The sixth co-ordination position is occupied by the Sb atom. Interestingly, one of the N atoms of the monodentate tpym- anion behaves as electron donor to Sb and a Sb-N linkage forms (d((sb-N)), 2.846 angstrom). As a consequence, an unusual totally hetero-atomic five-membered co-ordination ring of the type RhSCNSb(Rh) forms. A molecular modelling analysis at the hybrid density functional level DFT-B3LYP was performed on selected coordination molecules.