New rhodium complexes with phosphonate-phosphane ligands (RO) 2 P(O)-X-PPh 2 (1a-d), a, X ) CH 2 ; b, X ) (CH 2 ) 2 ; c, X ) (CH 2 ) 3 ; d, X ) p-C 6 H 4 , have been synthesized and characterized. Both, open-chain structures [ClRh(cod)PPh 2 -X-P(O)(OR) 2 ] (2a-d) and cyclic complexes [(cod)-Rh(PPh 2 -X-P(O)(OR) 2 )]A (3a-c, A ) BF 4 , PF 6 ) were isolated after stoichiometric reactions at mild conditions. FTIR investigations of 2b supported on silica at temperatures between 150 and 250 °C suggest that the phosphonate-phosphane ligand stabilizes rhodium monocarbonyl species and allows also the formation of free coordination sites to form dicarbonyl species, which is in accord with the proposed hemilabile behavior of the complexes in methanol carbonylation. During the catalytic cycle, the phosphane group is assumed to be strongly coordinated to the rhodium, while the phosphoryl oxygen of the phosphonate group is supposed to change between a free and a coordinated state, thus vacating or occupying a coordination site. This is supported by the findings that the activation enthalpies for the open-chained catalyst precursors 2a-d increased significantly with growing distance between the phosphonate and phosphane groups. The new rhodium-phosphonate-phosphane complexes 2a and 2b afforded also heterogeneous catalysts with enhanced activity in vapor-phase carbonylation. Activated carbon has been found to be a suitable support for hemilabile rhodium complexes, but normal diffusion of reactants begins to limit the high intrinsic carbonylation rate over the supported catalysts. Experimental SectionMaterials. Methanol, methyl iodide, and methyl acetate (Merck) were used as received. Dichloromethane was dried