Ortho-metalation reactions in binuclear dirhodium compounds: molecular structure and reactivity of a monometalated compound with the phosphine P(o-ClC6H4)(C6H5)2

The acetic acid adduct of Rh2(O2CCH3)3 (C6H4)P(o-ClC6H4)(C6H5) serves as a model Rh4(II) complex with a single ortho-metalated phosphine bridging two Rh atoms. The acetic acid can be displaced in solution by P(p-XC6H4)3. On heat treatment, a bimetallated compound with an additional (XC6H3)P(XC6H4) forms. The use of excess P(p-XC6H4) speeds up dimerization and allows total CH3COOH substitution. The character of X determines the reaction rate, increasing in the order of X = Cl to H to CH3. Both head-to-head and head-to-tail bridgings occur. Two reaction routes are proposed to account for these differences.