Synthesis, X‐Ray Structural Analysis, and Thermal Decomposition of the Platinum(II) Carboxylic Acid (Hydroxycarbonyl) trans‐Pt(CO2H)(C6H5)(PEt3)2. Formation of a Diplatinum(II) Complex Containing Carbon Dioxide.

Carbon monoxide reacts at atmospheric pressure and temperature with trans-Pt(OH)(C6H5)(PEt3)2 (1) to give a metallacarboxylic acid, trans-Pt(CO2H)(C6H5)(PEt3) 2 (2). Crystals of 2 are monoclinic, space group C2/c, a = 15.006 (2) A, b = 11.520 (2) A, c = 27.135 (6) A, b = 89.90 (1)d, Z = 8, at 175 K. The structure was solved by heavy atom methods and refined by least-squares techniques to R = 0.033 (Rw = 0.038) for 3070 unique data (I g 3s(I)). It consists of a dimer in which two planar trans-Pt(CO2H)(C6H5)(PEt3) 2 units are joined by hydrogen bonding between the carboxylate groups. This structure, the O-H⋯O distance of 2.695 A, and the metrical data for the carboxylate group [r(C=O) = 1.238 (11) A, r(C-O) = 1.334 (10) A, and O-Ĉ-O = 117.7 (8) A] are similar to those found for many organic carboxylic acids. Solutions of 2 in organic solvents contain monomer and dimer in equilibrium; in formamide and N-methylformamide, dissociation to [Pt(C6H5)(CO)(PEt3)2]OH takes place. This dissociation accounts for many of the reactions of 2, e.g., facile exchange in the CO2H group with CO and reaction with HBF4, HCl, CH3OH, and (CH3)2NH to give, respectively, [Pt(C6H5)(CO)(PEt3)2]BF4, [Pt(C6H5)(CO)(PEt3)2]Cl, Pt(CO2Me)(C6H5)(PEt3)2, (3), and Pt(CONMe2)(C6H5)(PEt3)2 (4) (trans isomers in all cases). Although 2 does not form salts with KOH, KHCO3, or tertiary amines, it does react reversibly with its precursor 1 in a kind of acid-base or esterification reaction to give a dinuclear platinum(II) complex containing a m2-CO2 ligand, [trans-Pt(C6H5)(PEt3)2]2 (m2-CO2) (5), which has been identified by its NMR (13C, 31P, and 195Pt) and IR spectra. Complex 5 is also formed, together with ca. 0.5 mol of CO, when 2 is heated at ca. 100dC. This mode of decomposition contrasts with that normally observed for metallacarboxylic acids, viz. formation of a metal hydride and loss of CO2.