TRANSITION-METAL HYDRIDES AND CARBONYL ANIONS AS LIGANDS TOWARD A PT CENTER IN RHENIUM-PLATINUM TRIANGULAR CLUSTERS - A QUALITATIVE ORDER OF THERMODYNAMIC NUCLEOPHILICITY

Six novel mixed-metal spiked-triangle complexes [Re2Pt(mu-H)(2)(CO)(9) {X}] have been obtained. The metallo-ligands X bound to the Pt vertex are either transition-metal hydrides, such as HMn(CO)(5), HRe(CO)(4)(P Ph3), HRe(CO)(3)(PPh3)(2), or carbonyl anions, such as [Mn(CO)(5)](-), [WCp(CO)(3)](-) and [Co(CO)(4)](-). Two main synthetic routes have be en used to prepare these complexes: (i) replacement of the labile liga nd 1,5-cyclooctadiene (COD) of [Re2Pt(mu-H)(2)(CO)(8)(COD)] with CO an d X; (ii) substitution of the labile 'ligand' HRe(CO)(5) in [Re2Pt(mu- H)(2)(CO)(9){HRe(CO)(5)}] by the ligands X. The neutral species [Re2Pt Mn(mu-H)(3)(CO)(14)] has been obtained by protonation of [Re2Pt(mu-H)( 2)(CO)(9){Mn(CO)(5)}](-). Variable temperature H-1 NMR investigations showed that it exists in solution as two isomers a and b, likely diffe ring in the location of one hydride, a having the structure [Re2Pt(mu- H)(2)(CO)(9){HMn(CO)(5)}], and b the structure [Re2Pt(mu-H)(2)(CO)(9){ Mn(CO)5}]. The ligand HMn(CO)(5) of a shows a high lability, being in fast exchange with free [HMn(CO)(5)] even at 193 K, At higher temperat ure interconversion between the two isomers and exchange between the h ydrides bound to Pt in b is observed. The treatment of [Re2Pt(mu-H)(2) (CO)(9){WCp(CO)(3)}](-) with strong acids failed to give the protonate d derivative. The strong nucleophile [FeCp(CO)(2)](-) reacted with [Re 2Pt(mu-H)(2)(CO)(9){HRe(CO)(5)}] as a Broensted base rather than as a nucleophile, giving deprotonation instead of substitution of the labil e HRe(CO)(5) ligand bound to Pt. The complex [Re2Pt(mu-H)(2)(CO)(9){HR e(CO)(4)(PPh3)}] has been characterised by X-ray single crystal analys is. It crystallises in the monoclinic space group P2(1),le (No, 14) wi th a = 9.229(3), b = 30.700(8), c=12.915 (3) Angstrom, beta=98.05 (2) Angstrom, V=3623(2) Angstrom(3) and Z=4. A series of displacement reac tions of the type [Re2Pt(mu-H)(2)(CO)(9){X}] + X' double left right ar row [Re2Pt(mu-H)(2)(CO)(9){X'}] + X allowed the following qualitative order of affinity of the metallo-ligands X for the Pt atom (a scale of 'thermodynamic nucleophilicity') to be established: [Re(CO)(5)](-) > [HRe2(CO)(5)](-) approximate to [Mn(CO)(5)](-) > [WCp(CO)(3)](-) > [Co (CO)(4)](-) > [HRe(CO)(4)(PPh3)] > [HRe(CO)(3)(PPh3)(2)] > [HRe(CO)(5) ] > [HMn(CO)(5)] > [HWCp(CO)(3)].