SUBSTITUTION OF CHLORIDE IN [PTCL2(DPPE)] BY THE CHIRAL ANIONIC LIGAND [MO(CO)(5)(PPHH)](-) - SYNTHESIS AND X-RAY STRUCTURE OF [PT(MU-PPHH)(2)(MO(CO)(5))(2)(DPPE)] AND A P-31(H-1) NMR ANALYSIS OF ITS ISOMERIC PRODUCTS AND FLUXIONALITY

Reaction of the chiral anionic ligand [Mo(CO)(5)(PPhH)](-) with [PtCl2 (L-L)] affords the neutral trimetallic monophosphido bridged complexes [Pt(mu-PPhH)(2){Mo(CO)(5)}(2)(L-L)] (L-L = dppe, Ph(2)PCH(2)CH(2)PPh( 2), 2, dpae, Ph(2)AsCH(2)CH(2)AsPh(2), 3; dppe', cis-Ph(2)PCH=CHPPh(2) , 4). Compounds 2-4 are the first examples of heterometallic complexes that contain two chiral primary phosphido bridges existing as pairs o f diastereoisomers. [P(mu-PPhH)(2){Mo(CO)(5)}(2)(dppe)] (2) has been c haracterized spectroscopically and by a single-crystal X-ray analysis. The molecular structure of 2 can be considered either as Mo2Pt phosph ido-bridged trimer or a square-planar Pt-II complex bonded to one chel ating dppe and two anionic phosphine ligands. The latter PtP4 descript ion successfully accounts for the line broadening observed in the P-31 {H-1} NMR spectra of 2-4, which can be understood in terms of the exis tence of several interconverting rotameric forms arising from restrict ed rotation about the Pt-phosphido bond. The Pt-P-Mo angles of 122.1(1 ) and 123.5(1)degrees are some of the largest ever to be reported for phosphido-bridged heterometallics, reflecting the long Pt-Mo separatio ns [4.349(2) and 4.320(2) Angstrom] and the electronic and structural flexibility of the phosphido bridge. The PtMo compound [Pt(mu-PPhH)(2) {Mo(CO)(4)}(dppe)] (5), comprised of one neutral and one dianionic che late ligand, was prepared from [Li](2)[Mo(CO)(5)(PPhH)(2)] and [PtCl2( dppe)] and was shown to contain a conformationally rigid PtP4 structur e with characteristically sharp P-31 resonances. The P-31{H-1} NMR spe ctrum of 5 is rich with information analysing as two overlapping AA'XX ' spin systems together with their AA'XX'M (M = Pt-195) counterparts. The values of (2)J(P-P) and (1)J(Pt-195-P) are significantly lower for PPhH{Mo(CO)(5)}(-) than dppe, a difference that cannot be accounted f or solely on the basis of bond length variations. Meso-[Pt(mu PPhH)(2) (Mo(CO)(5)}(2)(dppe)] (2) is thermodynamically unstable, undergoing a rapid intramolecular elimination of [Mo(CO)(6)] to generate [Pt(mu-PPh H)(2){Mo(CO)4}(dppe)] (5), while its racemic diastereoisomer, that whi ch was observed in the crystal, decomposes to unidentified phosphorus- containing products.