ISOCYANIDE AND DIISOCYANIDE COMPLEXES OF A TRIPLATINUM CLUSTER - FLUXIONALITY, ISOMERISM, STRUCTURE, AND BONDING

The reaction [Pt3(mu3-CO)(mu-dppm)3]2+, 1, with RNC (R = t-Bu, cyclohe xyl or o-xylyl) gives an intermediate characterized as [Pt3(mu-CO)(CNR )(mu-dppm)3]2+, 2, which then isomerizes to the stable complex [Pt3(mu 3-CO)-(CNR)(mu-dppm)3]2+, 3. When R = o-xylyl, reaction of 3 with more RNC gives an intermediate [Pt3(mu-CO)-(CNR)2(mu-dppm)3]2+, 4, which t hen loses CO to give [Pt3(CNR)2(mu-dppm)3]2+, 5, the first example of a Pt3 cluster complex having no bridging atom. The cluster 5 has been characterized by an X-ray structure determination. [The solvated salt crystallizes in the monoclinic space group P2(1), cell dimensions a = 17.639(2) angstrom, b = 19.704(3) angstrom, c = 14.5444(12) angstrom, and beta = 101.99(2)degrees with Z = 2. Full matrix least-squares refi nement on F2 of 558 variables converged at a conventional R factor of 0.0393 for 6334 data with I > 2sigma(I).] The two xylyl isocyanide lig ands are trans-bonded to Pt(1), and as a result the Pt(2)-Pt(3) distan ce of 2.548(1) angstrom is significantly shorter than the mean of the two distances to Pt(1), 2.649(1) angstrom. Complexes 3 and 5 are fluxi onal; the RNC ligands can migrate easily around the face of the Pt3 tr iangle. The diisocyanides 1,4-C=NC6R4N=C, R = H or Me, react with 1 to give polymeric complexes {[Pt3(mu-1,4-C=NC6R4N=C)(mu-dppm)3][PF6]2}n, 7, in which the diisocyanide bridges between Pt3 cluster units. The l ocal stereochemistry at platinum is similar to that in 5, and the flux ionality appears even easier in 7 than in 5. Evidence is presented for diisocyanide complexes analogous to 2 and 3, namely [Pt3(mu-CO)(CNC6H 4NC)(mu-dppm)3]2+ and [{Pt3(mu3-CO)(mu-dppm)3}2(mu-CNC6H4NC)]4+, 8. Ca lculations of the EHMO type on model complexes give insight into the f actors which influence the preferred binding mode (terminal or triply bridging) of the isocyanide at the Pt3 triangle.