5-COORDINATE PLATINUM(II) ALKYNE COMPLEXES - SYNTHESIS, AB-INITIO CALCULATIONS AND CRYSTAL AND MOLECULAR-STRUCTURE OF [PTI2(ME(2)PHEN) (ETA(2)-PHC-CPH)]CENTER-DOT-CHCL3

[PtX2(Me(2)phen)] complexes (1) (Me(2)phen = 2,9-dimethyl-1,10-phenant hroline; X = Br, ly; I, it), having a highly distorted square-planar g eometry, undergo alkynes uptake in chlorinated solvents to give the fi ve-coordinate trigonal-bipyramidal species [PtX2(Me(2)phen)(alkyne)] ( 2) (alkyne = ethyne, a; propyne, b; 2-butyne, c; 1-pentyne, d; phenyla cetylene, e; methylphenylacetylene, f; diphenylacetylene, g). The equi librium constants for formation (K-f) decrease in the order (I> Br) al ready observed for the analogous reaction with alkenes to give the [Pt X2(Me(2)phen) (alkene)] complexes (3) (I > Br > Cl) although the diffe rences in K-f,values are much greater in the case of alkynes. In contr ast, the number and bulk of substituents at the metal-coordinated carb ons have practically no effect on the K-f,of the alkyne complexes, whi le they greatly reduce the stability of the five-coordinate olefin com pounds. Finally, the Delta G of activation for rotation around the Pt- alkyne bond calculated by variable temperature H-1 NMR spectroscopy (7 1.5 +/- 0.8 kJ mol(-1) for 2zb) is considerably smaller than the value found for the corresponding five-coordinate propene complex (86.2 +/- 0.8 kJ mol(-1)). The crystal and molecular structure of the chlorofor m solvate of [PtI2(Me(2)phen) (eta(2)-PhC=CPh)] (2zg . CHCl3,) has bee n determined by X-ray diffraction methods. In the crystals two crystal lographically independent complexes and chloroform molecules are prese nt: space group Cmc2(1),, a = 14.367(3), b = 15.529(3), c = 26.459(5) Angstrom Z=8, R = 0.0444. The average bending back of the phenyl group s with respect to the C=C triple bond axis is 25(1)degrees. Spectrosco pic and crystal data show that the conformation of the coordinated alk ynes in these five-coordinate complexes is intermediate between those found for the trigonal Pt(O) and the square-planar Pt(II) alkyne compl exes. Ab initio Hartree-Fock calculations and constrained space orbita l variation analyses have shown that in the five-coordinate species et hylene exhibits better sigma-donating and pi-accepting properties than acetylene. (C) 1998 Elsevier Science S.A. All rights reserved.