Activation of a coordinated alkyne by electron transfer: crystal structures of [Pd{PPh2CH = C(Bu-t)NN = C(Bu-t)CH2PPh2}-{C(CO2Me)= CH(CO2Me)}] and [Pd{(Z,Z)PPh2CH2C(Bu-t)= NN = C(Bu-t)CH2PPh2}-{C(CO2Me) C(CO2Me)}]

A new zerovalent complex [Pd{PPH2CH2C(Bu ')double bond NN double bond C(Bu ')CH2PPh2}{C(CO2Me)drop C(CO2Me)}] (1) was prepared and its crystal structu re was determined by X-ray diffraction analysis, which shows it crystallise s in the C2/C monoclinic space group with a = 10.930(1) Angstrom, b = 22.08 6(1) Angstrom, c = 19.042(2) Angstrom and beta = 92.692(9)degrees. The azin e diphosphine ligand chelates the metal in a (Z,Z) configuration with a P-P d-P bite angle of 114.63(4)degrees. The alkyne carbon atoms lie essentially in the P-Pd-P plane. The electrochemical reduction of 1 promoted the conve rsion of the alkyne ligand to a vinyl species, and of the azine diphosphine to ene-hydrazone diphosphine, with formation of [Pd{PPh2CH double bond C(B u ')NN double bond C(Bu ')CH2PPh2}{C(CO2Me)double bond CH(CO2Me)}] (2). X-r ay diffraction analysis of complex 2 shows it crystallises in the triclinic space group <P(1)over bar> with a =11.302(1) Angstrom, b=12.528(1) Angstro m, c = 16.028(2) Angstrom, alpha = 107.64(2)degrees, beta = 92.27(1)degrees and gamma = 111.79(2)degrees. It displays a square-planar geometry with th e two phosphorus atoms in trans position. Extended Huckel MO calculations w ere performed in order to elucidate the redox process. (C) 2000 Elsevier Sc ience S.A. All rights reserved.