Structural systematics of some trinuclear alkynyl and diynyl Group 11 complexes containing dppm [dppm = CH₂(PPh₂)₂]

In this review the molecular structures of a series of trinuclear alkynyl and diynyl Group 11 cations [{M₃(μ-dppm)₃}(X)_n]^{(3− n )+} (M = Cu, Ag; n = 1, 2; where X is an alkynyl or diynyl group, an inorganic anion or solvent) are considered from the points of view of (i) the dimensions and geometries of the M₃(P–P)₃ cores, (ii) the conformations of the dppm ligands, and (iii) the attachment of the alkynyl and diynyl ligands. In the crowded [M₃(μ-dppm)₃]³⁺ core, the dppm ligands are arranged so that there is always one CH₂ group up and two down, to give pseudo mirror symmetry perpendicular to the M₃ plane (crystallographic in some cases). Attachment of the alkynyl or diynyl substituent(s) occurs roughly normal to the M₃ plane; according to their perpendicularity, the C(1) atom may be μ₂ or μ₃. In most cases where only one alkynyl or diynyl ligand is present, a second ligand is also attached to the M₃ core. Unusual and interesting dispositions/conformations of the dppm ligands are widespread, among the mono–diynyl complexes in particular, whereby some phosphorus donor atoms lie at unusual distances out of the M₃ planes, a concomitant of strong agostic interactions between phenyl H atoms and the atoms of the open M₃ face, and weak M⋯M interactions. With one X group, C–H⋯M interactions persist on the other face, with C–H⋯X interactions with the alkyne affecting the inclination of the alkyne and the conformation of the Ph rings. With two substituents (one of which may be a loosely bound anion), similar interactions may occur, accompanied by twisting of the dppm chelate ring to displace P atoms from the M₃ plane. These factors possibly inhibit formation of the bis(diyndiyl) complexes, which are only obtained under more strongly basic conditions.