Coordinatively unsaturated 16-electron centres

More interestingly, the same authors used their procedure for highly selective couplings of alkenes and aldehydes (Equation (10.32)). The catalytic system combined a strong a donor IPr and a strong tt acceptor P(OPh)3. This combination was proposed to produce a synergistic relationship by reducing the electron density at the coordinatively unsaturated Ni centre, which would accelerate the elimination step in the catalytic cycle. 

The Pt2S2 core is formally coordinatively and electronically saturated since the 16-electron d centre is effectively stable to addition reactions. However, upon metallation, it is possible to introduce unsaturation to the system when the metal carries along a labile ligand such as cycloocta-1,5-diene (cod). This is exemplified in the synthesis of [Pt3(cod)(PPh3)4( 3-S)2] and [Pt2Pb-(N03)2(PPh3)4(/i3-S)2]. This transformation from a Lewis basic to a Lewis acidic complex is accompanied by alteration of its chemical reactivity and coordination... 

Throughout this section, the role of coordinatively unsaturated 16-electron species (see Section 23.7) and the ability of the metal centre to change coordination number (essential requirements of an active catalyst) should be noted. 

For the elimination to proceed, a vacant coordination site at the metal centre is required. In the example quoted the square planar platinum complex has only a 16-electron configuration, so it is coordinatively unsaturated. In 18-electron compounds this site has to be provided by dissociation or by change of hapto number of a ligand. 

The first step in most palladium-catalysed cross-coupling reactions is the oxidative addition of an organohalide (or pseudohalide) to a coordinatively unsaturated Pd species. While numerous catalytic systems exist for the activation of C(sp )-I and C(sp )-Br bonds, activation of C(sp )-Cl bonds is a greater challenge owing to their increased stability. There is also an issue in that sterically unhindered electron-rich Pd centres undergo oxidative addition most readily, yet electron-poor and sterically-hindered Pd intermediates... 

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