Tt-bonding, effect

Group IV unsaturated molecules, indicates the significance of tt-bonding effects. However, no differentiation of the orbitals involved in the bonding and, thus, no differentiation between d-orbital effects and hyperconjugative effects is possible. Recently, several groups have performed semiempirica SCF-MO calculations which, to some extent, overcome this difficulty (29, 85, 112). 

It is now well established in platinum(II) chemistry that Pt—Cl bonds are substantially weaker when trans to ligands such as H and CHf than when they are tram to Cl . The bonds cis to H in trans-[PtHCl(EtPh2P)2] (9) are of very similar length to those cis to Cl in tram-[PtCl2(Et3P)2] (10), so the bond weakening effect of H compared with Cl is directed specifically at the ligand trans to H . Since neither H or Cl Tt-bonds effectively with platinum, this trans influence of H must stem from its effect on the (t-electrons of the complex. 

B. Eight-Electron Systems, 143 TT-Bonding Effects of Ligands, 145 Shapes of AII4 Systems, 148... 

In the reaction of tetrakistriethylphosphitenickel(o) with cyclohexyl isocyanide, in n-hexane or benzene solution, the kinetics are first-order with rates independent of isocyanide concentration. Rates are, however, reduced by the addition of excess triethyl phosphite. Here the forward reaction of the initial equilibrium must be rate-determining, and the second reaction fast. Triethyl phosphite exchange with Pd[P(OEt)a]4 or Pt[P(OEt)a]4 also takes place by rate-determining loss of one ligand molecule. Activation energies are in the order Ni < Pd > Pt, as are the stabilities to thermal decomposition of M(PF3)4. A delicate balance of (7- and TT-bonding effects is needed to rationalise this trend. ... 

F > OH > NH2, which parallels the ability of these substituents to act as cr-electron acceptors. The strong effect of the trifluoromethyl group is a combination of both O - and TT-bond effects. 

It is well-known that the halides of the Group III elements are good electron acceptors and previous work (1,2) has explored the thermochemistry of adduct formation in order to obtain information about the strength of the donor-acceptor bond. The results of this work have been interpreted in terms of the steric, inductive, and tt-bonding effects of the groups attached to the donor and acceptor atoms. The dominant effect, at least in the halide... 

Substitution is, of course, much slower in [Ru(pc)L2] than in [Fe(pc)L2]. A D mechanism operates in both series of complexes the five-coordinate intermediates show very little discrimination. Variable-temperature proton nmr studies of axial ligand exchange in 1-methylimidazole- and 4-r-butylpyridine-benzyl isonitrile-ruthenium-tetraphenylporphyrin complexes show that tetraphenylporphyrin has a much smaller cis effect here than in analogous iron(II) systems. Again tt-bonding effects are important in determining kinetic parameters. ... 

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