7r-Donation

Two types of NO coordination to ruthenium are known linear Ru-N—O 180° and bent, Ru-N-O 120°. Since NO+ is isoelectronic with CO, linear Ru-N-O bonding is generally treated as coordination of NO+, with bent coordination corresponding to NO- thus, in the former an electron has initially been donated from NO to Ru, as well as the donation of the lone pair, whereas in the latter an electron is donated from the ruthenium to NO (making it NO-) followed by donation of the lone pair from N. Though an oversimplification, this view allows a rationale of metal-nitrogen bond lengths, as with the Ru—NO+ model 7r-donation is important and a shorter Ru—NO bond is predicted - and, in fact, observed. 

Bent metal-NO bonding is traditionally associated with NO bonding as NO , whereas linear coordination is associated with NO+. The latter is predicted to involve shorter M-N bonds as both a- and 7r-donation can be involved. 

The effect of an aryl ring as substituent is similar to that of an amino group, since it too has the potential to 7r-donate, but the net effect is much smaller. 

The associated optical cis effects at the porphyrin system can be understood according to the inductive transmission path shown in Figure 1, Case A Rh,u causes a hypso spectrum via Rh - (P) 7r-donation increasing in the series I < Br < C =CPh < Ph Me for X which is roughly the series of increasing basicity towards the proton, as expected. A common feature (see Sect. 6) is the small effect of the exchange NH3 - HjO in the a-band the Soret band shows a stronger shift. 

S-Donor Ligands. The full account of the crystal and molecular structure of tetrakis-(AfJV-diethyldithiocarbamato)titanium(iv), [Ti(S2CNEt2)4], has been published. The difference in the Ti—= 260.6(8) and Ti—Sg = 252.2(8) pm bond lengths in the trigonal-dodecahedral structure could be due to 7r-donation... 

The nature and position of substitution has a profound influence upon the oxepin-arene oxide equilibrium position. The effect of substituents on the relative energies of each valence tautomer has been calculated (80JA1255) and these theoretical results are in accord with the limited experimental data which are available. In general terms, oxepins substituted at the 3-position are less favored than the corresponding arene oxides, while the reverse obtained for 2- and 4-substituted oxepins. This substituent effect has been rationalized in terms of a preference for the maximum number of alternative resonance contributors. The influence of both 7r donating and v withdrawing substituents oh the oxepin contribution is summarized in Scheme 2. This latter effect may be considered as an electronic substituent effect. 

Thus, the net impact of a substituent such as fluorine, which is inductively withdrawing and 7r-donating, will result from a complex interplay of these disparate interactions. To make matters even more complicated, die combined influences of multiple fluorine substituents is not additive, and cannot be readily derived from an understanding of the effect of a single fluorine substituent. 

The X-ray-excited O(ls) photoelectron spectra of various compounds of the type RMn(CO)5 have been correlated with infrared carbonyl stretching frequencies, and conclusions have been drawn about the extent of 7r-donation from metal to ligands in particular, it is inferred that the SiCl3 group in Cl3SiMn(CO)5 is a rather poor it-acceptor (265). 

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