Bent linear complexes

A cation (or other electrophile) will interact principally with the HOMO of the carbonyl, i.e. the oxygen p lone pair. Hence, if all other interactions are negligible, the optimum geometry for the ate complex should be 17. However, ab initio calculations13 predict structures 18 (if M+ = H+ or Me+) and 19 (if M+ = Li+ or BH2+), where the cation complexes the two lone pairs simultaneously. The bent structure 18 is favored if the Lewis acid has a o acceptor orbital, whereas the linear complex 19 is preferred if an additional rc-type acceptor orbital is available. The HOMO-LUMO interaction is slightly diminished, but this is compensated by the interaction with the s lone pair. 

The chemistry of transition metal nitrosyls has been reviewed, with spectra of many types used to study the electronic structure. Bonding, as described in Chapter 13, can be thought of as a linear complex of NO, isoelectronic with CO and with NO stretching frequencies of 1700 to 2000 cm or a bent complex of NO , isoelectronic with O2 and with NO stretching frequencies of 1500 to 1700 cm . The number of electrons on the metal ion and the influence of the other ligands on the metal provide for changes from one to the other during reactions. 

IR spectrometry shows the existence of two equilibria for the complexation of phenols with carbonyl bases in CCI4 (equations 30 and 31) . Two different 1 1 stereoiso-meric complexes are formed the planar bent n complex a and the planar bidentate linear n complex b. The complex b has also been given the structnre c (ont-of-plane n complex). Experimentally, an overall complexation constant is determined which is the sum of the individual complexation constants and Kj, for each stereoisomeric complex. Massat and coworkers ° have proposed an IR method for evalnating the constants Kn and K of phenol-alkyIketone complexes. They have shown that the n vs. n complex competition depends on the alkyl branching, measured by the number of methyls alpha to the carbonyl, and on the phenol acidity, measured by pATa (equations 32 and 33). 

The narrower ranges when combined with isotope shifts for and (NO) have been used to distinguish linear and bent nitrosyl complexes, and it was noted that isotope shift differences are more discriminating than isotope frequency ratios. The review also analyses the data for bridging nitrosyl and analyses environmental and solvent effects. Infrared spectroscopy has proved particularly useful for identifying complexes which have structural isomers in the sohd state. For example. 

Enemark and Feltham noted that it is quite misleading to describe all linear complexes as derivatives of NO" and all bent complexes as derivatives of NO , but did not provide a notation for indicating the M-N-O geometry and did not connect the notation to the 8 and 18 EAN rules. 

To clarify the assignments of electrons and formal charges, examples of electron counting in the bent nitrosyl complex Mn(NO)(CO) and the linear nitrosyl complex [Co(NO) (NHj) ] are given below using both the covalent and ionic formalisms. 

Reactions conducted with the more basic phosphine, PEtj, occur differently and afford the bent nitrosyl complex at the bottom left of Scheme 9.6. The reactions of this bent nitrosyl complex are inhibited by added phosphine, and ties inhibition suggests tiiat tiie bent NO is unreactive toward insertion. The linear nitrosyl in the starting complex appears to be more reactive. Some data suggest that the insertion of NO is slower than the insertion of CO. For example, MeFe(NO)(CO)3 reacts with CO to form the acyl complex [MeC(0)]Fe(N0)(C0)3. ... 

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