Bent bonds, reviews

Structure. The straiued configuration of ethylene oxide has been a subject for bonding and molecular orbital studies. Valence bond and early molecular orbital studies have been reviewed (28). Intermediate neglect of differential overlap (INDO) and localized molecular orbital (LMO) calculations have also been performed (29—31). The LMO bond density maps show that the bond density is strongly polarized toward the oxygen atom (30). Maximum bond density hes outside of the CCO triangle, as suggested by the bent bonds of valence—bond theory (32). The H-nmr spectmm of ethylene oxide is consistent with these calculations (33). 

In the course of investigating multiple bonds in molecules and complexes by the valence bond approach, we have recently found that such multiple bonds are more accurately described as bent bonds rather than as a and tt bonds (7-5). In order to understand the potential implications of these results for multiple metal-metal bonds, it is important to brieffy review the basic assumptions of the valence bond model and compare them to those of the more familiar molecular orbital model of bonding. 

Protonated cyclopropane has been reported in the gas phase" "" to be ca 8 kcal mol" in energy above the isopropyl cation. The bent bonds of the cyclopropane ring are susceptible to electrophilic attack leading to the expectation that cyclopropane will be more basic than saturated alkanes and that protonation will occur on the C—C bond, i.e. the edge-protonated isomer will have the lowest energy. There is, however, considerable evidence from solution chemistry that corner-protonated cyclopropanes exist as intermediates in 1,2-alkyl shifts in carbocations. There have been several reviews of protonated cyclopropanes " and, in the current work, only the very recent theoretical work will be reviewed. 

Chemisoq)tion bonding to metal and metal oxide surfaces has been treated extensively by quantum-mechanical methods. Somoijai and Bent [153] give a general discussion of the surface chemical bond, and some specific theoretical treatments are found in Refs. 154-157 see also a review by Hoffman [158]. One approach uses the variation method (see physical chemistry textbooks) ... 

As a direct consequence of Bent s rule,56 carbon compounds with a significant amount of s-character in their bonds to an electronegative element, e.g., fluoro-alkynes, are unstable. Thus, reactions which involve change in hybridization at a carbon atom bearing such an acceptor can be promoted when the amount of s-character at the reactive atom decreases. The role of rehybridization in a variety of chemical and supramolecular processes has been extensively analyzed and reviewed recently.55,58... 

By writing about complexes containing triple bonds between phosphorus and transition metals, one has to take into account the triple-bond character of phosphinidene complexes which are in a nearly linear coordination mode (type C) in contrast to the usual bent coordination mode D possessing typical double-bond features. Due to the additional r-donation bonding ability of the PR moiety to the metal atom in type C and the observed bond lengths, this type of complexes has to be included into the classes of metal-phosphorus triple bond compounds. Thus, at the end of this review will appear a chapter highlighting the appropriate compounds of type C. 

Coulson has recently discussed the bond angles in these molecules in a review paper,550 using two simple models. An ionic model would predict these molecules to be linear, but in a second model, in which valence states arising from s2- sp, s2- sd excitations were permitted, the first would lead to linear, the second to bent structures. From a consideration of the excitation energies, it was shown that when the central atom is heavy and the attached halogens are as electronegative as possible, the bent situation is favoured, in agreement with the experimental results. 

The series [ReCp ( 0)2)2 (/r-E) (96) (E = O, S, Se, Te) and related complexes have also been prepared from the elements or from EH2 and (92). Slow decomposition of solid (92) in a glovebox produces the dimer Cp Re(CO)2=Re(CO)2Cp (see Section 11). Reaction of (91) with PhC=CH gives the vinylidene ReCp(CO)2(=C=CHPh), but the structure of the coordinated PhC=CPh in ReCp(CO)2( -PhC=CPh), in spite of being bent, has an almost unperturbed C=C bond with a rather short length. In contrast, the azobenzene derivative ReCp(CO)2( 7 -PhN=NPh) has an almost pure single N N bond and is best described as a metallacyclic structure. The extensive research on the coordination of unsaturated N N bonds to the fragment [Re] = ReCp(CO)2 as related to N2 reduction involving species sketched in Scheme 22 has been reviewed. ... 

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. 

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