Bent unsaturated bonds

Bending of unsaturated bonds reduces the overlap between the p-orbitals and weakens the interaction. The n orbital hes high in energy and the Jt orbital lies low. Bent unsaturated bonds are electron acceptors as well as donors. The energy gap between Jt and Jt is small. Bent unsaturated bonds are readily pseudoexcited to undergo [2+2] cycloaddition reactions. 

Thermolysis of m-PtH(CH2CMe3)(cy2PC2H2Pcy2) at 45-80°C yields a bent platinum(0) complex (Figure 3.13) that is intensely reactive to a whole range of unactivated C—H bonds in saturated and unsaturated hydrocarbons. 

The nonequivalent n orbital extension or the higher electron density in the exo face pyramidizes the unsaturated carbons The -H bonds are bent in the endo face. 

The NO+ and NO- modes of coordination differ by two electrons in terms of formal charge. Interconversion of these two bonding modes becomes feasible when the bound metal ion possesses two complementary oxidation states. It has been proposed that this interconversion, which corresponds to an intramolecular redox reaction, represents a unique and facile way to achieve coordinative unsaturation at the metal center with the nitrosyl acting as an electron pair reservoir (201). Interconversion of linear and bent nitrosyls has been reported in the unusual complex Ru(NO)2C1-(PPh3)2+ that possesses one linear and one bent nitrosyl, structure (39) (202). 

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. ... 

All available data appear to point to the same underlying feature, that fluorine prefers not to be attached to unsaturated carbon this most probably stems from repulsion between electron pairs on fluorine and those of the ir-systems. Earlier discussions showed that similar repulsions are important in determining the stability of fluorocarb-anions (see Chapter 4, Section VllA), and that these repulsive forces appear to be critically interdependent with stereochemistry. Similar effects on fluoroalkenes are represented in Figure 7.9. The formation of a double bond involving -hybridised carbon, 7.9A, would lead to greater electron-pair repulsion than when formed from sp -hybridised carbon (cf. 7.9A and 7.9B) the latter would lead to bent bond formulation 7.9B. 

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