Antibonding orbital, olefinic

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

According to Dewar (41) the metal-to-olefin bond in such complexes consists in part of the overlap of the 7T-electron density of the olefin with a cr-type acceptor orbital of the metal atom and in part of the back-donation of electrons from filled metal or other d-n-pn hybrid oribitals into the antibonding orbitals on the carbon atoms. 

The antibonding orbital of ethylene (and other olefins) is a also a proper MO, highly localized to the two carbon atoms. It is the linear combination of the two 2p orbitals which is A with respect to reflection in the bisecting plane and S w.r.t. a 180° rotation about the C2 axis which contains that plane. 

Reaction of an alkene with a nitronium ion involves donation of the n -electron pair of the alkene into an orbital of the nitronium ion. Donation of a bonded electron pair necessarily means that the bond from which it comes is broken. Likewise population of an antibonding level by electron donation generally results in breaking of the bond to which the antibonding orbital corresponds. In this case electron donation of the olefinic r-electron pair results in the rupture of file olefinic n bond and acceptance into file N-O n orbital results in breakage... 

The same group of coordination polymerisations in which alkene undergoes re complex formation with the metal atom includes the copolymerisation of ethylene, a-olefins, cycloolefins and styrene with carbon monoxide in the presence of transition metal-based catalysts [54-58], In this case, however, the carbon monoxide comonomer is complexed with the transition metal via the carbon atom. Coordination bond formation involves the overlapping of the carbon monoxide weakly antibonding and localised mostly at the carbon atom a orbital (electron pair at the carbon atom) with the unoccupied hybridised metal orbitals and the overlapping of the filled metal dz orbitals with the carbon monoxide re -antibonding orbital (re-donor re bond) [59], The carbon monoxide coordination with the transition metal is shown in Figure 2.2. 

Ti-backbonding from the metal to the ti antibonding orbital of the olefin weakens the C=C bond by up to 140 cm. ... 

In both schemes cis insertion is the most hypothetical step. Cossee has assumed 164) that similar processes take place during olefin polymerization in the presence of Ziegler-Natta catalysts, and in some other reactions catalyzed by the transition metal compounds. This author came to the conclusion 165) that the d orbitals of the metal combine with the migrating group to facilitate such processes. In the course of 7r-allyl transfer the palladium orbitals overlap with the antibonding orbitals of double bond of the <7-bonded allyl group so as to favor an insertion reaction. 

The antibonding orbital of the fr-dllyl system is 3 and it correlates with the n orbital of the parent olefin. For simple ground-state olefins, the 7T orbital is empty. In olefins coordinated to transition metals, however, electronic population of the v orbital is possible through interaction (back-bonding) with d orbitals of that symmetry (XXXIX). 

M. J. S. Dewar proposed a model (Figure 1) to describe the bonding of an olefin to silver(i) or copper(i). The model suggested that, in addition to cr-donation of olefin 7r-bonding electrons to the metal, d electrons on the metal would also interact with antibonding orbitals of 7r-symmetry on the olefin. No experimental evidence was provided to support this proposal nor, indeed, was explicit mention made of Zeise s salt or other platinum-olefin complexes. From a study of Chemical Abstracts 41 (1947)-75 (1971), it would appear that Dewar did not follow up his proposal with more detailed studies, possibly indicating that this was not a prime focus of his own interests in MO theory and its application to... 

First, there are the N-> V transitions which are from the bonding orbital in the ground state of a molecule to a higher energy orbital (antibonding orbital)2. The class of N -> V transitions is large and one or more may occur for a molecule. For a transition between a orbitals as in the case of paraffins, the N V transition may be written as a 7r transitions. The <7 -> ultraviolet region. While some of the 7r r transitions are observed in the far... 

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