Back-donation theory

As a matter of fact, even now the back donation theory bases itself mostly on chemical evidence, while the physicochemical data are at least controversial. However, it appears to be fully plausible, also from a quanto-mechanical point of view, because the value of the overlap integrals confirmed an interaction between the filled d orbitals of the metal and the vacant p or d orbital of the ligand in different types of coordination 62). In conclusion, this was the leading hypothesis for all subsequent research work on the chemistry of low oxidation numbers— and especially on nickel (0), palladium (0), and platinum (0). The results obtained in the years following 1951 were numerous, important, and often completely unexpected. 

Due to its importance in many industrial processes, the prototypical reaction of CO binding to metal surfaces has received much attention. Using Hiickel molecular orbital theory, Blyholder showed that CO bonding at top sites consists of the donation of electrons from the filled CO 5a HOMO to the metal d 2 orbitals with a back-donation of electrons from the metal dxz caddy orbitals to the CO 2n LUMO. Consequently,... 

The bonding interaction between the metal and CO may be described with simple MO theory in which only the frontier orbitals (19) of M and CO are considered (Fig. 1). A a bond results from interaction of an empty metal orbital of a symmetry along the MCO axis with the carbon lone pair of CO. The tt bonding, also called back donation, results from interaction of a filled metal orbital of IT symmetry (e.g., the or dyz where the z-axis is taken as the molecular axis) with a r orbital of CO. These effects lead to the frontier molecular orbitals shown in Fig. 1 and described energetically by the MO scheme (Fig. 2). 

In apparent contradiction to the electroneutrality principle, there are many complexes in which the metal exists in a low oxidation state and yet is bonded to an element of fairly low electronegativity. Among the most prominent examples are the transition metal carbonyls, a large class of compounds in which the ligand (CO) is bound to the central metal through carbon. The source of stability in these complexes is the capacity of the carbon monoxide ligand to accept a back donation of electron density from the metal atom. Within valence bond theory, this process can be described in terms of resonance ... 

The insertion of acetylene into the Pd-CH3 bond of the complex PdCl(NH3)(CH3) has been studied by de Vaal and Dedieu [63] by using the valence double-zeta basis sets. The geometries of the prereaction complex [PdCl(NH3)(CH3)(C2H2)], the transition state, and product have been optimized at the SCF level, and their energetics has been improved at the CASSCF and Cl level. It has been shown that acetylene is quite weakly bound (5.8 kcal/mol) in the square-planar Pd(II) complexes because of weak 7T back-donation from Pd to the tt orbital of C2H2. The insertion barrier calculated relative to the acetylene complex is 20.5, 22.6, and 17.1 kcal/ mol at the SCF, CASSCF, and Cl levels of theory, respectively, and the transition state corresponding to this barrier displays monohapto coordination of acetylene. The entire insertion reaction is calculated to be exothermic by 26.0, 19.3, and 22.4 kcal/mol at the SCF, CASSCF, and Cl levels, respectively, relative to the acetylene complex. 

In 1951, Michael J. S. Dewar (1918-97) published a molecular orbital (MO) theory of bonding between unsaturated compounds and transition metals later augmented by Joseph Chatt (1914-94) and Leonard A. Duncanson. It recognized o-type overlap of an occupied 7t-orbital on the ligand (e.g., ethylene, benzene, cyclopentadienide) with a vacant d-orbital of appropriate symmetry on the transition metal, coupled with back-donation through 7t-type overlap of an occupied d-orbital on the metal with a vacant (antibonding) 7t -type orbital on the ligand. 

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