Square planar complex valence bond theory

Valence bond theory describes the bonding in complexes in terms of two-electron, coordinate covalent bonds resulting from the overlap of filled ligand orbitals with vacant metal hybrid orbitals that point in the direction of the ligands sp (linear), sp3 (tetrahedral), dsp2 (square planar), and d2sp3 or sp3d2 (octahedral). 

As stated by Chatt and co-workers, their early speculation on the role of trans-n bonding groups in ligand substitution of platinum(ii) complexes was based on the assumption that the reactions proceed by an 8 2 mechanism. However, at the time (1955) most of the observations reported on such reactions were qualitative and little had been done to use detailed kinetic studies in attempts to elucidate the mechanism of ligand substitution. Since the valence bond theory in use then assigned dsp hybridisation to the square-planar plati-num(ii) complexes, coordination chemists believed an entering nucleophile would readily attack the low energy vacant p orbital on the metal and substitution would take place by an 8 2 mechanism. Furthermore, a coordination... 

According to valence bond theory, what set of orbitals is used by a Period 4 metal ion in forming (a) a square planar complex (b) a tetrahedral complex ... 

It is in the realm of cupric square planar complexes where the valence bond theory gives not only an inadequate, but an incorrect description of the bonding. Valence bond theory ignores the presence of antibonding orbitals and invokes the use of a linear combination of d -y, Px, Py, and s orbitals to form the a bonds. For d ions, the ninth electron can only... 

Pauling s valence bond theory is likewise of only limited value in its application to transition metal complexes. In the VBT, the ligand electrons are accommodated in hybrid orbitals localized on the central metal. The orbitals of interest for transition metals are the nd, n -f 1), n + l)p, and n + )d. An octahedral configuration arises from d sp hybridization of the metal orbitals, while dsp hybridization gives the square planar structure and sp hybridization results in a tetrahedral geometry. 

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. 

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