Complexes of Dinitrogen, Dioxygen, and Dihydrogen

Because NH3 has no low-energy orbitals of suitable symmetry, 7r bonds do not form between Pt and NH3. Sulfur has empty d orbitals available to accept electron density from the Pt2 +. Therefore, to the extent that t bonding occurs, it is to the sulfur end of SCN. When it is bonded to Pt2+ opposite NH3, SCN- bonds to Pt2+ through the sulfur atom. 

In the second case, the phosphorus atom in the phosphine also has empty d orbitals that can accept electron density donated from the Pt2+. In fact, it is more effective in this regard than is the sulfur atom in SCN-. This results in the more stable bonding to SCN- being to the nitrogen atom when PR3 is in the trans position. In essence, the presence of tt bonding ligands in trans positions that compete for back donation leads to a complex of lower stability. As will be discussed in Chapter 20, this phenomenon (known as the tram effect) has a profound effect on the rates of substitution reactions in such complexes. 

FIGU RE 16.11 The molecular orbital energy level diagrams for 02 and N2. 

The first preparation of a complex containing oxygen was carried out by Vaska (1963) utilizing the reaction 

Another milestone in coordination chemistry dates from 1980, when the first complex containing dihydrogen was reported by G. J. Kubas. ft was found that hydrogen reacts with [W(CO)2(Pchx3)2] 

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