Ligand-to-metal electron donation

The overlap between the metal and ligand orbitals in the 5i>2g orbital (LUMO for Z = 0) provides an efficient pathway for ligand-to-metal electron donation and confers electrophilic character on the ligands in neutral complexes. This finding affords a reasonable explanation for the electrophilic mechanism proposed by Wang and Steifel for the nonclassical reaction of neutral Ni-bis(dilhiolene) complexes wilh olefins, which can be separated and purified by a novel melhod. The LUMO of Ihe dilhiolene complex... 

The six coordinated titanium(IV) compounds, Ti(acac)2(X)2, where X is methoxy, ethoxy, isopropoxy, -butoxy, or chloro, all adopt the cis-configuration. This is beheved to result from the ligand-to-metal TT-electron donation (88,89). 

When the structures for many ligands (e.g., H20, NH3, C032-, and C2042-) are drawn, there is no question as to which atom is the electron pair donor. Ligands such as CO and CN normally bond to metals by donation of an electron pair from the carbon atom. It is easy to see why this is so when the structures are drawn for these species and the formal charges are shown. 

In this case, the product is the fac isomer, in which all NH3 ligands are trans to the CO molecules. Ammonia does not form ty bonds to metals because it has no orbitals of suitable energy to accept electron density. Thus, the back donation from Cr in Cr(NH3)3(CO)3 goes to only three CO molecules, and the bond order is reduced even more than it is in Cr(CO)s, where back donation occurs equally to six CO molecules. There is, of course, an increase in Cr-C bond order and stretching frequency in Cr(NH3)3(CO)3 compared to Cr(CO)s. Based on the study of many mixed carbonyl complexes, it is possible to compare the ability of various ligands to accept back donation. When this is done, it is found that the ability to accept back donation decreases in the order... 

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