The Distribution of Charge in Complex Compounds

Abstract

The charges on metal atoms and ligands in complex compounds of transition metals may be calculated theoretically or measured empirically, but there is no reliable method of doing either. This review is concerned mainly with two experimental methods, dipole moment measurement and X-ray photoelectron spectroscopy, applied to tertiary phosphate and such complexes of the heavier transition metals. It is concluded that the charge is rarely greater than ±0.3e and that some so-called neutral or positive ligands, e.g. N2 or NO+, may carry more negative charge in a complex than formally anionic ligands such as H−. Small ligands of high formal charge, e.g. O2−, NR2−, and N3−, appear to carry no more negative charge than Cl−, but monoanionic ligands vary greatly in their capacity to contribute to the dipole moments. The ligands NO, CO, PF3, CH, and H− are all nearly neutral to slightly negative, whereas pyridines, tertiary phosphanes, etc. are the most strongly positively charged.