Hexacarbonyls, electronic structure

The Electronic Structures of Octahedral Metal Complexes. I. Metal Hexacarbonyls and... 

The instability of the electronic structure caused by the unfilled inner orbit of a transition metal leads to a very variable valency. Molybdenum exhibits valencies of 2,3,4,5 or 6 in different compounds, and it is considered that it has zero valency in its hexacarbonyl Mo(CO)g. Because of this variability in its valency, many of its reaction products are mixtures of compounds in which it has different valencies. In solid form such products may be quite homogeneous in composition, and best represented by a nonstoichiometric molecular formula. The hypothetical simple compounds present in such products cannot be separated readily from one another because valency shifts occur in processing. 

Brown made a theoretical study of the electronic structure of rr-complexes of heteroaromatic systems, and predicted rr-complexes of borazoles to be less stable than the corresponding complexes of benzene (95). Although many attempts to synthesize a 7r-borazole complex were unsuccessful, Et3B3N3Me3 and Me3B3N3Et3 derivatives were later prepared from the hexacarbonyl by irradiation. [Cr(CO)3(MeftB3N3)] (LV) was eventually... 

A. Jost, B. Rees, and W. B. Yelon, Acta Crystallogr., Sect. B, 31, 2649 (1975). Electronic Structure of Chromium Hexacarbonyl at 78 K. 1. Neutron Diffraction Study. 

Gray HB, Beach NA (1963) The electronic structure of octahedral metal complexes. I. Metal hexacarbonyls and hexacynanides. J Am Chem Soc 85 2922-2927... 

Chromium forms a white solid, hexacarbonyl, Cr(CO)6, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [Cr(CO)5I] . 

Compound 388 is an acylating agent for electron-deficient alkenes, in a Michael addition process. It is formed by treating molybdenum hexacarbonyl with an organolithium compound, followed by quenching the intermediate 387 with boron trifluoride (equation 104). The structure of 388 (R = Ph) can be elucidated by NMR spectroscopy. Other examples of enantioselective and diastereoselective Michael-type additions involving lithium-containing intermediates in the presence of chiral additives can be found elsewhere in the literature . 

A mixture of molybdenum hexacarbonyl and 4-chlorophenol is effective in performing al-kyne metathesis of dipropynylated dialkylbenzenes. Alkyne metathesis of these precursors leads to the clean formation of dialkyl poly( paraphenyleneethynylene)s (PPEs) in high yield and with high molecular weights. This facile yet effective access to the PPEs has allowed study of their spectroscopic, structural, and thermal properties. While PPEs have been made before, the dialkyl-PPEs turned out to have particularly interesting optical and liquid-crystalline properties that can be explained in terms of the conformation of the main chains. The PPEs have also been utilized to construct light-emitting diodes and other semiconductor devices. This chapter discusses the interplay of structure, chromicity, and electronic properties of the dialkyl-PPEs. 

Complexes of N6P6(NMe2)12 with other metal chlorides and nitrates have essentially similar structures as indicated by infrared and electronic spectroscopic data (98). Compounds N4P4Me8 and N5P5Me10(L ) react with molybdenum and tungsten hexacarbonyls... 

The electron configurations for the transition metals discussed here and in Appendix B are for individual metal atoms in the gas phase. Most chemists work with the transition metals either in the metallic state or as coordination compounds (see Chapter 25). A solid transition metal has a band structure of overlapping d and s orbital levels (see Section 13-7). When transition metal atoms have other types of atoms or molecules bonded to them, however, the electronic configuration usually becomes simpler in that the d orbitals fill first, followed by the next higher s orbital. This is illustrated by Cr, which has a 4s 3d electronic configuration as a free atom in the gas phase. But in the compound Cr(CO)5, chromium hexacarbonyl, which contains a central Cr atom surrounded by six neutral carbon monoxide (or carbonyl) groups, the chromium atom has a 3d electronic configuration. 

---