Nonclassical carbonyls

Carbonyls bound to very weakly ir-donor metals, where 4.1 is predominant, have very high v(CO) bands. Some, termed nonclassical carbonyls, even appear to high energy of the 2143 cm band of free CO. Even (f species can bind CO, for example, the formally Ti(IV) carbonyl, [Cp2Ti(CO)2], has v(CO) bands at 2099 and 2119 cm One of the most extreme weak ir-donor examples is [Ir(CO)6] with v(CO) bands at 2254, 2276, and 2295 cm l The X-ray structure of the related complex [IrCl(CO)5] shows the long M-C [2.02(2) A] and short C-O [1.08(2) A] distances expected from structure 4.1. The highest oxidation state carbonyl known is tra 5-[0s02(C0)4], with v(CO) = 2253 cm . Conversely, carbonyls with exceptionally low v(CO) frequencies, found... 

Noyori, R., Yamakawa, M. and Hashiquchi, S. Metal-Ligand Bifunctional Catalysis A Nonclassical Mechanism for Asymmetric Hydrogen Transfer between Alcohols and Carbonyl Compounds. J. Org. Chem. 2001, 66, 7931-7944. 

Most nonclassical condensed thiophenes that have been prepared, (53) being an exception, are quite reactive and undergo reactions characteristic of 1,3-dipolar intermediates. The 1,3-dipole need not, however, be a thiocarbonyl ylide (55) has been generated and it undergoes cycloadditions as a carbonyl ylide, this being the 1,3-dipolar form illustrated (72JA6214, 74JA1817). 

The known nonclassical A,B-diheteropentalenes consist of compounds containing an annelated thiophene or selenophene ring, the only uncharged nonradical representations of which contain a tetravalent sulfur or selenium, while the charged structures represent carbonyl, azomethine, thiocarbonyl or selenocarbonyl ylides. The parent systems have not yet been synthesized, only substituted compounds being known. The properties of these substituted derivatives provide a good measure of understanding of the reactivities of the parent systems. 

Lupinetti, Anthony J., Strauss, Steven H., and Frenking, Gernot, Nonclassical Metal Carbonyl. 49 1... 

The electron counting rules of Wade (S3), Williams (117), and Rudolph (118) can serve as a useful concept to explain structure and bonding in a variety of systems which at first glance are very different Zintl phases, boranes and carboranes, transition metal n complexes and carbonyl clusters, nonclassical carbocations, and also n complexes of main-group elements. According to... 

In the presence of low equilibrium pressures of CO, M +-CO (M = Fe, Co, Ni, Mn, and Cr) complexes are formed that are characterized by v(CO) stretching frequencies higher than that of CO gas. This means that we are dealing with nonclassical metal carbonyls and that the M +-CO bond has predominantly a donor character. 

Liquid and supercritical noble gases are ideal media for studying H2 species since H2 is completely miscible under these conditions. The group VI metal carbonyl hydrides, M(CO)5(H2), and cis-Cr(CO)4(H2)2 were formed photochemically in liquid Xe/If2 mixtures. H2 and N2 complexes of a number of half-sandwich compounds were formed in supercritical Xe. These included (C Rn)M(CO)2Y, where R = H, Me, M = Mn, Re (n = 5), Cr (n = 6), and (C4H4)Fe(CO)2Y, where Y = H2 or N2. In all cases except Re, nonclassical H2 complexes were formed. ... 

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