Fivefold bonds

Synthesis of a Stable Compound with Fivefold Bonding between Two Chromium(I) Centers. 

T. Nguyen et al., Synthesis of a stable compound with fivefold bonding between two chromium centers. Science 310, 844—847 (2005)... 

Fig. 9.6 Structural representation of Power s diaryldichromium(I) compound 20 with fivefold bonding between the two chromium atoms...

The appreciation of the theoretical work notwithstanding, the greatest achievement, however, was undoubtedly the isolation and precise characterization of the novel dinuclear compound 20. Not too long ago, probably nobody would have assumed that simple molecules of the general composition A2B2 with a fivefold bonding between the two atoms A could exist as stable entities under normal conditions. It is a relatively safe bet to predict that Phil Power s chromium dimer was just a start and, as he proposed [152], could be the forerunner of other quintuple bonds . 

T. Nguyen, A. D. Sutton, M. Brynda, J. C. Fettinger, G. J. Long, and P. P. Power, Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers, Science 310, 844-846 (2005). 

Compounds with fivefold bonds between transition metals have been reported (Figure 1.3), accompanied by debate as to whether these bonds merit the designation quintuple. ... 

The bond order in complexes with very short metal-metal bonds has been controversial. Various calculations have given bond orders as low as 3.3 for fivefold bonded chromium(I) complexes. Factors such as the influence of bridging ligands on metal-metal distances, nonlinearity in Power s original complex that may reduce orbital interactions, and more complex interactions beyond the scope of this text are factors that merit consideration. The reader is encouraged to cousult the sources cited for discussions of the numerous factors involved in Cr—Cr multiple bonds in these dimers. Explorations of the reactivity of these new bonds are underway, with a particular emphasis on small molecule activation chemistry. " ... 

One objective of exploring the reactivity of dichromium complexes with proposed fivefold bonding is to obtain additional data to support these high bond orders. 

X-ray studies at 22.5 A resolution of murine polyomavlrus by 1. Rayment and D.L.D. Caspar at Brandeis University confirmed the presence of these 72 capsomers at the expected positions, but even at low resolution the pentagonal shape of all 72 capsomers was evident (Figure 16.22). They concluded that each capsomer must be a pentameric assembly of the major viral subunit, known as viral protein 1 (VPl). Each of the 60 icosahedral asymmetric units contains 6 VPl subunits, not 7, and the complete shell contains 360 VPl subunits. The 12 VPl pentamers centered on icosahedral fivefold axes are identically related to their five neighbors, but the 60 pentamers centered on pseudosixfold positions "see" each of their 6 neighbors quite differently (Figure 16.23). How can such diversity of interaction be incorporated into the bonding properties of just one type of protein subunit, without compromising specificity and accuracy of assembly ... 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

Fig. 27. Schematic diagram illustrating (a) a fivefold coordinated floating bond and (b) a threefold coordinated dangling bond (Pantelides, 1986).

The 12 RP fragments cap alternately the Cu4 faces of the Cu24 polyhedron, resulting in fivefold-coordinated phosphorus atoms. This structure resembles that of the recently described [Cu24(NPh)i4]4 anionic cluster (40). The Cu-P and Si-P distances are unremarkable. The construction principle of parallel Cu layers to form a metal-like package has also been observed for other Cu clusters (41). The main reason for the different structures of Cu2PR and Li2PR clusters is the covalent character of the Cu-P bond, with the additional involvement of favorable Cu-Cu interactions. The latter are probably due to relativistic d10-d10 interactions (dispersion-type of interaction) (42, 43). 

This approximate fivefold excess is necessary to reduce flic chromium higher valent salts and thereby eliminate overoxidation and double bond cleavage. The zinc dust used was obtained from Allied Chemical Corporation. 

---