Quintuple bonds

As an improved metric of M—M quintuple-bond strength, let us instead consider the energetics of hydrogenation of HMMH to form two MH6 metal hydrides,... 

We present in Table 3 the excitation energies needed to produce a valence state with all orbitals singly occupied. The largest excitation energy is for Ac. The price to pay for forming a triple bond between two Ac atoms is 2.28 eV for Th, only 1.28 eV is needed, which can then, in principle, form a quadruple bond. Note that in these two cases only 7s and 6d orbitals are involved. For Pa, 1.67eV is needed, which results in the possibility of a quintuple bond. The uranium case was already described above where we saw that, despite six unpaired atomic orbitals, only a quintuple bond is formed with an effective bond order that is closer to four than five. 

The strongest bond is formed between the Pa atoms in Pa2. Here the contribution of 6d is maximum, and we see a complete promotion to the atomic state with five unpaired electrons. A quintuple bond is formed with a short bond distance and a bond energy close to 5 eV. The bond contains the (7sag)2(6dnu)4 triple bond plus a 6dag two-electron bond and two 6dhg one-electron bonds. The 5/population is increased to one electron, but we still do not see any molecular orbital dominated by this atomic orbital. They are all used but rather in combination with the 6d orbitals. 

The Quintuple Bond between Two Chromiums in PhCrCrPh (Ph — Phenyl). Trans-Bent Versus Linear Geometry A Quantum Mechanical Study. 

Recently, structural evidence for the first quintuple bond between two metal atoms was found in the dichromium(I) complex Ar CrCrAt7 (where Ar is the sterically encumbering monovalent 2,6-bis[(2,6-diisopropyl)phenyl]phenyl ligand). This complex exists as air- and moisture-sensitive dark-red crystals that remain stable up to 200°C. X-ray diffraction revealed a centrosymmet-ric molecule with a planar trans-bent C-Cr-Cr-C backbone with measured structural parameters Cr-Cr 183.51(4) pm, Cr-C 213.1(1) pm, and C-Cr-Cr 102.78(3)°. Characterization of the compound is further substantiated by magnetic and spectroscopic data, as well as theoretical computations. Figure 19.2.6 shows the molecular geometry and structural formula of Ar CrCrAr. ... 

Power and co-workers have utilised the steric demands of bulky monodentate m-terphenyls to protect highly reactive transition metal centres, whilst minimising the number of orbitals involved in metal-ligand bonding in order to form complexes with very high M-M bond orders. The first complex to feature a quintuple bonding interaction between two... 

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 . 

M. Brynda, L. Gagliardi, P.-O. Widmark, P. P. Power, and B. O. Roos, A Quantum Chemical Study of the Quintuple Bond Between Two Chromium Centers in... 

In the 1960s quadruple bonds between metallic atoms were discovered in species such as Re2Clg . There would be no obvious way to explain quadruple bonds with cubic atoms or tetrahedra. In 2005, evidence was presented for a quintuple bond between the two chromium atoms in a crowded dichromium complex (RCrCrR). ... 

Ritter, Steve R. Quintuple Bond Makes Its Debut. Chemical ... 

Engineering News 83 (September 26, 2005) 9. This is a very brief review of the article describing the first example of a quintuple bond. 

During the succeeding decades, thousands of transition-metal cluster compounds have been synthesized, including hundreds containing quadmple bonds and some that may contain quintuple bonds. Therefore, we need to consider how metal atoms can bond to each other and, in particular, how high bond orders between metals can be achieved. 

Stable multibonded transition metals followed [6] and led to the first experimental evidence of quintuple bonding in 2005 [7]. 

The other five pairs form the 3d bonds in the Cr-Cr complexes and they present quite a different binding power. The 3do-orbital pair creates a bond with an EBO of 0.77, whereas the 3dtt bonds exhibit an EBO of 0.81 each, and the 3d5 bonds an E BO of 0.58 each. The total EBO then adds up to 4.45. Therefore, the Cr-Cr bond is closer to a quadraple bond, rather than a hextuple or quintuple bond, because the 5 bonds are very weak and could also be considered as two pairs of antiferromagnetically coupled 3d5 atomic orbitals. The CASSCE/CASPT2 conclusions presented by Roos [25] yield a bond distance of 1.66 (1.68) A and a bond energy (Dg) of 1.65 (1.53) eV (experimental values are given within parentheses). 

The Cr-Cr quintuple bond in Ar CrCrAr has been recently investigated with a newly developed energy and density decomposition scheme by Ndambuki and... 

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