The Many Ways To Have a Quintuple Bond

Abstract

The existence and persistence of five-fold (quintuple) bonding in isomers of model RMMR molecules of quite different geometry are examined theoretically. The molecules studied are RMMR, with R = H, F, Cl, Br, CN, and CH3; M = Cr, Mo, and W. The potential energy surface of these molecules is quite complex, containing two, three, even four local minima. The structural preferences in these molecules are rationalized, and electronic factors responsible for these preferences are elucidated. The linear geometry is always a minimum, but almost never the global minimum; there is a definite preference in RMMR for either a trans-bent conformation or perturbations of the trans-bent isomer with at least one of the R groups in a bridging position about the MM bond. The potential energy surface of these RMMR molecules is relatively flat, the lowest energy conformation being that which for a given molecule attains the best compromise between maximization of the MM bonding and minimization of orbital interactions that are MR antibonding. A surprising low-symmetry C(s) structure is identified, which along with the trans-bent isomer is one of the two most popular choices for the global minimum. Regardless of what isomer of the RMMR molecule is preferred, the MM quintuple bond persists.

Experimental and Theoretical Studies on Arene‐Bridged Metal–Metal‐Bonded Dimolybdenum Complexes

“…Thus, the MoMo distances have almost the same value within experimental error, as they range from 2.105(1) Å in 3⋅C 6 H 5 Me to 2.117(1) in 3⋅C 6 H 4 F 2 . This bond length is longer than in Tsai’s complex by nearly 0.1 Å, and even if it lies in the upper part of the 2.02–2.12 Å interval calculated theoretically for fivefold MoMo bonding,26 it is clearly signaling that the effective metal–metal bond order in compounds 3 must be substantially lower than five (see below).…”

Chromium to chromium quintuple bonds in a trigonal lantern configuration

“…The 4 å sieves and Celite were dried in vacuo overnight at a temperature just above 200°C. Compounds Cr 2 [ μ ‐ η 2 ‐HC(N‐2,6‐R 2 C 6 H 3 ) 2 ] 2 (R = i Pr ( 3 ), Et ( 4 )) were prepared according to the literature method . All other compounds were used as received.…”

“…Initiated by theoretical investigations, a D 3h ‐symmetric quintuply bonded complex was first proposed by Hoffmann et al in 1979, although the metal identity was not sure . Later on in 2000s, the group VI metal elements became the focus in the hypothetical quintuply bonded compounds M 2 X 2 , which have been shown to display various conformations, of which the trans ‐bent geometry is the most energetically favorable. On the experimental side, the breakthrough in the quintuple bonding came from the characterization of the isolable quintuply bonded chromium terphenyl dimer Cr 2 Ar′ 2 (Ar′ = 2,6‐[2,6‐ i Pr 2 C 6 H 3 ] 2 C 6 H 3 ) by Power and coworkers in 2005 .…”

Strong Chemical Bonds

“…Mo 2 molecule has also been observed in the gas phase at low temperatures [99][100][101][102], and the Mo-Mo bond length was determined to be 1.93 Å [103]. M-M bond in these cases was proposed to be a sextuple bond corresponding to filling two σ-type orbitals (formally, one from the d block and one from the s orbital), plus two π-type and two δ-type orbitals (σ 4 π 4 δ 4 ) [3,[104][105][106][107]. In reality, there is substantial configuration mixing in such compounds.…”