2- Butyne, bond distances

Figures 11.3.1(a) and 11.3.1(b) show the structures and hydrogen bonding distances of 2-butyne-HCl and 2-butyne-2HCl, respectively. In 2-butyne-HCl,...

The single bond distance in 2-butyne is 146.8 pm, suggesting that the bonding radius of sp hybridized C atoms is about 6.3 pm smaller than the radius of sp hybridized, i.e. about 70.3 pm. This estimate, in turn, implies that the single bond distance in 1,3-butadiyn should be 140.6 pm as compared to an observed value of 138.8 pm. Addition of the radii of sp and sp hybridized carbon atoms yields an estimated single C-C bond distance in l-butene-3-yn equal to 144.4 pm as compared to an observed distance of 143.4 pm. 

Complex condensation products are obtained by reaction of the alkyne complex with excess of 3,3-dimethyl-l-butyne, which yields two isomeric products of formulas Ru3(CO)6[HC2C(Me)3-COCH2CMe3][HC2CMe3]2 (125). The X-ray structure of one of those adducts (Fig. 19) shows that both dimerization of two alkyne molecules and the insertion of carbon monoxide into the alkyne metal bonds have occurred. The Ru-Ru distances of 2.820,2.828, and 2.686 A in the ring are of interest. The value of 2.686 A is one of the shortest found in a... 

The principal bonding interaction occurs between metallic u and cr frontier orbitals and the a C2 orbitals. You expect this because the [Mn(CO)s]+ fragment is isolobal with [CH3]+ and the product is isolobal with 2-butyne. However, the metal fragment provides the opportunity for weaker interactions between the tt-symmetry orbitals. These are of two types. A backbonding interaction (occupied metallic 77 orbitals with unoccupied C2 tt ) transfers charge from the metal to the C2 unit, thereby reducing the C-C bond order. For this compound, the observed C-C distance is consistent with a bond order of three hence, the interac-... 

Hock and Mills, in 1958, established that the best representation of the complex obtained from 2-butyne was as shown in formula (CIV) (6, 118). One iron atom and two of its three carbonyl ligands are very nearly coplanar with the four-carbon chain and its two methyl substituents. The other iron atom is approximately equidistant from the four ring carbon atoms and is apparently analogous to that in the butadiene-Fe(CO)3 complexes. An Fe—Fe bond, which would preserve the inert gas structure of the ring Fe atom, is assumed, the distance being 2.49 A. The molecular orbital structure of this system has been discussed 119). 

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