Pyramidal carbon molecular structure

Its molecular structure (Figure 37) consists of a centrosymmetric dimer with a bridging H2Al(OR)( U-OR)2Al(OR)H2 entity. The Ta atoms are approximately square pyramidal, with the four phosphorus atoms forming the basal plane (Ta lies 0.64 A out of it). The relatively short Ta—A1 distances are comparable to those found in other transition metal aluminum complexes (Ta—Al 2.79-3.13 A). The hydrogen atoms have not been located, but were evidenced by chemical and spectroscopic techniques (IR 1605, 1540 cm 1 HNMR 16.30p.p.m.). The Ta—(ju-H2)A1 unit is relatively stable, and (54) is inert to carbon monoxide or trimethylamine. It is a poor catalyst in the isomerization of 1-pentene. Formation of complexes analogous to (54) may explain the low yields often obtained from alkoxoaluminohy-drides and metal halides. 

Bridgehead Double Bonds. Typical bridgehead olefins represented by 44 (cf. Table 4) have been extensively discussed by Bredt (3). Inspection of molecular models, which do not allow for rehybridization at the carbon atoms of the double bond, suggest that structure 68 should prefer extensive torsional deformations. Extended Hiickel calculations showed, however, that the structure of a bridgehead double bond is best described by the overlap of a spx hybridized orbital at the bridgehead with an orbital at the adjacent carbon atom of high p character. This type of n bond is favored over a structure with two strongly pyramidalized carbon atoms (79). 

Figure 2. The molecular structure of [Au2Ru3(//-H)(/(3-COMe)(//-dppf )(CO)9] (3). The C, and carbons on each of the phenyl rings have been omitted for clarity. The clusters [A112RU3-( U-H)(//3-COMe) /i-Ph2P(CH2)nPPh2 (CO)9] [n = 1 (1) or 2 (2)] adopt similar square-based pyramidal skeletal geometries. ...

An interesting example of Reaction 1 has just been discovered in a study of the reaction between Mo2(OR)g compounds and carbon monoxide (13). The compound Moo(OBu )6 reacts reversibly with carbon monoxide in hydrocarbon solvents at room temperature and 1 atm to give a deep purple crystalline compound Mo2(OBu )eCO, v(CO) = 1670 cm The molecular structure deduced from x-ray studies (13) is shown in Figure 1. The molecule has virtual C2v symmetry. The coordination polyhedron about each metal atom is approximately a square pyramid with the bridging carbonyl carbon at the common apex. The short metal-metal distance, 2.498(1) A (cf. Mo—Mo = 2.222(1) A in Mo2(OR)e), the diamagnetic nature of the compound, and electron counting require the existence of a metal-metal double bond. 

The C4v structure is a transition state leading to the relative-ininimum C2v species [32], Structures 6, 7 and 8, rather than housing a pyramidal carbon, also turn out to be of relevance to the question of how nearly two bonds at a tetracoordinate carbon can approach linearity (the relevant CCC angle of 7 is calculated to be 178°). Another molecule that, drawn intuitively, looks like it may have a pyramidal carbon, but on subjection to computation turns out to be otherwise, is 1,3-dehydroadamantane geometry optimization by molecular mechanics,semiempir-ical, or ab initio methods show the deceptive atom to be tetrahedral, albeit strongly distorted ... 

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