Square planar carbon

Translate these very poor diagrams of molecules into more realistic structures. Try to get the angles about right and, whatever you do. don t include any square planar carbon atoms or other bond angles of 90° ... 

The central 77-type lone pair, a characteristic feature of square planar carbon systems, will be surely destabilized. Vacating this MO will result in C5- for which there are... 

A square planar carbon configuration can be achieved only if the HOMO a u is substantially stabilized by good r-acceptor ligands. For example, di- and trilithio-methane are calculated to have a planar structure 9.33a only 2.5 and 3.5 kcal/mol, respectively, higher than the tetrahedral structures 9.33b [18]. As indicated in 9.33c,... 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

In the cis isomer, the two CH3 groups (or the two H atoms) are as close to one another as possible. In the trans isomer, the two identical groups are farther apart. The two forms exist because there is no free rotation about the carbon-to-carbon double bond. The situation is analogous to that with cis-trans isomers of square planar complexes (Chapter 15). In both cases, the difference in geometry is responsible for isomerism the atoms are bonded to each other in the same way. 

Each step includes elementary acts that require different properties of the metal, for example, sufficiently low ionization potential to favor oxidative addition, sufficiently weak metal-carbon bonds, tendency to form square-planar complexes and to reach pentacoordination to allow insertion, a sufficiently high electron affinity to allow reductive elimination, and so on. Some properties are conflicting and a compromise has to be reached. 

As anticipated by its ML2X2 formulation (Table 4.52), the computed structure of singlet nickelocene approximates a square-planar di-allylic coordination mode. We can deconstruct each r 3-Cp to Ni interaction into an electron-pair bond (M—X) with the radical carbon and a dative interaction (M—L) with the 7icc bond, symbolized as shown below with a half-filled circle ( >) to represent the radical site and a filled circle ( ) to represent the dative 7t-bond site ... 

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