Square-planar carbon atom

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 cyclododecatrienylnickel complex [Li(tmeda)]2Ni(cdt) (65) (92) is structurally analogous to 64 each lithium atom is apparently tetrahedrally surrounded by two olefinic carbon atoms and two nitrogen atoms with a possible additional interaction with the nickel atom. The nickel atom in 65 adopts a distorted trigonal-bipyramidal geometry with the lithiums occupying the apices, whereas 64 involves a square-planar nickel atom. 

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. 

The reactivity of neutral square-planar d butatrienylidene complex 11 (Scheme 3.8) strongly deviates from that of cationic d ruthenium complexes. The deviation is readily understood when considering the orbital contributions of the metal and the carbon atoms of the chain to the LUMO. In d and d complexes the LUMO is predominantly localized at the metal, at Cl and C3. However, the relative contribution of the metal in d and d complexes is significantly different. In d complexes the metal contributes considerably less than Cl and C3, in d complexes its contribution is approximately equal to that of Cl and C3. 

The LUMO in d pentatetraenylidene complexes is predominantly localized on the odd carbon atoms and to a lesser extent on the metal. The coefficients on Cl and C3 are very similar, independent of the metal-ligand fragment and the terminal substituent. The coefficient at C5 is somewhat larger. In square-planar d rhodium and iridium complexes the coefficient at the metal is comparable to that on C5 and is larger than those on Cl and C3. Thus, a nucleophilic attack at the metal of d complexes has also to be taken into account. 

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