Three-coordinate trigonal pyramidal molecules

The coordination of CU3 resembles that in the peroxide form. The threefold coordination by histidines is a very flat trigonal pyramid. The coordination sphere around CU4 is not affected. CU2 is fivefold coordinated to the NE2 atoms of the three histidines, as in the reduced form, and to the two azide molecules. The two azide molecules are terminally bound at the apexes of a trigonal bipyramid. Both azide molecules bind to the copper atom CU2, which is well accessible from the broad channel leading from the surface of the protein to the CU2—CU3 copper pair. It is not unexpected that the second azide molecule (az in Fig. 13) binds similarly to the peroxide molecule, as azide is regarded as a dioxygen analogue. There is no bound azide molecule bridging either CU2 with CU4 or CU3 with CU4. 

The possible stroctures may be classified in terms of the coordination number of the central atom and the symmetry of the resulting molecule (Fig. 6.17). Two groups about a central atom will form angular (p2 orbitals, symmetry) or linear (sp hybrid, D j, symmetry) molecules three will form pyramidal (p. C3,) or trigonal planar (jp2, Dj,) molecules four will usually form tetrahedral (sp, TJ) or square planar (dip, 4 ,) hve usually form a trigonal bipyramidal (Dyf,), more rarely a square pyramidal (C4,.) molecule (both dsp hybrids, but using different orbitals, see Table 6.2) and six groups will usually form an octahedral molecule (d sp, Oi,). 

Two singlet isomers for the three-coordinate M(I) complexes (cis-1, trans-1) have been identified in the previous section. Addition to either complex of a dihydrogen molecule with the formation of two M-H bonds formally oxidizes the metal atom to M(in). Assuming a least-motion pathway, the cis-1 reactant will lead to the square pyramidal (SQP) product cis-2, reaction (3) similarly, addition of H2 to trans-1 produces the trigonal-bipyramidal (TBP) product trans-2, reaction (4). 

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