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23 Transition Metals and Solutions to Exercises Coordination Chemistry a stronger electron pair donor and Lewis base than en. Dmpe creates a stronger ligand field and is higher on the spectrochemical series. Structurally, P has a larger covalent radius than N, so M-P bonds are longer than M-N bonds. This is convenient because the two -CH₃ groups on each P atom in dmpe create more steric hindrance (bumping with adjacent atoms) than the H atoms on N in en. (b) CO and dmpe are neutral, 2CN⁻ = 2-, 2Na+ = 2+. The ion charges balance, so the oxidation state of Mo is zero. (c) The symbol P P represents the bidentate dmpe ligand. CN CO P CO P CN Mo Mo P CO P CN CN CO CN CN P CN NC P Mo Mo P CO OC P CO CO optical isomers 23.72 (a) In a square planar complex such as if one pair of ligands is trans, the remaining two coordination sites are also trans to each other. Ethylenediamine is a relatively short bidentate ligand that cannot occupy trans coordination sites, so the trans isomer is unknown. (b) The minimum steric requirement for a bidentate ligand is a medium-length chain between the two coordinating atoms that will occupy the trans positions. In terms of reaction rate theory, it is unlikely that a flexible bidentate ligand will be in exactly the right orientation to coordinate trans. The ligand with the four- carbon chain is more able to occupy trans positions, although four carbons may still not be long enough. And, the flexibility of its backbone means that it is still an unlikely trans bidentate ligand. 23.73 We will represent the end of the bidentate ligand containing the CF₃ group by a shaded oval, the other end by an open oval: 3+ 3+ 3+ 3+ Co Co Co Co 23.74 (a) Hemoglobin is the iron-containing protein that transports O₂ in human blood. (b) Chlorophylls are magnesium-containing porphyrins in plants. They are the key components in the conversion of solar energy into chemical energy that can be used by living organisms. 709