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23 Transition Metals and Solutions to Exercises Coordination Chemistry 23.18 Ferromagnetic, antiferromagnetic and ferrimagnetic materials all have atoms or ions with unpaired electrons. Furthermore, the spins of these electrons are influenced by (coupled with) the electron spins at neighboring atoms or ions. In ferromagnetic materials, coupled electron spins are aligned in the same direction. In antiferromagnetic materials, coupled spins are aligned in opposite directions and the opposing spins exactly cancel. In ferrimagnetic materials coupled spins are aligned in opposite directions (like antiferromagnets) but the opposing spins do not cancel. 23.19 Analyze/Plan. Consider the orientation of spins in various types of magnetic materials as shown in Figure 23.5. The diagram shows a material with misaligned spins that become aligned in the direction of an applied magnetic field. This is a paramagnetic material. 23.20 (a) has all Fe atoms in the +3 oxidation state, while contains Fe atoms in both the +2 and +3 states. Each Fe₃O₄ formula unit has one Fe(II) and two Fe(III). (b) In an antiferromagnetic material, spins on coupled atoms are oppositely aligned, producing a net spin of zero. This is only possible for where all Fe atoms have the same oxidation state, d-electron configuration and number of unpaired electrons. In Fe(II) and Fe(III) atoms have different d-electron configurations and different numbers of unpaired electrons. Assuming an Fe(II) is coupled to an Fe(III), even if spins on coupled centers are oppositely aligned, their spins do not fully cancel and the material is ferrimagnetic. Transition-Metal Complexes (section 23.2) 23.21 (a) In Werner's theory, primary valence is the charge of the metal cation at the center of the complex. Secondary valence is the number of atoms bound or coordinated to the central metal ion. The modern terms for these concepts are oxidation state and coordination number, respectively. (Note that "oxidation state" is a broader term than ionic charge, but Werner's complexes contain metal ions where cation charge and oxidation state are equal.) (b) Ligands are the Lewis base in metal-ligand interactions. As such, they must possess at least one unshared electron pair. NH₃ has an unshared electron pair but BH₃, with less than 8 electrons about B, has no unshared electron pair and cannot act as a ligand. In fact, BH₃ acts as a Lewis acid, an electron pair acceptor, because it is electron-deficient. 23.22 (a) Coordination number is the number of atoms bound directly to the metal in a metal complex. (b) Ligands in a metal complex usually have a lone (unshared, nonbonding) pair of electrons. is neutral, CN- is negatively charged. (c) No, ligands with positive charges are not common. Metal atoms in a complex usually have a positive charge, so a positively charged ligand would neither be electrostatically attracted to the metal nor provide the electron density required to form a metal-ligand bond. 697