Chapter 23 Transition Metals and Coordination I Chemistry A 23 1 The Transition Metals Minerals metallic elements found in nature as solid inorganic compounds Metallurgy the science and technology of extracting metals from their natural sources and preparing them for practical use o 1 Mining the relevant ore mixture of minerals o 2 Concentrating the ore or otherwise preparing it for further treatment o 3 Reducing the ore to obtain the free metal o 4 Purifying the metal o 5 Mixing it with other elements to modify its properties o The last process produces an alloy a metallic material composed of two or more elements Increasing effective nuclear charge favors a decrease in radius as we move from left to right across each period Metallic bonding strength increases until the middle of each period and then decreases as antibonding orbitals become full A bond shortens as it becomes stronger The filling of 4f orbitals through the lanthanide elements causes a steady increase in effective nuclear charge producing a size decrease called the lanthanide contraction The lanthanide contraction just offsets the increase we expect as we move from period 5 transition metals to period 6 Most transition metal ions contain partially occupied d subshells which are responsible in large part for three characteristics o 1 Transition metals often have more than one stable oxidation state o 2 Transition metal compounds are colored o 3 Transition metals and their compounds often exhibit magnetic properties Magnetic moment the spin an electron possesses which cause the electron to behave like a Diamagnetic solid on in which all the electrons in the solid are paired the spin up and spin tiny magnet down electrons cancel on another Diamagnetic substances are generally nonmagnetic but when a diamagnetic substance is place in a magnetic field the motion of the electrons causes the substance to be very weakly repelled by the magnet Paramagnetic substance one in which the atoms or ions have one or more unpaired electrons I In a paramagnetic solid the electrons on one atom or ion do not influence the unpaired electrons on neighboring atoms or ions As a result the magnetic moment on the atoms or ions are randomly oriented A paramagnetic substance is attracted a magnetic field Ferromagnetism a form of magnetism much stronger than Paramagnetism This arises when unpaired electrons of the atoms or ions in a solid are influence by the orientation of electrons in neighboring atoms or ions When placed in a magnetic field electrons tend to align strongly with the direction parallel to the magnetic field Antiferromagnetism when unpaired electrons on a given atom or ion align so that their spins are oriented in the direction opposite from the spin direction on neighboring atoms Ferrimagnetism a substance that has both ferromagnetic and antiferromagnetic properties B 23 2 Transition Metal Complexes Metal complexes species that are assemblies of a central transition metal ion bonded to a group of surrounding molecules or ions Coordination complexes compounds that contain complexes Ligands The molecules or ions that bond to the metal ion in a complex Primary valence the oxidation state of the metal Secondary valence the number of atoms bonded to the metal ion Also called the coordination number Coordination sphere the central metal and the ligands bound to it The charge of a complex is the sum of the charges on the metal and on the ligands C 23 3 Common Ligands in Coordination Chemistry Donor atom the ligand that binds to the central metal ion in a coordination complex Monodentate ligands ligands having only one donor atom Bidententate ligands ligands having two donor atoms Polydentate ligands ligands having three or more donor atoms Chelating agents bidentate and polydentate ligands Chelate effect the trend of generally larger formation constants for bidentate and polydentate ligands Ligands can also be called sequestering agents D 23 5 Color and Magnetism in Coordination Chemistry Color depends on the identity of the metal ion its oxidation state and on the ligands bound to it Complementary colors are opposite each other on the color wheel Absorption spectrum the amount of light absorbed by a sample as a function of wavelength 23 6 Crystal Field Theory E Crystal Field Theory A theory that accounts for the colors and the magnetic and other properties of transition metal complexes in terms of the splitting of energies of metal ion d orbitals by the electrostatic interaction with the ligands Because some d orbitals point right at the ligands whereas other point between them the ligands split the energies of the metal d orbitals For an octahedral complex the d orbitals are split into a lower energy set of three degenerate orbitals the t2 set and a higher energy set of two degenerate orbitals the e set The gap between the two sets is called the crystal field splitting energy Because transition from a t2 set to an e set involves exciting an electron from one set of d orbitals to another we call it d d transition Spectrochemical series when ligands are arranged in order of their abilities to increase splitting energy Ligands at the low change end of the spectrochemical series are called weak field ligands Ligands at the high change end of the spectrochemical series are called strong field ligands Spin pairing energy the difference between the energy required to pair an electron in an occupied orbital and the energy required to place that electron in an empty orbital High spin complex when electrons are arranged so that they remain unpaired as much as possible Low spin complex when electrons are arranged so that they remain paired as much as possible