Chapter_20_Notes

Chapter_20_Notes - Reactions of coordination complexes...

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1 Reactions of coordination complexes Shriver, Chapter 20 Naming conventions for complex ions: 1. Name the cation first, then the anion 2. When naming the complex ion, name the ligands first (in alphabetical order), then the metal, then give the oxidation state of the metal • metals in a positive formal oxidation state keep their periodic table names • metals in negative formal oxidation states take the –ate suffix K 3 [Fe(CN) 6 ] potassium hexacyanoferrate(III) Ag becomes argentate Au becomes aurate Pb becomes plumbate Sn becomes stannate Sb becomes stibate 3. Anionic ligands take an –o suffix (chloro, cyano…). Neutral ligands retain their usual name (except NH 3 , ammine; H 2 O, aqua) 4. Use the prefixes mono-, di-, tri-, tetra-, penta-, hexa- etc., to indicate the number of identical ligands. If the ligand name already contains one of these prefixes, use instead bis, tris, tetrakis, pentakis, hexakis, etc., and separate the prefix from the ligand name with brackets. [Co(NH 2 CH 2 CH 2 NH 2 ) 2 Cl 2 + ] dichlorobis(ethylenediamine)cobalt(III) Ligand substitution in square planar complexes General reaction: Y + M-X Y-M + X By analogy to organic chemistry: Y is the nucleophile and X is the leaving group. The other ligands in the square planar complex (those which do not leave) are called spectator ligands . Strong nucleophiles (those that are good σ -donors) and good leaving groups (those that are weakly bonded to the metal) accelerate the rate of the reaction. In square planar complexes, which are coordinatively unsaturated (having only 16 valence e- on M, instead of 18), the mechanism of ligand substitution is a ssociative , or A. This means that the incoming nucleophile Y forms a bond to the metal first (ligand association), followed by departure of the leaving group X:
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2 L M X L L Y L M X L L L M Y L L X Y L M X L L Y L M X L L Y 16e- 18 e- 18 e- 18 e- 16 e- square planar square pyram. trigonal bipyram. square pyram. square planar Note that the mechanism is reversible and symmetrical (same sequence of steps from right to left and left to right). This is required by the Principle of Microscopic Reversibility . In other words, substitution of X by Y must occur by exactly the same mechanism as the substitution of Y by X. Double ligand substitution in the square planar d 8 complex [PtCl 4 ] 2- by NH 3 gives exclusively the cis product: Cl Pt Cl Cl Cl 2- NH 3 Cl Pt NH 3 Cl Cl - Cl Pt NH 3 NH 3 Cl NH 3 - Cl - - Cl - cis In contrast, double ligand substitution in the square planar d 8 complex [Pt(NH 3 ) 4 ] 2+ by Cl - gives exclusively the trans product: H 3 N Pt NH 3 NH 3 H 3 N 2+ NH 3 H 3 N Pt NH 3 Cl H 3 N + H 3 N Pt NH 3 Cl Cl NH 3 - Cl - - Cl - trans This is a manifestation of the kinetic trans effect . In both reactions, substitution trans to chloride is faster than substitution trans to NH 3 . Another way of stating this is that the ligand trans to chloride (the leaving group) is more labile (i.e., more rapidly removed) than the ligand trans to ammonia. The ligand trans to ammonia is, by contrast, more inert (slower to be removed).
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