Lecture_5_CoordinationBasic

Lecture_5_CoordinationBasic - Chem 310 Lecture Module 5...

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Chem 310 Lecture Module 5 Coordination chemistry: Structure
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General trends for high and low coordination numbers A) Low coordination numbers. 1. Soft ligands, metals in low oxidation states. Extensive π bonding may compensate for less σ bonding. Valence-electron rich metals do not need additional electrons om more ligands (see electroneutrality principle ). from more ligands (see electroneutrality principle ). 2. Large, bulky ligands. Steric hindrance prevents coordination of more ligands. 3. Counterions of low basicity. Cationic complexes with small C.N. are electron- poor, i.e. Lewis acids. To prevent the anionic counterion to react as a Lewis base, it must have a low basicity. Good counterions are the triflate anion, CF 3 OSO 2 - , BF 4 - , PF 6 - , SbF 6 - . B) High coordination numbers. )g 1. High oxidation states and hard ligands. Both serve to maximize electrostatic interactions. See spinel for a comparison of M II and M III . 2. Small ligands. Enables the presence of many ligands without steric difficulties. idi ti l ith li d t i ll l d 3. Large, non-acidic cations. Complexes with many ligands are typically large and anionic despite the high oxidation state of M. Cationic counterions of a similar size will enhance the lattice energies.
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Radius ratios r M / r L limit geometries, cf. ZnS vs NaCl vs CsCl inimum radius ordination C. N. minimum radius ratio coordination polyhedron 4 0.225 tetrahedron 414 uare planar 4 0.414 square planar 6 0.414 octahedron 6 0.528 trigonal prism 7 0.592 capped octahedron 8 0.645 square antiprism 8 0.668 dodecahedron 8 0.732 cube 732 icapped trigonal prism 9 0.732 tricapped trigonal prism 12 0.902 icosahedron 2 000 boctahedron 12 1.000 cuboctahedron
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Coordination number (C.N.) 1 •Rarely observed. Examples are ion pairs (e.g., Na + Cl - ) in the gas phase, or the ordination entities M h) = Cu Ag) with a highly sterically hindered ligand: coordination entities M-C 6 H 2 (Ph) 3 (M Cu, Ag) with a highly sterically hindered ligand: Coordination number (C.N.) 2 •few examples, basically limited to +1 ions of group 11 (Cu I , Ag I , Au I ) and Hg II , with ligands NH 3 , CN - , Cl - . Low stability however; i.e. could react with additional ligands if present: [Ag(NH 3 ) 2 ] + + 2 NH 3 -> [Ag(NH 3 ) 4 ] + [Hg(CN) ] + 2 CN - -> [Hg(CN) ] 2- •Sterically hindered ligands (e.g. [N(SiMePh 2 ) 2 ] - ) can build two-coordinate complexes with other ions (Mn II , Fe II , Co II , Ni II ) by preventing approach of additional ligands. • avored ligand arrangement is linear; (consider electrostatics and/or ybridization).
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This note was uploaded on 02/28/2011 for the course CHEM 310 taught by Professor Nazar during the Fall '09 term at Waterloo.

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Lecture_5_CoordinationBasic - Chem 310 Lecture Module 5...

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