Chem_173B_HW2_2008_Key

Chem_173B_HW2_2008_Key - Chem 173B/268B Prof S Scott...

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Chem 173B/268B Homework assignment #2 Answer Key Prof. S. Scott Due in class on Monday, February 11, 2008 1. For each cluster, assess the cluster geometry as closo, nido or arachno, and predict the arrangement of the skeletal atoms: (a) [B 5 H 9 ] (b) [B 10 H 14 ] 2- (c) [C 2 B 9 H 12 ] - (a) F = (5x3) + (9x1) – (5x2) = 14, or 7 bonding pairs, or (n+2) structure is nido , octahedron minus one vertx which is a square pyramid (b) F = (10x3) + (14x1) – (-2) – (10x2) = 26, or 13 bonding pairs, or (n+3) structure is arachno , icosahedron minus two vertices which is a pentagonal antiprism (c) F = (2x4) + (9x3) + (12x1) – (-1) – (11x2) = 26, or 13 bonding pairs, or (n+2) structure is nido , icosahedron minus one vertex 2. The most common lead-containing ore is galena (PbS), whereas Ge and Sn are both found in nature as oxides. Explain. The hard Ge(IV) and Sn(IV) cations form stronger bonds to the hard O 2- anion, while the soft Pb(II) cation forms stronger bonds to the soft S 2- anion. Note that softness is a result of the larger size of Pb as well as the lower (II) oxidation state.
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