Intro Inorganic Chem
Intro Inorganic Chem CHM 218
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Date Created: 11/01/15
131 Chapter 13 The Group 13 Elements Common Features No simple trend in melting points is observed mp C bp C B 2180 3650 A1 660 2467 Ga 30 2403 In 157 2080 T1 303 1457 Each element has a dilferent solid state structure For example the most stable allotrope of boron has icosahedra of B atoms Al has a fcc structure Ga In and T1 adopt dilferent structures Ga has the longest liquid range of any known substance Decrease in boiling point with increasing Z in normal behavior Covalency is common due to high chargetovolume ratio Those compounds regarded as ionic typically have hydrated cations eg AlOH253 The 3 oxidation state is the only important oxidation state for Al but the 1 oxidation state becomes increasingly important going down the group This is a manifestation of the inertpair effect Inert Pair Effect involves the existence of an oxidation state two less than the group oxidation state It is a manifestation of the resistance of the us electrons to be lost or participate in covalent bonding The second and third ionization energies of A1 are small enough that the hydration enthalpy can almost compensate However this is less and less the case as we descend the group Only the 3 and 1 oxidation states are known Although compounds of the formula GaC12 exist they actually involve combinations of the 1 and 3 oxidation states GaC12 is actually properly formulated as Ga GaC439 Boron ls the only elanemln Group lELhat ls notclassl ed as a m Ills classl ed as a semlrmetalbutm some respects it resembles nonmetals The chefsoumes ofbomn are ores sueh as borax Na2 B050H 8 H20 and kemlte Na1 30 4 H20 whreh e contain mgorral planar 1303 goups and tetrahede BO goups m large borate amons Boron has several allotxopes all based or the B reosahedrorr dlffenng only m the mngement of the lcosahedm wthm um eells All are somewhat dafflmlt o punfy due to the hlgh melnng pomt and eorroslye nature of the llquld or Ch mlstxy lsumque among he Group 13 elemeth and shows U ehemrsory Bor e slmllanues to sllleorr 1 n AD In 2 Bomtes and sllcateshave slmllarstxuctures wth eham and nngsbang eommorr hydrolyzed 4 Hydndes ofBand 5r ar p l as e yolaale mohlaheously ammable mdreadly hydrolyzed ALH ls o ymenc and no reamve Isolanorl oflhequurd l Redredorr of B10 by Mg results m an amorphous powder whreh ls e 953 pure B2033Mg lt2B3Mg0 Bcb 3er a 213 3zrrCl1 Bcb 3H2 a213 6 HO Uses ofbomn Twin M H Pyrex Increasedthermal shockresmance comparedto sodarhmersllma glasses um 141 Composmons and charmevinis of some at the Commun ommerdaf Glasses Camposmun was GkssType 51o N310 Cso 11101 1110 my havadensheand A 11 mm Fuscd silxca gt915 exp on muck muslin 95 5mm 95 A Thsrmuuy sh k an wean chemcally msxsmmi laboratory Bnms icnlc x1 3 s 25 13 hermally mu 1 mm chznucally mmquot 7 mcnwarc Container 15 5 1 my Law melxing ampemmm 1soda41me sasny worked aha 111mm berglass 1s 1s 111 my Emil drawn mm bers 7 s1 5 rssm canpoms opnm 111m so 1 Who High dens y nd hlgh 0 1 dex cfrcfx m lanus emscnrmm 70 Is np ca Eas1ly mama 5mm mm umth mask 7 mums Also usedm cleanmg agents mummy sodaum peroxoborate NaB ox 134 Boranes Compounds of boron and hydrogen collectively referred to as boranes are unusual in their electron de cient bonding The simplest borane is diborane B2 H5 which is produced by the reaction of boron tri uoride with sodium hydride 2 BF3 g 6NaH s a B2 H6 g 6 NaF Diborane is a highly reactive toxic and colorless gas It is spontaneously ammable and explosive with pure dixygen 13sz g 3 02 g A 13203 S 3 H20 g It also reacts with trace moisture to give boric acid and dihydrogen 13sz g 6 H20 0 v 2 H3303 211 3 H2 g Diborane is used extensively in the reduction of various organic compounds Higher boranes are known and constitute several classes closo nido and arachno hypho and conjuncto boranes Closoboranes a modi cation of the Greek clovo cage are complete ie closed polyhedra Nidoboranes from Latin nidus nest have structures that may be described as polyhedra that lack one vertex Arachnoboranes from the Greek word for a spiderweb have structures describable as polyhedra from which two adjacent vertices are missing Hyphoboranes from the Greek word for net have even more open structures o en visualizable as polyhedral fragments formed by removal of three contiguous vertices of a complete polyhedron Conjunctoboranes from Latin implying the joining of subunits have structures in which two or more of the preceding types are linked For these the possibilities are very numerous and nomenclatural problems can be formidable Linkage may be through a B B bond by way of one or more B H B bonds 3c2e or by having one or more boron atoms belong simultaneously to two subunits 135 Borane Nomenclature In naming the boranes the number of boron atoms is indicated using the normal pre xes di tti tetra etc with the word borane and the number of hydrogen atoms is given by an Arabic numeral in parentheses For example BGHIO is hexaborane10 B5H11 is pentaboranel l B10H14 is decaboranel4 Electron Counting for Boranes and Other Framework Substances Wade39s Rules It is now instructive to review the structures of the numerous boranes and carboranes which as noted earlier fall into the 01030 m39do and arachno categories The type of structure adopted by a particular compound has been shown to be related to the number of electrons that are available in the compound for bonding within the polyhedral framework that is the number of quotframework electronsquot A way of correlating the number of framework electrons with structure was rst articulated by K Wade hence the name quotWade s rulesquot Wade39s Rules as Applied to Boranes and Carboranes We start by de ning the quantity F the number of electrons available for framework bonds F3b4chxe 2n where b the number of boron atoms c the number of carbon atoms h the number of hydrogen atoms x the amount of negative charge on the ion n the number of vertices that is b 0 Note that x is de ned so as to be a positive quantity for anions Thus the number of valence electrons available for the framework bonds F is the number that remains a er providing for n exoframework 2c2e terminal B H or C H bonds For boranes c 0 and we will restrict our examples to these 136 RULE 1 When the value of F is equal to the quantity 2n 2 the substance should have a closo structure that is the framework geometry is based on an n vertex triangulated regular polyhedron This result is obtained for all of the borane dianions Banz39 for the carborane anions CBfr 1H39 and for the neutral carboranes Can2Hn since substitution of a BH39 group by the isoelectronic CH unit does not change the value of F as defined in the equation An example readily illustrates this result For B6H5239 the value of F is F3x64x0627 2x614 Since the quantity 2n 2 is also equal to 14 we have identi ed a closo situation The structure for BGHGZ39 is thus a six vertex polyhedron namely the octahedron as shown In closo compounds the bonds to hydrogen are only of the terminaltype B H or C H 2c 2e bonds RULE 2 When the value of F is equal to the quantity 2n 4 the substance should have the m39do structure that is an n l vertex polyhedron with one vertex missing as illustrated For example for B5H9 the quantity F is equal to F3x54x0907 2x514 Since this is equal to the quantity 2n 4 the structure of B5H9 is that of an n l 6 vertex polyhedron with one vertex missing This structure is illustrated In m39do compounds there are B H B b1idge bonds at those edges le open by the missing vertex atom The other hydrogen atoms are bonded in the 2c2e terminal fashion It is characteristic then of m39do compounds that we find two types of groups 7 terminal B H hydrogen atoms and B H B indges for the remainder RULE 3 When the value of F is equal to the quantity 2n 6 the compound falls into the arachno category and the preferred structure is that of the n 2 vertex polyhedron with two vertices missing The compound B5H11 and the ion B9H1439 provide use ll examples For B5H11 the quantity F is equal to F3x54x0ll02x5l6 This is equal to the quantity 2n 6 The structure is based on an n 2 vertex polyhedron with two adjacent vertices missing For ByHu39 the value ofF s F3x94x0141 2x924 13H2 groups The compound 35H provides moths example ngum 677 Srucmms nfsome bumncs go 450 Amemu W Nido 0 Closo bornneq Bridglng II3JI ogeIIs and EH groups In not shown Lilies linking bnran moms we only meant an illustrate chum geometry 13 9 Trihalides The trihalides are Lewis acids by virtue of the vacant p orbital As many F lm boron compounds they are electron de cient with respect to a complete F B F octet Lewis acidbase adduct formation BX3 is trigonal planar sp2 hybridized Even weak Lewis bases form adducts with BX3 On the basis of electronegativity diiTerences we might expect that BF3 would be the strongest Lewis acid of the lot However the observed order is BF3 lt BC13 lt BBr3 Enthalpies of reaction as well as rapid quantitative hydrolysis of BCl3 and BBr3 indicate that these are stronger Lewis acids than BF3 BF3 is partially hydrolyzed 4 BF3 6 H20 a 3 H3O 3 BF439 BOH3 BCIS and BBr3 are completely hydrolyzed BX3 3 H20 3 HCl 130H3 X 6 X 5 5 O 5 a 5 5 B X H H x B o H 3 0 H c X 6 I I gt H 6 5 XH 5 6 5 Experimental evidence suggests a degree of double bond character in F e 43 the BiX bond In BF3 rBF 130 pm and in BF439 rBF 145 pm The B F bond energy in BF3 is 613 ldmol F Since pi overlap increases as size and energy match improve and BiX bond length decreases the observed order of pi overlap is BF3 gt 13013 gt BBr3 1310 Boric Acid H3BO3 or BOH3 Starting point for much of the boron chemistry In aqueous solution boric acid acts not as a Bronsted acid but as a Lewis acid BOH3 2 H20 a BOH439 H3O Remaining Elements Preparation Aluminum is produced by the HallHeroult Process A1203 is dissolved in molten cryolite Na3A1F6 and electrolyzed using a graphite anode In this process C is oxidized at the anode rather than 0239 providing a huge energy savings 2A1203 e4Al 3 02 AG 3164ldE 273V 2A12033C e4Al 3C02 AG l9821dE l7lV At the elevated T the product might actually be CO but the point is that the oxidation of C provides a tremendous savings in the energy required GaInandTl Electrolysis of aqueous solutions of salts is made possible by large hydrogen overpotentials Oxides and Hydroxides The only oxide of Al is A1203 but this compound exists as a number of diiTerent polymorphs and hydrated materials Anhydrous alumina occurs in two forms 06 A1203 and y A1203 Both consist of close packed arrays of oxide ions but diiTer in the placement of the cations 06 A1203 is the higher temperature form of the material but also metastable at lower temps aka corundum Moh s hardness 9 7 only diamond is harder 06 A1203 is hard resistant to hydration and attack by acids 13 l l y A1203 absorbs water and dissolves in strong acid 7 used in chromatography A10 39 OH to AlOH3 is the range of stoichiometties for hydrated aluminas These vary from 1 H20 to 3 H20 per A1203 A1203 H20 2 A10 OH A1203 3 H20 2 AlOH3 Not to be con lsed with the true hydroxide Several gemstones natural and synthetic are based on alumina with traces of other metals ruby Cr doped into A1203 blue sapphire Fe Fe and Ti doped into A1203 GaIn oxides are similar T1203 exists but decomposes to T120 at 100 C Halides Halides of each element are known but T113 is actually W131 Tl3 is too oxidizing to coexist with I39 Some of the halides have CN 6 and some have CN 4 Fluorides are all CN 6 and high melting F Cl Br consistent with an ionic bonding model There is a correlation between melting point and CN CN 4 AI 6 consist of discrete molecular units dimers with a more covalent nature while CN 6 correspond to higher melting ionic compounds 60 6 In the vapor phase dimers exist but these dissociate In 6 at higher temperatures TI 6 X X x MI I39 39II M X X 1 3 1 2 Thallium halides vary in thermal stability T1F3 is stable up to 500 C T1C13 decomposes to TlCl C12 at 40 C T1Br3 is unstable with respect to TlBr2 which is actually Tf T1Br439 T113 cannot be isolated as T1 Aqueous Chemistry All elements form well de ned octahedral aqua ions MOH253 which are fairly acidic MOH263 H20 a MOH25OH2 H3O Ka varies from 10395 to 10391 and salts that contain anions of weak acids cannot coexist in aqueous solution Hydroxides of Al and Ga are amphoteric and dissolve in acids as well as bases Oxides and hydroxides of In and T1 are strictly basic Spinels Spinel is magnesium aluminum oxide MgA1204 although the term is used to refer to a number of compounds that adopt the same crystal structure The general formula is AB2X4 where A is a divalent cation B is a trivalent cation and X is a divalent anion usually 02 The crystal structure involves a cubic close packed array of anions and the unit cell includes 32 anions making the unit cell composition ASBMX32 In a normal spinel the 8 A ions occupy c of the tetrahedral holes and the 16 B ions occupy of the octahedral holes Subscripts oft and o are used to describe occupancy so the formula of spinel can be written as Mg2t 2 Al 02 4 In an inverse spinel half of the trivalent cations occupy tetrahedral holes and the remaining cations occupy octahedral holes Magnetite Fe304 adopts an inverse spinel structure with the arrangement Fe3t Fez Fe 0274 1313 Other Compounds 1 N A14C3 Aluminum Cmbide 4A1 3 C a A14C3 T 1000 20000C appears to contain discrete C439 centers and is sometimes considered a methanide but this is probably an oversimpli cation Nitrides Only Al reacts directly with N2 2 A1N2 a 2 AlN T 1000 7 2000 C GaN and InN are made by indirect routes All are fairly hard and stable as might be expected on the basis of their close structural relationship with cubic BN and diamond
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