中国科技大学：《无机化学》第十五章硼族元素 The Boron Family Elements

一、 General Properties 1.硼的化学性质与Si有某些相似之处(对角线相似原则),通常硼呈现+3氧化态,负氧化态的情况很少。硼与金属形成非化学计量的化合物( nonstoichiometric compounds), M4B、M2B、MB、M3B4、MB2、MB6等。

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216 1971 年美国结构化学家 K.Wade 在分子轨道理论基础上提出了一个预言硼烷，硼烷衍生物及其它原子簇化合物结构的规则，通常称为“Wade 规则”，其含义是硼烷、硼烷衍生物及其它原子簇化合物的结构是由其骨架成键电子对数决定的。若以 b 表示骨架成键电子对数，n 为骨架原子数，则 b = n + 1 为闭式结构 (closo)，b = n + 2 为巢式结构（nido），b = n + 3 为网式结构（arachno），b = n + 4 为链式结构（hypho）。这意味着随着硼烷骨架上的成键电子对数目增加，其完整程度逐一降低，开口程度不断增加。对于中性硼烷、硼烷阴离子，碳硼烷的结构均可用下式表示： [(CH)a(BH)pHq] d− CH 提供给骨架 3 个电子，BH 提供给骨架 2 个电子，S 提供给骨架 4 个电子， P 提供给骨架 3 个电子给骨架，因为当硫原子和磷原子取代硼烷中的硼原子时， S、P 原子上必有一对价电子指向骨架外侧，成为孤对电子对，所以键合时，S 原子提供 4 个电子，P 原子提供 3 个电子。则多面体骨架成键电子数 M = 3a + 2p + q + d ，由于 a + p = n ，则： M = 2n + a + q + d ∴b (骨架成键电子对数) ( ) 2 1 = n + a + q + d 对于 2− BnHn ：a = 0，q = 0，d = 2，，所以 b = n + 1，为闭式结构 B11SH11，B10CPH11 为闭式，B3C2H7，B5SH10 为巢式。 Practice Exercise：应用 Wade 规则，指出下列物质所属的结构类型： 4 2 6 3 8 10 14 2 9 2 11 9 2 11 2 B5H9，B6H6 ，B C H ，B C H ，B C H ，B H ，B H − − − B10H15，B10H18，B3C2H7，B9SH11 − §15-2 铝及其化合物 Aluminum and its Compounds 铝是两性金属，有钝化性。 Al2O3 有两种变体， −Al2O3 称为刚玉，非常致密,硬度仅次于金刚石和金刚砂 (SiC)，  −Al2O3 可制作刚玉坩埚（2100K），在 −Al2O3 中掺 Cr3+离子，成为人造宝石－红宝石；γ −Al2O3 为活性氧化铝，表面积大，可作为吸附剂和催化剂载体。在 AlF3 晶体中，Al 的配位数为六。在 Al2Cl6 和 Al2I6中，Al 的配位数为四。 Al I Al I I I I I Al2Cl6 易水解，所以 AlCl3 只能用干法制备： 2Al + 3Cl2 2AlCl3 2Al + 6HCl(g) 2AlCl3 + 3H2(g) Cl Al Cl Cl Cl Al Cl Cl

An Alternative View of Bonding in Boron Trifluoride The chemist, Ron Gillespie, has proposed an alternative bonding model for boron trifluoride, He argues that the bonding is essentially ionic First of all, he notes that as we progress across the Period 2 fluorides from LiF through Bef and BF, to CF it is assumed that there is an abrupt change in bonding from ionic to covalent between beryllium and boron, Second. he contends that the boron - fluorine bond distance is that expected for three close-packed fluoride ions around a boron 3+ on.The greater bond distance in the tetrahedral tetrafluoroborate ion BF, in his view, confirms this hypothesis; four fluoride ions cannot ap- proach the tiny boron ion as closely as three can. In fact, the ratio of B-F bond lengths between BF, and BF, is that predicted by the close packed model. To those who would argue that ionic compounds should not be gases at room temperature, he contends that ionic substances can indeed be gases, provided they are fully coordinated. Thus alu minum fluoride, AlFa, is a solid because six fluorides can surround the aluminum ion, resulting in a three-dimensional (high melting) lattice Boron, by virtue of its small size, can only accommodate the three fluo ride ions. So which is the "correct"model? In chemistry, we often for get that explanations are used to account for observed facts, not vice ersa. Sometimes we use simplistic models that have good predictive alue even when they have limited experimental validity. It would be nice to have the "correct"model but chemistry is rarely as definitive as it is sometimes portrayed in general chemistry courses. Inorganic Fibers In our everyday lives, the fibers we encounter are usually organic, for example, nylon and polyester. These materials are fine for clothing and similar purposes, but most organic fibers have the disadvantages of low melting points, flammability, and low strengths. For materials that are strong and unaffected by high temperatures, inorganic materials fit the specifications best. Some inorganic fibers are well known, for ex ample, asbestos and fiberglass. However, it is the elements boron, car bon, and silicon that currently provide some of the toughest materials for our high-technology world. Carbon fiber is the most widely used-not just for tennis rackets and fishing rods but for aircraft parts as well. The Boeing 767 was the first commercial plane to make sig nificant use of carbon fiber; in fact, about I tonne is incorporated into the structure of cach aircraft. Aircraft constructed with newer technol ogy, such as the Airbus 320, contain a much higher proportion of car bon fibers Fibers of boron and silicon carbide, SiC, are becoming increasingly important in the search for tougher, less fatigue-prone materials. The boron fibers are prepared by reducing boron trichloride with hydrogen gas at about1200°C: 2 BC1(0)+3 H,C-2 B(0)+6 HCI(a) The gaseous boron can then be condensed onto carbon or tungsten microfibers. For example, boron is deposited onto tungsten fibers of 5 um until the diameters of the coated fibers are about 100 um. The typical inorganic fiber prices are several hundred dollars per kilogra so even though production of cach type is mostly in the hundreds of tonnes range, inorganic fiber production is already a billion-dolla

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中国科技大学：《无机化学》第十五章硼族元素 The Boron Family Elements

中国科技大学：《无机化学》第十五章 硼族元素 The Boron Family Elements

中国科技大学：《无机化学》第十五章硼族元素 The Boron Family Elements