Hydride acidity trends

The most important trend to be noted in the covalent hydrides is the change in acid-base behaviour as we cross a period from Group IV to Group VII. In Period 1, we have... 

Systematic studies of the thermodynamic and kinetic acidity of metal hydrides in acetonitrile were carried out by Norton et al. [10, 11]. A review of the acidity of metal hydrides presents extensive tabulations of pKa data [12] only a few of the trends will be mentioned here. Metal hydrides span a wide range of pKa values considering only metal carbonyl hydrides shown in Table 7.1, the range exceeds 20 pfCa units. As expected, a substantial decrease in acidity is... 

Molecules in polar liquids such as water, liquid ammonia, sulfuric acid, and chloroform are held together by dipole-dipole and hydrogen bonding interactions. For molecules of comparable size, these are stronger than London forces resulting in the familiar trends in boiling points of nonmetal hydrides. For the heavier molecules, such as H2S, H,Se, PH3, and HI, dipole effects are not particularly important (the elec-... 

FIGURE 6-16 Trends in Acidity and Electronegativity of Binary Hydrides. 

Binary compounds (compounds with only two elements) containing hydrogen are called hydrides. Hydrides can be basic, acidic, or neutral. On the periodic table, the basic hydrides are to the left, and the acidic hydrides are to the right. For instance, NaH is basic H,S is acidic. Following this trend, metal hydrides are either basic or neutral, while nonmetal hydrides are acidic or neutral. (Ammonia, NH3, is an exception to this rule.) The acidity of nonmetal hydrides tends to increase going down the periodic table. H20 < H2S < H,Se < H2Te... 

Statement (c) is only partially correct— while Lewis acidity of BX3 does increase from F to Br, it is not due to stronger B-Br n bonding (as stated) but exactly due to the opposite trend the tc bonding is decreasing in the same order Lewis acidity is increasing, that is, n bonding in B-Br is weaker than in B-F (the latter explains the former). Statement (d) is incorrect—arachno boron hydrides are less stable than nic/o-borohydrides, also, their skeletal electron count is n+3 not 2(n-t-3). 

Bronsted acidity arises from the possibility of transferring a proton to a base, which may sometimes be the same compound (see Topic E2 for discussion of trends). Basicity is possible when nonbonding electron pairs are present (see Topics Cl and C9). Basicity towards protons decreases towards the right and down each group in the periodic table, so that ammonia is the strongest base among simple hydrides. 

The MW distribution of the polymer obtained from Cr/titania is shown in Figure 113, and compared with that made with Cr/silica and Cr/silica-titania. The polymer obtained with Cr/titania has extremely high MW, and this trend is opposite to that observed with Cr/silica-titania. Again, this finding is an evidence that it is the acidity of the mixed oxide that is important. As noted earlier, this acidity probably pulls electron density away from the chromium, possibly making it more likely to coordinate agostic (3-hydride, a necessary prelude to chain termination. 

These trends agree with the frontier orbital analysis. In particular, the delivery of hydride from carbon breaks a relatively unpolarised bond, making the hydride notably soft, as we saw earlier in its capacity to attack pyridinium salts preferentially at the 4-position. The metal hydrogen bond will be more polarised, and metal hydrides should therefore be harder. Similarly, the delivery of hydride from boron will make it softer than when it is delivered from the more electropositive metal, aluminium. It also seems that, among a,(3-unsaturated carbonyl compounds, the susceptibility to conjugate reduction increases in the sequence ketones < esters < acids < amides but there are too few examples to be sure. 

Various trends have long been noted in the acid strengths of many binary hydrides and oxoacids. Values for some simple hydrides are given in Table 3.4 from which it is clear that acid strength increases with atomic number both in any one horizontal period and in any... 

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