Covalent character

Simple metals like alkalis, or ones with only s and p valence electrons, can often be described by a free electron gas model, whereas transition metals and rare earth metals which have d and f valence electrons camiot. Transition metal and rare earth metals do not have energy band structures which resemble free electron models. The fonned bonds from d and f states often have some strong covalent character. This character strongly modulates the free-electron-like bands. 

B-0 and B-C bonds are shorter and stronger than the corresponding Li-O abd Li-C bonds (more covalent character)- therefore tighter more organized transition state. 

Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

The covalent character of mercury compounds and the corresponding abiUty to complex with various organic compounds explains the unusually wide solubihty characteristics. Mercury compounds are soluble in alcohols, ethyl ether, benzene, and other organic solvents. Moreover, small amounts of chemicals such as amines, ammonia (qv), and ammonium acetate can have a profound solubilizing effect (see COORDINATION COMPOUNDS). The solubihty of mercury and a wide variety of mercury salts and complexes in water and aqueous electrolyte solutions has been well outlined (5). 

The tertiary metal phosphates are of the general formula MPO where M is B, Al, Ga, Fe, Mn, etc. The metal—oxygen bonds of these materials have considerable covalent character. The anhydrous salts are continuous three-dimensional networks analogous to the various polymorphic forms of siHca. Of limited commercial interest are the alurninum, boron, and iron phosphates. Boron phosphate [13308-51 -5] BPO, is produced by heating the reaction product of boric acid and phosphoric acid or by a dding H BO to H PO at room temperature, foUowed by crystallization from a solution containing >48% P205- Boron phosphate has limited use as a catalyst support, in ceramics, and in refractories. 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

Silver bromide crystals, formed from stoichiometric amounts of silver nitrate and potassium bromide, are characterized by a cubic stmcture having interionic distances of 0.29 nm. If, however, an excess of either ion is present, octahedral crystals tend to form. The yellow color of silver bromide has been attributed to ionic deformation, an indication of its partially covalent character. Silver bromide melts at 434°C and dissociates when heated above 500°C. 

From the analysis of the DOS combined with the interpretation of the spectral details in FeAl, CoAl and NiAl (Botton et al., 1996a) the features at the edge threshold (71-73 eV) have been shown to arise from d character being introduced at the Al sites by the strong interaction with the TM d bands and thus by the presence of "covalent" character. This interaction causes the... 

One way to distinguish between ionic and covalent character of the bonding in a compound is to examine the decomposed DOS. If the bonding is strongly covalent, the states from different atoms are strongly mixed and one would expect the decomposed DOS to be very similar on all sites. If the bonding is primarily ionic the decomposed DOS would be very dissimilar on sites with different atoms. To deduce the amount of hybridization in the DOS we decomposed the DOS into site contributions. 

A discrepancy which is certainly related to the covalent character of bonding is the sign of the Cauchy pressures Ci2-C66 and C]3-C44. The experimental values of these pressures are -6.4 and -34.2 MPa, respectively, while the potentials give 31 and 30 MPa, respectively. It is a general feature of the Flnnls-Sinclair type potentials and the EAM that negative Cauchy pressures cannot be reproduced. Such Cauchy pressures... 

The complex cyanides of transition metals, especially the iron group, are very stable in aqueous solution. Their high co-ordination numbers mean the metal core of the complex is effectively shielded, and the metal-cyanide bonds, which share electrons with unfilled inner orbitals of the metal, may have a much more covalent character. Single electron transfer to the ferri-cyanide ion as a whole is easy (reducing it to ferrocyanide, with no alteration of co-ordination), but further reduction does not occur. 

From the ionization energies, predict which solid substance involves bonds having the most ionic character BeCl2, MgCl2, CaCl2, Srd2, BaQ2. Which substance is expected to have most covalent character in its bonds ... 

Cations with completely filled d sub-shells. Typical of this group are copper(I), silver(I) and gold(I) which exhibit Class B acceptor properties. These ions have high polarising power and the bonds formed in their complexes have appreciable covalent character. 

The Au-S bond length at 2.30 A is very similar to that in the gold(III) analogue (2.299-2.312 A) and other gold(III) complexes like Au(toluene-3,4-dithiolate) (2.31 A) suggesting substantial covalent character in the bond. 

All ionic bonds have some covalent character. To see how covalent character can arise, consider a monatomic anion (such as Cl-) next to a cation (such as Na+). As the cation s positive charge pulls on the anion s electrons, the spherical electron... 

Self-Test 2.13A In which of the compounds NaBr and MgBr2 do the bonds have greater covalent character ... 

J 6 Predict which of two bonds has greater ionic or covalent character (Self-Tests 2.12 and 2.13). 

Ions stack together in the regular crystalline structure corresponding to lowest energy. The structure adopted depends on the radius ratio of cation and anion. Covalent character in an ionic bond itnposes a directional character on the bonding. 

---