Arsenic bond type

Fig. 1.6 The breaking of three-bonds about each simple cubic site that leads to the black phosphorus and arsenic structure types. After Burdett and Lee (1985).

Thus, we expect the puckered graphitic sheet with 90° bond angles to have the smallest normalized fourth moment and shape parameter, s, and hence to be the most stable structure for the half-full p band as is indeed observed in the middle panel of Fig. 8.5. We should also note that if the n bonding is neglected then this three-atom contribution is identically zero for = 90°, so that 5 = 0 and we have the total bimodal behaviour of the p eigenspectrum that is observed in the lower panel of Fig. 8.1 for the arsenic structure type. 

The structure of this methylarsenic polymer (Figure 7) is very important with respect to colour phenomena and intermolecular interactions. The structure contains two types of As-As distances. The shorter distances correspond to normal covalent arsenic-arsenic bonds and the longer distances result from semibonding interactions. 

This is sometimes described as a trend from covalent, molecular Asia through intermediate SW3 to ionic Bils, but this exaggerates the difference in bond-type. Arsenic, Sb and Bi have very similar electronegativities (p. 550) and it seems likely that the structural trend reflects more the way in which the octahedral interstices in the hep iodine lattice are filled by atoms of gradually increasing size. The size of these interstices is about constant (see mean M-X distance) but only Bi is sufficiently large to fill them symmetrically. 

Atoms of elements that are characterized by a valence greater than four, eg, phosphoms or arsenic (valence = 5), are one type of dopant. These high valence dopants contribute free electrons to the crystal and are cabed donor dopants. If one donor atom is incorporated in the lattice, four of the five valence electrons of donor dopants are covalentiy bonded, but the fifth electron is very weakly bound and can be detached by only ca 0.03 eV of energy. Once it is detached, it is available as a free electron, ie, a carrier of electric current. A sibcon crystal with added donor dopants has excess electron carriers and is cabed n-ty e (negative) sibcon (Fig. Ic). 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

It was found that the platinum(II) complexes of these ligands were of two types. All isomers form compounds of the t)rpe PtBr2 (arsine) 2, some of which are illustrated in Fig. 5. In these compounds, coordination to the platinum is effected through the arsenic atom alone. In addition, the ortho isomers of both arsines form 1 1 complexes with platinum(II) bromide in which the ligand is bidentate. Both the arsenic atom and the double bond are coordinated to the platinum as shown in Fig. 6. 

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