Isoelectronic centres

In semiconductors containing isoelectronic centres with an attracting potential for electrons or holes mentioned in Sect. 1.3.2, free excitons can be trapped because of the preferential interaction of these centres with the electron (or hole) part of the exciton. The hole (resp. electron) part of the exciton is then comparable to a hole (resp. electron) bound to a neg-... 

It was mentioned in Sect. 1.3.2 that in semiconductors, isoelectronic impurity centres could present a relatively strong attracting potential for electrons or holes. Excitons can be trapped by or created at these isoelectronic centres to form an isoelectronic bound exciton (IBE). The electron (hole) of this exci-ton is also more strongly bound to the isoelectronic centre than in classical excitons and the second constituent of the exciton, hole (electron) can be considered to be bound to a compound negative or positive ion. These structures are similar to those of neutral donors or acceptors and they are called isoelectronic donors or acceptors [104]. When formed by near band-gap or above band-gap laser illumination, the long lifetimes of these IBEs result in sharp PL lines, and this has for some time aroused interest for these centres as potential near IR radiation emitters. 

The values of the hole binding energies of the (S,Cu) isoelectronic centre are 137 and 292meV for Sa and Sb, respectively. The ID ionization energies associated with this centre (65.28 and 66.21 meV) are significantly larger than those of the pseudo-donor (C,0) complexes associated with lines C and P, and this has been attributed to the (S,Cu) centre for a central-cell potential which is also attractive for electrons, but to a lesser extent than for holes [18]. 

The BH4 ion is essentially non-coordinating in its alkali metal salts. However, despite the fact that it is isoelectronic with methane, BH4 has been found to act as a versatile ligand, forming many coordination compounds by means of 3-centre B-H M bonds to somewhat less electropositive metals. " Indeed, BH4 ... 

Typical examples are carbon interstitial carbonyl clusters such as the octahedral Co6C(CO) -2, the trigonal prismatic Co6C(CO)i52- and its isoelectronic (mononegative) nitrogen interstitial Co6N(CO)15 or the icosahedral clusterNi12Ge(CO)222 with the interstitial atom (Ge or Sn) in the centre of aNi icosahedron. 

Open-chain systems containing conjugated N and C basic centres are found in enamines [150] (sometimes called vinylogous amines) and the isoelectronic hydrazones [151]. 

The boron-centred biradicaloid (R2PBR )2 (5.17, R = Pr, R = Bu) is an isoelectronic analogue of the (RPCR)2 biradicaloids, 5.14 and 5.15. Both of these four-membered rings contain 22 valence electrons for R=H. The P2B2 system 5.17 is synthesised by the reaction of a 1,2-dichlorodiborane with two equivalents of LiP Pr2. The P2B2 ring is presumably formed by rearrangement of the initially formed acyclic P-B-B-P skeleton (Scheme 5.2). 

S03 An electron centre of pyramidal AB3-type molecule having 25-electron molecules isoelectronic with C033 in CaS04. Quartet hyperfine splitting of 33S (I = 3/2,0.76%) is observed. 

Pyridine is isoelectronic with benzene in which C — H is replaced by N. The two electrons which formed the w-bonds in — CH are now localized in a nonbonding hybrid orbital centred on the nitrogen atom. The substitution of N lowers the overall symmetry of the molecule from D to Cs, and the degenerate eu and e, orbitals are split into ax and ht type orbitals. The MOs of pyridine are given in Figure 2.20. 

All of these metal complexes are isoelectronic and isostructural with each other. Upon coordination of the carbon monoxide to the nickel(O) centre, the metal ion acquires negative charge. The carbon monoxide ligand possesses empty 7r -orbitals, and back-donation from the metal to the ligand reduces the electron density on the metal. 

Notice that the nitronium ion (NO2) is linear with an sp hybridized nitrogen at the centre. It is isoelectronic with CO2. It is also very reactive and combines with benzene in the way we have just described. Benzene attacks the positively charged nitrogen atom but one of the N=0 bonds must be broken at the same time to avoid five-valent nitrogen. 

---