PN Bond

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

However, the important new feature of metal alkylidenes (4.51) is metal-carbon pi-bonding. As discussed in Section 2.8, pi bonds between transition metals and main-group elements are of d -p type, much stronger than corresponding p —pn bonds between heavier main-group elements. Compared with simple metal hydrides and alkyls, metal-carbon pi-bonding in metal alkylidenes affects the selection of metal d orbitals available for hybridization and skeletal bond formation, somewhat altering molecular shapes. 

The bonding is intermediate in type between purely covalent and ionic 164). There is some d-n—pn bonding between silicon and oxygen. 

The two P atoms could form two p bonds to produce a S+ ground state. If the valence excited state is important as argued above, these two states could also couple to S+ interacting strongly with the two pn bonds. 

Table 3 shows measurements of this effect, taken from a paper by Dixon and Bloembergen (4). The experimental results are compared with calculated values of the effect of the electric field on the quadrupole resonance frequency in which it was assumed that no jr-bonding is present. These calculations are rather crude but predict the correct order of magnitude for the Carbon-Chlorine bond. For the SiCl bond, however, the experimental Stark splittings are both much less than those for the C—Cl bond and almost an order of magnitude less than the calculated value and once more a ready rationalisation of this phenomenon lies in An—pn bonding. 

These descriptions preserve octet configurations about the central atoms. However, alternative descriptions are possible if we promote electrons to nd orbitals, and use these to form dn-pn bonds ... 

In the following tables, the valence state of the central atom is described in terms of orbital occupancy. Thus, for example, h h I p1 denotes a valence state in which two hybrid orbitals (of specified type) are singly occupied (and are used in bond formation) while a third accommodates a lone pair. The singly-occupied pure np orbital forms a p -p bond. Empty hybrid orbitals (denoted h°) always function as acceptor orbitals in the formation of coordinate (or dative) bonds. Doubly-occupied hybrid orbitals (h2) may be lone pairs, or may function as donor orbitals in coordinate bonds, in which case the h is underlined. A doubly-occupied np orbital always forms a dative n bond, while a singly-occupied np orbital always forms an ordinary ji bond. Singly-occupied nd orbitals form d -pn bonds. For the reasons discussed in Section 6.1, you will find few stable species where np orbitals are left empty. 

As another inorganic analogue of benzene reference may be made to the constitution of the polymeric phosphorus nitrile chlorides, in which the PN bonds in the ring also appear to be... 

Use of d Orbitals. The S—Po elements employ d orbitals together with their s and p orbitals to form more than four form multiple bonds. Thus, for example, in the sulfate ion, where the s and p orbitals are used in o bonding, the shortness of the S—O bonds suggests that there must be considerable multiple-bond character. The usual explanation for this is that empty dir orbitals of sulfur accept electrons from filled pn orbitals of oxygen. Similar dir—pn bonding occurs in some phosphorus compounds, but it seems to be more prominent with sulfur. 

Many of the known complexes have OR or NR2 groups attached to carbon, and electron flow from lone pairs on O or N leads to a strong contribution in which there is O—CorN-C ptr —> pn bonding. However, several complexes without such groups are known. 

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