Co valency

The elements of Period 2 (Li—F) cannot have a co valency greater than 4, because not more than four orbitals are available for bonding. In Period 3 (Na—Cl) similar behaviour would be expected, and indeed the molecule SiH4 is tetrahedral like that of CH4, and PH3 is like NH3 with a lone pair occupying one tetrahedral position. 

In summary, we may say that the NBO/NRT description of partial proton transfer in the equilibrium H-bonded complex(es) is fully consistent with the observed behavior along the entire proton-transfer coordinate, including the transition state. At the transition state the importance of partial co valency and bond shifts can hardly be doubted. Yet the isomeric H-bonded complexes may approach the TS limit quite closely (within 0.2 kcal mol-1 in the present example) or even merge to form a single barrierless reaction profile (as in FHF- or H502+). Hence, the adiabatic continuity that connects isomeric H-bond complexes to the proton-transfer transition state suggests once more the essential futility of attempting to describe such deeply chemical events in terms of classical electrostatics. 

Co valency and polarisation effects compHcate calculations to be made on these problems. The polarisation effects that influence the crystal structures may be estimated more readily, as they depend on size and charge of ions and are easy detectable. Thus the increase in Me—F-distances, which always occurs if MeFe-octahedra are linked together, is a consequence of polarisation and contrapolarisation that cancel out in the shared fluoride ions. An instructive example of two sorts of Me—F-distances which may be explained by polarisation effects is that of the pentafluorides (page 27). The differences are not so extreme in other cases however. 

Co-valency, in which each of two adjacent atoms shares an electron with the other. The simplest examples of this are afforded by molecules such as Ha, 02, etc. 

From the second formula it is clear that three tautomeric structures are possible, according as both hydrogen atoms are ionisable (as in the formula), or one or both atoms are absorbed into the complex, being attached by co-valencies to either a sulphur or an oxygen atom. 

T - mean enthalpy of disruption to ground state products Af/co - valence reorganization energy of CO ... 

Our suggested assignment of the v(CO) valence vibrations is based not only on their positions but more so on the frequency separation of the bands within the isoelectronic series Fe°(CO)5 (52), [Mn-,(CO)5] (52), and [Cr-,I(CO)5]2-. Thus, this series may be compared to the similarly isoelectronic series of tetrahedral complexes Ni°(CO)4, [Co lCOjJ-, and [Fe-II(CO)4]2-. Ellis et al. (29) were later able to confirm that Na2(Cr(CO)5] prepared by us and by a different route by Kaska (27) is the same complex. This is also, of course, true for this salt which we prepared in 1959 by the reduction of Na2[Cr2(CO)10] with sodium in liquid NH3 (17). 

Saturated. Chemically combined to the limit of the most commonly exhibited valence containing no double or triple bonds (organic) incapable of further chemical combination (classical) broadly, combined with other atoms or molecules to the limit expressed by the theory of maximum co valency. 

Figure 13 Sketch of boundary surfaces for CO valence orbitals...

The Study of Co valency by Magnetic Neutron Scattering Bruce C. Tofield... 

The relative energies of CO valence orbitals and Ni atomic orbitals are sketched in Fig. 4.4b. The Ni atom has eight occupied 3d and two occupied 4s orbitals. 

Table 6,17 Half-height widths of CO-valence absorption of different carbonyl compounds (from ref [53])...

It is interesting that in his original report (1902) Meisenheimer described the preparation from 9-nitroanthracene and methoxide of an adduct which he formulated as 45. Confirmation of this structure has come from NMR measurements which show that the resonance of the proton at CIO shifts from — 8-93 to — 4-93 p.p.m. on complex formation consistent with a co valency change at this ring position (Foster et al., 1967). Again the NMR spectra (Foster et al., 1967 Fendler et al., 1968)... 

The joining of atoms into molecules in this particular manner is known by several names shared electrons electron pair bonds co-valences. A great many molecules—those of sugar, for example—are formed in this way. 

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