Orbital contraction

Since n bonding is believed to be more important in low oxidation states, as d orbitals contract with increasing oxidation state leading to poorer dw-pw overlap, this would not be expected on the basis of a 7r-bonding mechanism. Similarly, one can compare /(Pt-P) for pairs of isomers in the +2 and +4 states in a planar platinum(II) complex, the platinum 6s orbital is shared by four ligands whereas in an octahedral platinum(IV) complex it is shared by six ligands. Therefore, the 6s character is expected to be only 2/3 as much in the platinum(IV) complexes, correlating well with the 7(Pt-P) values, which can be taken to be a measure of the a-character in the bond. 

The obvious conclusion to be reached is that there are three kinds of spd orbitals hybrid bond orbitals, contracted d orbitals, and about 0.70 other orbitals. In 1938 I considered this 0.70 unstable orbital per atom to be unsuited for either bond formation or... 

This would not be expected simply on the basis of a crystal-field model, for the d orbitals will contract with increasing positive charge and hence interact less well with the ligand point charges . The modest decreases in bond length as one traverses the series (Eq. 6.9) are unlikely to compensate for, let alone override, the effects of such orbital contraction. Finally, to add to the confusion, we also note from Eq. (6.7) that zio -t values increase as we go down the periodic table (Eq. 6.10). 

Baerends, E.J., Schwarz, W.H.E., Schwerdtfeger, P. and Snijders, J.G. (1990) Relativistic atomic orbital contractions and expansions magnitudes and explanations. Journal of Physics B-Atomic Molecular and Optical Physics, 23, 3225-3240. 

Snijders, J.G. and Pyykko, P. (1980) Is the relativistic contraction of bond lengths an orbital contraction effect Chemical Physics Letters, 75, 5-8. 

The molecules CH20 and BH3CO show a wider variety of electronic sub-structures than the simple hydrids discussed above in particular CH20 contains a (polar) ir-bond and sp2 hybrids and BH3CO has 7r bonds, sp hybrids and a dative bond. It is of some interest therefore to see how the GHOs behave in these situations. Table 2 contains the relevant orbital exponents. The pattern of orbital contraction for GHOs involved in a X—H bonds is repeated and reinforced by the C—0 a bond orbitals. 

Bulky //-substituents force carbons to resonate at lower field due to orbital contraction, which causes an increase in the paramagnetic shielding term. 

Atomic parameters involving 5d electrons The trends for atomic parameters representing effects involving the 5d electron are markedly different from the trends for 4f 1 core parameters, as can be seen from table 1. Whereas the / (ff) and f (ff) parameters show a dramatic increase across the lanthanide series from Pr3"1" to Lu3+, due to the contraction of the 4f orbitals for the heavy lanthanides, the ( (dd) parameter increases much more slowly. This is due to the fact that the 5d orbitals contract much less across the lanthanide series than do the 4f orbitals. Also, the Fk(f ) and CF (fd) parameters decrease gradually, due to the reduced overlap between the 4f and 5d orbitals as the 4f orbitals contract. These calculations are useful for estimating the trends in the parameters across the lanthanide series, once they have been determined for a few ions. 

Crystal-field parameters would be expected to change across the series roughly in proportion to the radial integrals r2 and r4 (and r6 for 4f electrons). These integrals decrease dramatically across the lanthanide series for the 4f electrons (because their orbitals contract dramatically) but only by a few percent for the 5d electron. Thus, crystal-field parameters determined for Ce3+ may be used across the lanthanide series, with only a small scaling factor for the heavy ions. 

Calculated orbital contraction ratio R / NR as a function of atomic number Z. 

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