Vacant d-orbitals

It provides electrostatic stabilization of the carbanion formed upon removal of the C-2 proton. (The sf hybridization and the availability of vacant d orbitals on the adjacent sulfur probably also facilitate proton removal at C-2.)... 

Whereas d8 Ni selects the rectangular di-allylic hyperbonding pattern in (4.121), d6 Fe of ferrocene offers an additional vacant d orbital and hence opens up new geometrical possibilities of an additional cu bond. In concert with the three cu bonds and nominal sd2 (90°) hybridization (Table 4.52), the two Cp ligands are naturally expected to coordinate in r 5 (L2X) fashion to occupy the six octahedrally arrayed coordination sites of the metal. Visualization of this coordination mode is aided by considering the possible patterns of L-type (filled circles 7tcc) and X-type (half-filled circles radical) sites of L2X Cp... 

The active catalyst formed then has an octahedral structure, with the four Cl attached to the latice and the ethyl group attached to the Ti through a s bond, and leaving a vacant orbital at the position where originally the fifth chlorine was attached. Once the active catalyst is formed, the monomer is attracted towards the vacant d-orbital which then forms a transition p complex with the Ti, as shown below ... 

The difference in the properties of the silicon atom from that of the carbon is because of its comparatively bigger size, less electronegative nature and the presence of the available vacant d orbitals. 

In addition, it should be useful to mention that silicon as a spiro atom is able to conjugate its mutually orthogonal aromatic constituents, and thus, secures a release of an unpaired electron in a cation-radical. Scheme 3.61 shows such a phenomenon, revealed by Hirao et al. (2007). Obviously, vacant d orbitals of silicon provide the cation-radical with the possibility of delocalizing spin density. 

Cavell and Dobbie (214-216) have suggested that halogen transfer rearrangements in trifluoromethylphosphines arise from interactions of nonbonding fluorine p orbitals with vacant d orbitals on phosphorus. Such an explanation is consistent with observations for the Groups IV and V pentafluorophenyl derivatives, exclusive of carbon and nitrogen, and similarly fits the behavior of boron with its vacant p orbital. 

The propylene forms a pi-complex with the vacant d-orbital of titanium as shown in the following structure. 

The oxidative decoupling reaction of the tetramethylene ligand should be allowed whenever the metal carries a vacant low-lying symmetric orbital. Thus the reaction is shown at low temperature by (t) -CsH5)2 Ti(CH2)4 but XVII, in which the only completely vacant d-orbital on each metal is antisymmetric towards each(CH2)4... 

Exactly why this is, we are not certain. One can simply say that 0H is a hard base, whereas (Pt(II) is a class A metal or soft acid, and this hard-soft combination is unstable. It is also possible to suggest that the repulsive interaction between the filled d-orbitals on Pt(II) and the filled -orbitals on 0H make it a poor reagent. The same is true of F which is also a poor reagent. Other halide ions have low energy, vacant d-orbitals which can accept electrons and decrease the effect of the filled -orbitals. This makes these halide ions better reagents than F . 

Borane (BH3), boron trichloride (BCI3) and boron trifluoride (BF3) are known as Lewis acids, because boron has a vacant d orbital that accepts a pair of electrons from a donor species. For example, diethyl ether acts as a Lewis base towards BCI3 and forms a complex of boron trichloride. 

The compounds 43, 45, 48b and 48c gave the corresponding disiloxanes in quantitative yields, and BTSP is converted into hexamethyldisiloxane, 44, formed from difluorote-tramethyldisiloxane. The reaction of 44 with BTSP was not inhibited by 2,4,6-tri(/-butyl)phenol. Compound 48a did not react with di-r-butyl peroxide, which is the carbon analog of BSTP, suggesting an important role of vacant d-orbitals of the silicon atom. The Si—Si oxidation of compound 49 with BSTP proceeds quantitatively and in a stereospecific fashion (equation 72)63. 

In view of the large difference in the reactivity of Cr+2 and Mn+2 aquo complexes it seems that the reactivity of the metal complex toward eaq is not related to the energy required to remove a second 4s electron. The similar reactivity of aquo and amino complexes makes the correlation between reactivity and redox potentials rather unlikely. As an alternative mechanism, we suggest that the availability of a vacant d orbital on the central atom and the energy gain on adding an electron are the major factors which determine the reactivity of transition metal ions. 

Exceptions Sometimes atoms break the octet rule. Molecules with such atoms include molecules with an odd number of electrons, molecules with an atom having less than an octet, and molecules with an atom having more than an octet. Compounds containing boron and beryllium may contain less than an octet. Molecules with an atom containing more than an octet must contain an atom from the third period or greater in the periodic table because only these a tours have vacant d orbitals available for hybridization. 

The foregoing facts suggest that the unique spectral properties of polysilanes are due to the silicon-silicon bond acting as a chromophore, probably through the use of vacant d orbitals of the silicon atom. In addition, it seems likely that there exists an enhanced conjugation through overlap of d and it orbitals between substituents with 7r-electron systems, such as phenyl and vinyl, and polysilane chains (53, 81,153). 

The question of (p — d)-ir bonding and ring planarity also arises in compounds having an anthracene-like carbon skeleton substituted at either or both of the 9 and 10 positions by atoms bearing vacant d-orbitals (4). Heteroatom substitution in 9,10-bis(dimethylgermyl)-... 

Dative covalent bonds, or coordinate covalent bonds, are those in which electrons are shared (as in all covalent bonds), but in which both electrons involved in each bond are contributed from the same atom. Such bonds occur in organometallic compounds of transition metals having vacant d orbitals. It is beyond the scope of this book to discuss such bonding in detail the reader needing additional information should refer to works on organometallic compounds.12 The most common organometallic compounds that have dative covalent bonds are carbonyl compounds, which are formed from a transition metal and carbon monoxide, where the metal is usually in the -1, 0, or +1 oxidation state. In these compounds the carbon atom on the carbon monoxide acts as an electron-pair donor ... 

It is assumed that this behaviour can be explained by an additional interaction between the undivided p-electrons of the oxygen atom and the vacant 3d-orbitals of the silicon atom (the so called (p -dj-interaction).96 The ideas about the nature of the siloxane bond were developed by Voronkov.97 He stated that (pT-dT)-bonds of a donor-acceptor type are formed in a Si-O-Si system owing to the undivided electron pairs of oxygen and the vacant d-orbitals of the silicon atoms. In this case, the oxygen atom is in a state of hybridization, intermediate between sp2 and sp. 

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