F orbital involvement in bonding

Answer 4.16 Transition metal carbonyls have transition metals in low oxidation states where d orbitals can overlap well, f orbital involvement in bonding in lanthanide compounds is minimal and the high oxidation state will contract the orbitals even further. 

Trend (2) is more complex, but two factors contribute to inversion of the stability of Aj-X and A2-X. One factor is repulsion between nonbonding electrons. The F-F bond is much weaker than the Cl-Cl bond for this reason. The non-bonding pairs for fluorine lie much closer to the nucleus and thus closer to the non-bonding pairs on the other nucleus, causing more repulsion and weakening the F-F bond relative to the Cl-Cl bond. The second factor is the possibility of d-orbital involvement in TT-bonding for second-row elements. The second-row elements have vacant low-lying d-orbitals which can form a drc-pjc interaction with... 

The original (and most widely-used) versions of MNDO, AMI, and PM3 do not use d orbitals. Hence they might be expected to show reduced accuracy for elements in the second-row (computational chemists lingo) and beyond like, P, S, Cl, Br, and I, and cannot be used for transition metals. Actually, because of appropriate parameterization AMI and PM3 are able to treat monovalent Cl, Br and I as standard elements (C, H, O, N, F), and they handle divalent S reasonably well. To make them able to work better with elements in the second row and beyond, and/or to handle transition metals (note that in Zn, Cd, and Hg the d electrons are not normally involved in bonding), d orbitals have been incorporated into some SE methods. MNDO/d [53] uses d orbitals for some post-first row nonmetals and has been parameterized for several transition metals. Some versions of SPARTAN [54] have PM3 (tm), PM3 with d orbitals for many transition metals. PM3 (tm) geometries have been compared with experimental and ab initio ones ... 

The electrons in the outer shell, or valence shell, of an atom are the electrons involved in bonding. In most of our discussion of covalent bonding, we will focus attention on these electrons. Valence shell electrons are those that were not present in the preceding noble gas, ignoring filled sets of d and f orbitals. Lewis formulas show the number of valence shell electrons in a polyatomic molecule or ion (Sections 7-4 through 7-7). We will write Lewis formulas for each molecule or polyatomic ion we discuss. The theories introduced in this chapter apply equally well to polyatomic molecules and to ions. 

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