52
votes
Why do compounds like SF6 and SF4 exist but SH6 and SH4 don't?
TL;DR Fluorine is electronegative and can support the extra negative charge that is dispersed on the six X atoms in $\ce{SX6}$, whereas hydrogen cannot.
First, let's debunk a commonly taught myth, ...
- 71.1k
42
votes
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Why is [PdCl4]2- square planar whereas [NiCl4]2- is tetrahedral?
The geometry of the complex changes going from $\ce{[NiCl4]^2-}$ to $\ce{[PdCl4]^2-}$. Clearly this cannot be due to any change in the ligand since it is the same in both cases. It is the other factor,...
- 71.1k
32
votes
When are two orbitals orthogonal?
Unfortunately, the sense in which orbitals are orthogonal is more or less impossible to define rigorously without recourse to functions of some kind. So, I'll give an explanation a shot using some ...
- 17.3k
27
votes
Why do n AOs only form n MOs?
If you have $n$ functions (e.g. AOs) you can make a maximum of $n$ new linearly independent functions (e.g. MOs). If you try to make $n+1$ MOs, then any one of them can be expressed as a linear ...
- 4,799
24
votes
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Canonical MOs vs. Localized MOs: Do both represent reality in the same way?
NOTE: In the below, I'm implicitly discussing a ground-state, closed-shell wavefunction, where all occupied orbitals are doubly occupied. The discussion would be similar for open-shell wavefunctions, ...
- 17.3k
24
votes
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Cyclobutadiene - Jahn–Teller effect or not?
Very interesting question, and it kept me up despite daylight saving time cheating me of one hour of sleep last night... A good reference is Albright, Burdett and Whangbo, Orbital Interactions in ...
- 71.1k
22
votes
Accepted
How do atomic orbitals interfere both constructively AND destructively to give two molecular orbitals?
What does MO formation entail?
A very common misconception is that the formation of MOs involve addition, or subtraction, of two physical objects. And the confusion arises because, how can the same ...
- 71.1k
21
votes
Destabilization of antibonding MO vs stabilization of bonding MO
TL;DR
"The destabilisation of an antibonding MO is equal to the stabilisation of a bonding MO, relative to the constituent AOs."
Several key assumptions are inherent in this statement.
Born–...
- 71.1k
19
votes
Accepted
Why is the bond order of CO+ 3.5?
For a long time it was taught in school and universities that the HOMO of carbon monoxide is anti-bonding. Without more context it was also often taught that the bond order in CO is three, since there ...
- 43.8k
19
votes
Accepted
How can the gauche-effect be explained?
The gauche effect is commonly explained with LCAO-based bond orbitals. LCAO is short for linear combination of atomic orbitals and implies that we can take two atomic orbitals $\phi_1, \phi_2$ and ...
- 67.3k
19
votes
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How can the intense color of potassium permanganate be explained with molecular orbital theory?
Let’s take a look at a qualitative MO scheme for a tetrahedric transition metal complex whose ligands have three p-type orbitals each. On the left of figure 1 you have the metal orbitals ($\mathrm{3d}$...
- 67.3k
19
votes
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When are two orbitals orthogonal?
One atomic (or molecular) orbital is said to be orthogonal to another atomic (or molecular) orbital if there is no interaction between the electrons in one orbital with the electrons (wavefunction) in ...
- 84.2k
18
votes
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How can the Bonding in IF7 be explained using LCAO method?
No 6-centre-10-electron bond.
The bonding situation in $\ce{IF7}$ is explained by Christe et. al. (see ssavec's answer) as a 6-centre-10-electron bond. In the article itself I found no real ...
- 43.8k
18
votes
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Why/how is blood red? (colours of hemoglobin)
The $\ce{Fe}$-porphyrin complex is a vastly conjugated $\pi$-system, in which the oxidation state of the $\ce{Fe}$ plays a central role. The wavelength of light absorbed by hemoglobin exactly ...
- 23.9k
18
votes
Accepted
Can the idea of entropy be extended to orbitals?
No.
The reason why a gas particle in a large volume has a large entropy is not because it has a lot of space to move around per se. A better explanation is that for a given energy, there are many ...
- 71.1k
17
votes
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Jahn-Teller Distortions in Square Planar Complexes?
Introduction
Thank you for prompting me to look for square planar complexes that are not $\mathrm{d^8}$; I learnt some valuable stuff while researching the answer to this question! Do take this ...
- 67.3k
17
votes
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Molecular orbital diagram for BF3
I think you are most of the way to the answer, but I will start the process from scratch for the sake of a full explanation for future readers.
First, we determine the symmetry group of $\ce{BF3}$, ...
- 11.6k
17
votes
Does a neutral dimercury molecule exist?
There has been recent research on mercury dimers in its ground state. Bond energies, transition energies, band spectrum and other spectroscopic parameters has been calculated. Here are the abstracts ...
- 25.2k
16
votes
Accepted
Why a higher s character increases a carbon atom's electronegativity?
Electronegativity is the power of an atom to attract bonding pairs of electrons to itself. It clearly depends on the nuclear charge: the larger it is, the more strongly the nucleus attracts electrons ...
- 18.9k
16
votes
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After a unitary transformation, is Koopmans' theorem still valid?
Disclaimer: I like Tyberius' answer, but I'd like to go a little further.
$%
\newcommand{\ll}{\left\langle}\newcommand{\rr}{\right\rangle}
\newcommand{\lb}{\left|}\newcommand{\rb}{\right|}
\newcommand{...
- 43.8k
15
votes
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Mechanism for the Cleavage of Diborane?
Is it known how this cleavage occurs?
The mechanism proposed for the cleavage reaction involves an initial attack by the donor on one boron atom in diborane, leading to cleavage of one $\ce{B-H-B}$ ...
- 4,114
15
votes
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Using group theory to make molecular orbital diagrams
Group theory really just formalises the process that you're going through when you construct molecular orbital diagrams by inspection (though as the molecules get bigger it gets significantly harder ...
- 17.1k
15
votes
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Explanation for bond lengths in trans-hexatriene
If you derive the π-type molecular orbitals of hexatriene, the three lower-energy MOs which are filled would look something like this (image from p 33 of Fleming's Molecular Orbitals and Organic ...
- 71.1k
15
votes
How do atomic orbitals interfere both constructively AND destructively to give two molecular orbitals?
Unfortunately, in quantum mechanics there is rarely an explanation "in simple terms." Quantum mechanics is a mathematical construct that so far seems to predict the results of all ...
- 10.1k
14
votes
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Why is the inversion barrier larger in PH3 than it is in NH3?
Ammonia is the classic system for $\ce{sp^3}$ hybridisation save methane. The lone pair (and each of the $\unicode[Times]{x3C3}$-bonds) has almost $25~\%$ s-character which corresponds nicely to $\ce{...
- 67.3k
13
votes
Why is the inversion barrier larger in PH3 than it is in NH3?
Disclaimer: This answer neglects quantum tunnelling effects, which are significant in such compounds. The gist of it is that because nitrogen is smaller and lighter than phosphorus, the rate of ...
- 71.1k
13
votes
Accepted
How can off-diagonal Hamiltonian matrix elements be nonzero when the overlap matrix is diagonal?
I believe the confusion lies in some ambiguity in the use of the term overlap.
For the $S_{AB}=0$ approximation of Hückel theory, overlap is used in the specific mathematical sense of the "overlap ...
- 17.3k
13
votes
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Pi molecular orbitals of polyenes
These pi-type MOs are most commonly derived using Hückel MO theory. I've never been a fan of Wikipedia's technical articles, so for further reading I'd suggest using the QMUL resources online, which ...
- 71.1k
13
votes
Why do n AOs only form n MOs?
You're correct in saying that p-orbitals are identical, and it follows that the following configurations are the same:
You must only consider their relative alignments (i.e. their symmetry).
To ...
- 1,261
13
votes
Accepted
Molecular orbital diagram and irreducible representations for dinitrogen
Because this is a diatomic molecule, there are no group orbitals. Put another way, the group orbitals are the molecular orbitals. Knowing the nitrogen atomic orbitals (AOs) and their irreducible ...
- 7,438
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