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54 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, ...
orthocresol's user avatar
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33 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 ...
hBy2Py's user avatar
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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 ...
Jan Jensen's user avatar
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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, ...
hBy2Py's user avatar
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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 ...
orthocresol's user avatar
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22 votes
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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 ...
orthocresol's user avatar
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19 votes
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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 ...
Martin - マーチン's user avatar
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 ...
ron's user avatar
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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 ...
ringo's user avatar
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18 votes
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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 ...
orthocresol's user avatar
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17 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{...
Martin - マーチン's user avatar
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 $\ce{BF3}$, ...
Tyberius's user avatar
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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 ...
Nilay Ghosh's user avatar
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16 votes
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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 ...
Wildcat's user avatar
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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 $\ce{B-H-B}$ ...
Berry Holmes's user avatar
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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 ...
NotEvans.'s user avatar
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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 ...
orthocresol's user avatar
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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 ...
Andrew's user avatar
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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 ...
orthocresol's user avatar
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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 ...
orthocresol's user avatar
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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 ...
obackhouse's user avatar
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13 votes
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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 ...
pentavalentcarbon's user avatar
13 votes
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Pauli exclusion principle

Wikipedia has the correct definition: A more rigorous statement is that with respect to exchange of two identical particles the total wave function is antisymmetric for fermions, and symmetric for ...
Karsten's user avatar
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13 votes

What theory accurately explains metallic bonding in beryllium?

The usual MOT explanation for why the group IIA elements are metals is that the band caused by the overlap of the np orbitals is sufficiently wide to overlap that due to the ns orbitals (the one you ...
Ian Bush's user avatar
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13 votes
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Why does color of substances not disappear after some time?

If you are seriously interested in understanding why things are colored, there is a book called The Physics and Chemistry of Color: The Fifteen Causes of Color, by Kurt Nassau. It should be available ...
AChem's user avatar
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12 votes

Structure of NO2 compound?

According to Electronic Structure of NO2 Studied by Photoelectron and Vacuum-uv Spectroscopy and Gaussian Orbital Calculations J. Chem. Phys. 53, 705 (1970) : The highest molecular orbital $4a_1$ is ...
DavePhD's user avatar
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12 votes
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Precise definition of atomic orbital

In school I was taught that an atomic orbital is the 3-dimensional region in which the electron is located with a probability of 90%. This is not correct. An atomic orbital is a solution to ...
Zhe's user avatar
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12 votes
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What is the dipole moment direction in the nitrosonium ion?

In non-neutral species the dipole moment in calculations is dependent on the choice of the co-ordinate system. For this reason the origin is chosen to be the centre of mass. With $r=107.8~\pu{pm}$ on ...
Martin - マーチン's user avatar
12 votes
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"NBO diagrams" versus MO diagrams

A molecular orbital diagram is a schematic representation of how we interpret bonding in certain species. It is as much an accurate representation for a specific bonding situation as a Lewis structure ...
Martin - マーチン's user avatar
11 votes
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Molecular orbital diagram of a complex including an oxido ligand

Although chemists and chemistry students alike often indiscriminatorily use the $\mathrm{t_{2g}}$ and $\mathrm{e_g}$ descriptors for the d-orbitals of a d-block metal in coordination complexes, those ...
Jan's user avatar
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