47 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, ...
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  • 65.5k
42 votes
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Why is the bond angle H-P-H smaller than H-N-H?

Starting point: 2s orbitals are lower in energy than 2p orbitals. The $\ce{H-N-H}$ bond angle in ammonia is around 107 degrees. Therefore, the nitrogen atom in ammonia is roughly $\ce{sp^3}$ ...
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  • 81.6k
38 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,...
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  • 65.5k
32 votes
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What are t2g and eg in CFT?

As stated in both the links Geoff and Philipp have kindly commented (1, 2) they are to do with symmetry labels we chemists like to assign to orbitals. Knowing an orbitals symmetry class can lead to a ...
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  • 1,973
31 votes
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What is a non-classical carbocation?

Here is a picture of a "classical" carbocation, there is an electron deficient carbon bearing a positive charge. There are many examples of "non-classical" carbocations, but the 2-norbornyl ...
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  • 81.6k
30 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 ...
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  • 16.8k
28 votes

When is it true that more nodes equals higher energy?

General case There is indeed a mathematical theorem that deals with the number of nodes an eigenfunction corresponding to a certain eigenvalue can possess. It was laid down by Courant$^{[1, 2]}$ and ...
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  • 17.2k
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 ...
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  • 4,759
24 votes

Molecular orbital (MO) diagram for N2 and N2^-

I have been taught that the MO diagram is different for molecules with 14 or less electrons than the one used for molecules with 15 or more electrons. This is (partly) wrong because the change in ...
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  • 18.4k
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, ...
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  • 16.8k
23 votes

How can the dipole moment of carbon monoxide be rationalised by molecular orbital theory?

Unfortunately, nothing in the bonding situation in carbon monoxide is easily explained, especially not the dipole moment. According to the electronegativities of the elements, you would expect the ...
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22 votes

Physical intuition behind negative values for wave function?

The wavefunction of a particle actually has no physical interpretation to it until an operator is applied to it such as the Hamiltonian operator, or if you square it which gives its probability of ...
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22 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 ...
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21 votes

Why is the bond length of CO+ less than that of CO?

Unfortunately, the arguments presented by buckminst and Uncle Al aren't completely right. The MO schemes are correct but the ...
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  • 17.2k
21 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 ...
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  • 65.5k
19 votes
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What is actually the difference between valence bond theory and molecular orbital theory?

TL;DR VB theory treats atomic orbitals (including hybridized orbitals) as providing a good mathematical/physical description of the true form of the molecular wavefunction. MO theory uses atomic ...
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  • 16.8k
19 votes

Sp5 hybridization in cyclopropane?

Until this post, $\mathrm{sp^1,~sp^2~\text{and}~sp^3}$ meant for me, that the hybrid orbitals would consist from one s orbital and one, two or three p orbitals. I thought of it being so, as this seems ...
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19 votes
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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 ...
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  • 63.6k
19 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–...
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18 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 ...
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18 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 ...
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  • 81.6k
17 votes

Delocalization of π-electrons in nitrate ion

Unfortunately the key to understanding the delocalisation of electrons lies in understanding rudimentary molecular orbital theory. It is also necessary to understand that hybridisation is a ...
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17 votes
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What is the usage of orbitals more complex than f orbitals?

Surprisingly, I learned that there are also usages for orbitals g,h,i and even j. Actually, the letter "j" is not used, so it is s, p, d, f, g, h, i, k, l, etc. The higher angular momentum ...
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  • 38.8k
17 votes
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What does it mean when it is said that an sp³ orbital has 25% s character?

Hybridisation is a purely mathematical concept, which makes it possible to explain experimentally found structures. The most prominent example for this is methane, where you can consider the central ...
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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 ...
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17 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 ...
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  • 65.5k
16 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 ($\mathrm{3d}$...
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  • 63.6k
16 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 ...
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  • 23.2k
16 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 ...
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  • 21.7k
15 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 ...
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