69 votes
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Why is it wrong to use the concept of hybridization for transition metal complexes?

Tetrahedral complexes Let's consider, for example, a tetrahedral $\ce{Ni(II)}$ complex ($\mathrm{d^8}$), like $\ce{[NiCl4]^2-}$. According to hybridisation theory, the central nickel ion has $\mathrm{...
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57 votes
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Why isn't the American nickel magnetic?

There are many types of magnetic properties, including ferromagnetism, paramagnetism, diamagnetism, antiferromagnetism, ferrimagnetism, superparamagnetism, metamagnetism, spin glasses, and ...
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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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26 votes
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Can metals have a net negative charge

These species usually do not exist in nature, but they can be synthesized. Silver has been reduced in liquid ammonia to give $\ce{Ag-}$. A lot of anionic metal carbonyl complexes $\ce{M(CO)_{n}^{m-}}$...
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25 votes

Why isn't the American nickel magnetic?

None of the US coins are magnetic (ferromagnetic), except for the 1943 Lincoln penny (Steel Cents, made in steel and zinc to save copper for ammunition during wartime), which are considered magnetic. ...
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23 votes
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Why do transition elements make colored compounds?

You are absolutely correct, it all about the metal's electrons and also about their d orbitals. Transition elements are usually characterised by having d orbitals. Now when the metal is not bonded ...
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23 votes
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What is the oxidation state of Mn in HMn(CO)5?

On negative oxidation states, in general Although it's usually a topic that's covered relatively late in a chemistry education, negative oxidation states for transition metals[1] are actually quite ...
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22 votes
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Why do 3d orbitals have lesser energy than 4s orbitals in transition metals?

Disclaimer: I now believe this answer to be fully incorrect. Please consider un-upvoting it and/or downvoting it. I do not like seeing incorrect answers at +22. However, I will leave it up ...
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22 votes
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Why is WF6 stable whereas CrF6 is unknown?

The answer has to do with two things. Note that HSAB theory is dubious at best and doesn't have very useful predictive power, so I am going to avoid talking about it. (1) The accessibility of the high ...
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22 votes
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Why is manganese(II) coloured although the transition should be spin-forbidden?

Selection rules The intensity of the transition from a state $\mathrm{i}$ to a state $\mathrm{f}$ is governed by the transition dipole moment $\mu_{\mathrm{fi}}$ (strictly, it is proportional to $|\...
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20 votes
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Why does the Co³⁺/Co²⁺ couple have such a high reduction potential?

The electronic configuration has nothing to do with it. The reduction potentials of $\ce{Ni^3+}/\ce{Ni^2+}$, $\ce{Cu^3+}/\ce{Cu^2+}$ and $\ce{Zn^3+}/\ce{Zn^2+}$, if they have been/could be measured, ...
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19 votes

Why isn't the orbital angular momentum also considered while calculating the magnetic moments 3d transition elements?

Simplistically speaking orbital angular momentum is present when some conditions are satisfied: A set of orbitals are degenerate; These orbitals can be "interconverted" by rotation about a certain ...
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What is the origin of the colour of azo dyes?

Yes, it is all about the absorption of light at specific wavelength. Azobenzene, the parent compound has an absorption maximum around $\lambda$= 430 nm in the visible spectrum. The interesting part ...
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Why is tetraamminecopper(II) a square planar and not a tetrahedral species?

It is very convenient to use crystal field theory to discuss this. It is usually assumed that in octahedral coordination the energy levels of the five d-orbitals are split, with two orbitals ($d_{z^...
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17 votes
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Is iron the most stable element in the periodic table?

Yes, $^{56}\ce{Fe}$ has the most stable nucleus, and $\ce{He}$ is the most chemically inert element. These are different and unrelated qualities, pretty much like physical fitness and intelligence in ...
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Why copper(I) is unstable in aqueous medium?

You are quite correct in that it appears at first sight that $\ce{Cu+}$ should be more stable than $\ce{Cu^2+}$, but in aqueous media it isn’t. Stability in aqueous conditions depends on the ...
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What is the Structure of FeSO₄ • NO?

What is the structure of $\ce{FeSO4 \cdot NO}$ that is formed when $\ce{NO}$ is passed through ferrous sulphate solution? The structure is octahedral. The Fe ion is at the center of the octahedron. ...
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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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15 votes
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Cr(II) and Mn(III) - their oxidizing and reducing properties?

Related question with same answer but in a different context of the 4f block: Why don't we see these lanthanide species? You have a misconception regarding the stability of oxidation states. The ...
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15 votes
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While filling electrons, we follow Aufbau principle, but not while removing them. Why is this so?

Usually when adding electrons based on the Aufbau principle, you go from one element to the next highest one, e.g. from $\ce{Ti}: \ce{[Ar] 4s^2 3d^2}$ to $\ce{V: [Ar] 4s^2 3d^3}$. Thus you add not ...
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14 votes
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Why is zinc more reactive than copper?

You have to think about the whole process. When a metal loses electrons to make a metal ion the following happens: The metallic bonds holding the metal atoms together are broken. The metal atom ...
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Why can mercury(I) exist, but not zinc(I)?

On the contrary, zinc(I) compounds do exist, though they are rare, and relatively unstable. Most zinc(I) compounds contain a $\ce{[Zn2]^{2+}}$ core, which is analogous to the $\ce{[Hg2]^{2+}}$ cation. ...
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14 votes

Why is anhydrous copper(II) sulfate white while the pentahydrate is blue, even though both have one unpaired electron?

Although the question is a bit old, I think it still hasn't been answered yet. And by looking at the given answers it seems like the discussion went into a different direction at some point. So let's ...
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14 votes

Why is Ni[(PPh₃)₂Cl₂] tetrahedral?

We sometimes call this type of complex 'pseudotetrahedral' since there is an isomerism from a tetrahedral to a square planar complex possible. I was unable to find the original work here but this link ...
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13 votes

What is the origin of the colour of azo dyes?

What is the origin of colours? Most of the colours that we perceive are originate by the selective absorption of some spectral bands and the reflection of the others wavelength, some times with the ...
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13 votes
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What is the most common oxidation state of gold?

It's not obvious, but common oxidation state for gold is +3. It caused by destabilization of the $5d^{10}$ orbital. Detailed explanation you can find in The Chemistry of Gold, in Chapter 1.1.3.
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13 votes
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How do I find the ground state term symbol for transition metal complexes?

To find the ground state term symbol, you should be using symmetry and group theory arguments, you shouldn't have to resort to searching Tanabe-Sugano diagrams to get the answer. We'll start with ...
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13 votes
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Derivation of the Orgel diagram for octahedral d2 complexes

1. Weak-field and strong-field limits I will adopt the description used in Figgis and Hitchman's Ligand Field Theory and Its Applications (p 5), because I cannot really phrase it better: It is ...
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13 votes

Exchange energy of d6 configuration

As @orthocresol points out, the key is that you need to compare the exchange energy before vs after the ionization process. Anything that is unchanged by ionization cannot affect ionization energy. ...
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