- 18 and 16 electron rule pdf merge
- Electron counting
- 18 n 16 e rule ( must watch for NET/SET/GATE EXAM)
- Advanced Inorganic Chemistry/Electron counting and the 18 electron rule
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Though I did read the article on it on Wikipedia, I don't feel I've really grasped the concept well enough in short … I'm kinda lost. So I hope it isn't too much to ask but, is there someone here who's willing to spend some explaining the 18 electron rule, lucidly enough for a 17 year old to understand?
18 and 16 electron rule pdf merge
Or if that isn't possible, could someone recommend a site that explains the rule decently enough? Also could you touch upon some exceptions to the rule at least for period 4 transition metals?
This means that, the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding, and when metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period.
In some respect, it is similar to the octet rule for main group elements, something you might be more familiar with, and thus it may be useful to bear that in mind. So in a sense, there's not much more to it than "electron bookkeeping". As already mentioned in the comments, 18 electron rule is more useful in the context of organometallics. Oxidation state method: We first arrive at the oxidation state of the metal by considering the number of anionic ligands present and overall charge of the complex.
The major premise of this method is that we remove all of the ligands from the metal, but rather than take them to a closed shell state, we do whatever is necessary to make them neutral. Let's consider ammonia once again.
When we remove it from the metal, it is a neutral molecule with one lone pair of electrons. Therefore, as with the ionic model, ammonia is a neutral two electron donor. But we diverge from the ionic model when we consider a ligand such as methyl.
When we remove it from the metal and make the methyl fragment neutral, we have a neutral methyl radical. Both the metal and the methyl radical must donate one electron each to form our metal-ligand bond.
Therefore, the methyl group is a one electron donor, not a two electron donor as it is under the ionic formalism. Where did the other electron "go"? It remains on the metal and is counted there. In the covalent method, metals retain their full complement of d electrons because we never change the oxidation state from zero; i. Fe will always count for 8 electrons regardless of the oxidation state and Ti will always count for four. Bridging anionic eg.
Step 1: Determine the total valence electrons TVE in the entire molecule that is, the number of valence electrons of the metal plus the number of electrons from each ligand and the charge -- I'll call this T T for total, I'm making this up. If the number of electrons is 18, it indicates that there is no M—M bond; if it is 17 electrons, it indicates that there is 1 M—M bond; if it is 16 electrons, it indicates that there are 2 M—M bonds and so on.
Let's use the neutral atom method, W has 6 electrons, the carbonyls donate 12 electrons and we get a total of Of course there can be no metal metal bonds here. Complexes of this kind tend to obey the electron rule irrespective of their coordination number. Tetrahedral complexes cannot exceed 18 electrons because there are no low lying MOs that can be filled to obtain tetrahedral complexes with more than 18 electrons.
In addition, a transition metal complex with the maximum of 10 d-electrons, will receive 8 electrons from the ligands and end up with a total of 18 electrons.
Bulky ligands : eg. Additionally, for early transition metals, e. This is commonly seen with metals and ligands high in the spectrochemical series. Octahedral Complexes which disobey the 18 electron rule, but still have fewer than 18 electrons 12 to Thus, 18 electrons maybe exceeded.
18 n 16 e rule ( must watch for NET/SET/GATE EXAM)
The following weblinks proved useful to me while I was writing this post, especially handy for things like MO diagrams. The Wikipedia lead is as good an explanation as I could do actually, better.
Reading it carefully you'll note that it admits that the rule is violated. Of course, so is the octet rule, especially as you go down the table.
At some point in learning chemistry, simple "rules" are more trouble than they are worth. I suggest that this is one of those times.
Advanced Inorganic Chemistry/Electron counting and the 18 electron rule
I suggest that you hold it in the same regard, that it's better than nothing, but not by much and there are almost certainly better alternatives to use to predict coordination in any specific case you are given except on an exam, perhaps. What are the alternatives?
Well, similar complexes for the most obvious. By similar I mean isoelectronic or similarly sized or, even better, both! Home Questions Tags Users Unanswered.
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Jan It is more for organometallics. Would this rule apply fairly well to those? So in a sense, there's not much more to it than "electron bookkeeping" As already mentioned in the comments, 18 electron rule is more useful in the context of organometallics. Ligand Electron Contribution for neutral atom method a.
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Neutral Terminal eg. Anionic Terminal eg. Hapto Ligands eg. Bridging neutral eg. Bridging alkyne 4 electrons h.
The 18-electron rule and electron counting in transition metal compounds: theory and application
NO linear 3 electrons i. NO bent lone pair on nitrogen : 1 electron j. At this point, let's apply this method to a few examples a Tungsten Hexacarbonyl picture Let's use the neutral atom method, W has 6 electrons, the carbonyls donate 12 electrons and we get a total of A few examples where the "18 electron Rule" works I. Tetrahedral Complexes e. Violations of 18 electron rule I.
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