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Topic: coordination (semi organic question)  (Read 2050 times)

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Offline ajolly

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coordination (semi organic question)
« on: July 17, 2013, 11:29:10 PM »
How come in coordination compounds the bonding ligand does not receive a charge reflecting its newly established bond?  For example in cisplatin the amine groups are neutral in charge (however they have four bonds) but if the amine was bound to an organic molecule it would receive a formal charge of +1.

Furthermore, why would the oxygen in H2O ever want to coordinate as oxygen typically does not "like" to freely give/share electrons with other atoms?

Sorry to ask what might seem as obvious questions, I am but an undergrad who has taken only 5 semesters worth of chemistry.
CE27 + HE27 + OE27 + NE26 + PE25 + SE24 + CaE25 + KE24 + ClE24 + NaE24 + MgE24 + FeE23 +  FE23 + ZnE22 + SiE22 + CuE21 + BE21 + IE20 + SnE20 + MnE20 + SeE20 + CrE20 + NiE20 + MoE19 + CoE19 + VE18 ='S Me (not methane)

Offline gritch

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Re: coordination (semi organic question)
« Reply #1 on: July 18, 2013, 12:13:41 AM »
Oh oh! Inorganic chemistry question! Hooray!

Concepts such as formal charges can basically be thrown away when we're talking about coordination chemistry in general. Bonding with d-block metals is typically more a complicated affair than in organic molecules. Formal charges a nice shortcut and they help explain reactivity in simple terms but they're not always applicable to coordinate complexes.

In early inorganic classes they generally teach these bonds are dative, basically the metals centers are very electron efficient and only the ligand donates electron density toward the metal-ligand bond. That's not typically true though, there is quite often covalent contribution from the metal's d-orbitals especially if the metal in question is more electron rich (late d-block metals) and if the ligand has orbitals of similar energy to accept the electron density (such as thiols, phosphines, or carbonyl).

As for why water would bother to do this in the first place, we'd have to take a look at molecular orbital diagram of a relevant complex. I assume you'll eventually learn this (if I assume you'll be taking any upper level inorganic classes) so I'll just sort of lay out the general idea. Basically by interacting with the d-orbitals of the metal centers the energy of the metal center's orbitals can be lowered thus forming a more stable species. The benefit is mostly for stabilizing the metal rather than the ligand.

And to answer your last question there, it actually shouldn't be too surprising to see water "give/share" to another atom. Remember water itself will hydrolysis into hydronium and hydroxide spontaneously, in which the oxygen gives some of its electrons to a proton. A metal center is much more electron efficient than a proton and we would expect water to bind to it as well.

I like inorganic chemistry...

Offline ajolly

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Re: coordination (semi organic question)
« Reply #2 on: July 18, 2013, 12:47:28 AM »
gritch honestly that was a very informative post, thanks!! So am I right in assuming that where organic chemistry is based on "rules of thumb"...at least at the lower undergraduate level, inorganic chemistry is based more on MO theory? Also should I forget about the rules I have learned in organic and biochemistry regarding bonding (ie nucleophile, electrophile attacks and basicity of the leaving group and etc) when looking at coordination complexes.  In addition what do you mean by electron efficiency? This is not a term I am familiar with.  Sorry to ask so many questions..
CE27 + HE27 + OE27 + NE26 + PE25 + SE24 + CaE25 + KE24 + ClE24 + NaE24 + MgE24 + FeE23 +  FE23 + ZnE22 + SiE22 + CuE21 + BE21 + IE20 + SnE20 + MnE20 + SeE20 + CrE20 + NiE20 + MoE19 + CoE19 + VE18 ='S Me (not methane)

Offline gritch

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Re: coordination (semi organic question)
« Reply #3 on: July 19, 2013, 01:56:04 PM »
gritch honestly that was a very informative post, thanks!! So am I right in assuming that where organic chemistry is based on "rules of thumb"...at least at the lower undergraduate level, inorganic chemistry is based more on MO theory? Also should I forget about the rules I have learned in organic and biochemistry regarding bonding (ie nucleophile, electrophile attacks and basicity of the leaving group and etc) when looking at coordination complexes.  In addition what do you mean by electron efficiency? This is not a term I am familiar with.  Sorry to ask so many questions..

Inorganic chemistry does tend to emphasis MO more than organic, though MO definitely works its way into higher level organic classes as well. I wouldn't say you should forget your rules from organic chemistry entirely they're still a good background you just need to bear in mind the extra complications involved with adding a metal center into the equation. Especially when dealing with catalysis those organic concepts are often readily used to explain reactivity.

These organic rules are still useful as a reference when dealing with coordinate bonding, for example a good leaving group like trifilate is also a very weakly coordinating anion (if it bonds at all) in coordination complexes, and an electron rich aromatic ring such as cyclopentadiene binds strongly with an electron deficient metal center.

For your last question, it should be "electron deficient"  rather than efficient, meaning the metal centers have a lack of electrons relative to the ligands attached to them. That's a pretty bad typo on my part. My apologies.

But don't worry about asking so many questions, it's a good thing to want to learn more.

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