Classes of Organometallic Compounds - Where do I put Grignard Reagents?

[Note- I've already seen this question [Why do Magnesium and Lithium form \*covalent\* organometallic compounds?](https://chemistry.stackexchange.com/questions/429/why-do-magnesium-and-lithium-form-covalent-organometallic-compounds) as user Mithoron was kind enough to point out, but the problem that I face is different though. MY problem here, is that if I'm asked whether Grignard reagents are a)Ionic compounds of electropositive metals or are they b) Sigma bonded Complexes what should my answer be?]


My school textbook defines Organometallics (A section under the chapter 'Coordination Chemistry') as "*Compounds where the metal atom is directly linked to a carbon atom(s)* " , and then it proceeded to give the following classes of Organometallics:



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1) Ionic Compounds of Electropositive Metals

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Includes compounds like Alkyl Sodium, Phenyl Lithium and Alkyl Magnesium Halides (Grignard Reagents)



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2) Sigma Bonded Complexes

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Includes the likes of Trimethyl Aluminium



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3) Pi Bonded Complexes

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Includes Zeise's salt and Ferrocene



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4) Sigma and Pi Bonded Complexes

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Includes transition metal carbonyls like Ni[(CO)\_4]


Now the thing is, some other (relatively unheard of) reference material places Grignard reagents under Sigma bonded complexes. Now I'm not sure which one's right, or if both are right, or if both of them are wrong, since in my brain the R-Mg bond in the grignard reagents are *polar covalent*, i.e- smack in the middle, between covalent and ionic bonding.


So going by convention (if there even *is* a convention for something like this), I reiterate my question, which one's right? 


[Additionally if there are more classes of Organometallic compounds that I haven't mentioned, I'd really appreciate it if someone would add on to it...]


Thanks!


