Chemistry - Metal Compounds that bond covalently
Solution 1:
There is no sharp line between ionic, metallic or covalent bonds.
Most of transition metal oxides and sulfates have strong covalent characteristics. For example, the whole reason of using ligand field theory instead of crystal field theory is that the ionic description (CFT) breaks down, and the covalent effects are important in all but the simplest cases.
If you want to take even more covalent molecules, look for organometallic compounds, metal carbonyls, metallocenes, etc.
Solution 2:
There are examples of metal compounds that are regarded by a majority of chemists as covalent in organometallic chemistry. The problem is that not everyone regards some of these compounds as covalent, because the border between ionic and covalent isn't so strict and it doesn't matter.
Alkyllithium compounds and Grignard reagents ($\ce{R-Li}, \ce{RMgX}$) are considered to be covalent, and they are represented like this $\ce{CH_3CH_2-Li, CH_3-Mg^+I^-}$ (no +,- signs above the carbon and lithium and only the magnesium-halogen bond in Grignard reagents is ionic), while alkyl-sodium and potassium compounds are considered to be ionic (like sodium-acetylide).
An Organoaluminium example of a covalent compound is $\ce{Al(C_2H_5)_3}$.
Organocopper exhibits covalent bonds between copper and carbon, like in $\ce{(CH_3)_2CuLi}$.
There are many more examples in organometallic chemistry.
The d-block metals have significant polarizability and thus the bonds they participate in can have a significant covalent character like in $\ce{RuO_4}$, $\ce{Hg(CH_3COO)_2}$, $\ce{Pb(CH_3COO)_4}$, $\ce{CuI}$, etc.
Mercury(I) exists in solution as $\ce{Hg_2^{2+}}$ where the two mercury ions are covalently bonded.
Solution 3:
There is no sharp border between ionic and covalent bond. Generally, single-atom anions are not stable enough to justify full transfer of an electron from cation, so simple binary compounds usually have significant covalent character, even if their crystal structure suggests otherwise. For example, $\ce{CsCl}$ said to have 25% covalent character. On the opposite end, heteroatomic bonds usually have some ionic character. Given that, we don't have clear groups of ionic and covalent metal compounds, but rather a spectrum of them.
- $\ce{CsClO4}$ would be an almost pure ionic compound
- $\ce{CsCl}$ would be mostly ionic
- $\ce{AlCl3}$ would be highly polar, but significantly covalent
- $\ce{CuS}$ would be almost purely covalent
- $\ce{Li2}$ in gas phase is purely covalent
To get a structure with purely covalent bond we have to consider homoatomic bonds with atoms in identical neighborhood, otherwise some amount of polarity and consequently ionic bonding will arise. Such molecules with metal-metal bonds do exist.
Let's start with the most clear (and, admittedly, least chemically significant) example: group one metal diatomic molecules. Such molecules do exist in gas phase and can be detected spectroscopically, though the bond energy is quite low because of low density and relatively insignificant overlapping of the orbitals.
For group two there is a report of a rather complex compound with a Mg-Mg bond
For the third row there are reports of various compounds with the most simple being $\ce{((((CH3)3Si)2CH)2Al)2}$ with a direct Al-Al bond.
There are reports of $\ce{Sn2H6}$, i.e. $\ce{(SnH3)2}$
However, it must be noted that with atoms of the lower-left corner of periodic table homoatomic bonds are very unstable, to the point of impossibility to obtain stable compounds.
Many more examples are known for d-elements. For starters, $\ce{(Mn(CO)5)2}$ is a relatively stable compound with a singular Mn-Mn bond. However, d-elements favor complex structures with unusual delocalized, but clearly covalent bonding, similar to boranes. Such structures are especially common for late transition metals, say, from the Cr-W-Pt-Ni rectangle. An example would be $\ce{Rh6(CO)16}$, adopting structure with $\ce{Rh}$ atoms forming an (almost) perfect octahedron.
Looking from another side, we can look for molecules with metal as a central atom. They are much more common in regular chemistry, though the nature of bonding may be sometimes disputed. For example, copper acetate adopts an unusual paddle-wheel structure, but the copper-oxygen bond may be argued to be ionic. On the other hand, molecules with high d-element oxidation state usually have an effective charge of the central atom far below its formal oxidation state, so such compounds may be considered highly polar, but still covalent in nature. This said, many higher chlorides, oxochlorides and fluorides may be considered covalent, such as $\ce{TiCl4}$, $\ce{VCl4}$, $\ce{CrO2Cl2}$, $\ce{OsO4}$, $\ce{WF6}$, $\ce{AuF5}$.
Looking from third side we can argue to be mostly covalent compounds where metal is bound to radical, that clearly cannot stabilize a negative charge, such as $\ce{CO}$ molecule or $\ce{CH3}$ and some other organic fragments. Such compounds are known for most metals in the periodic table, say, $\ce{LiCH3}$, $\ce{CH3MgCl}$ (the nature of these compounds is more complex than is usually written), $\ce{Al2(CH3)6}$, $\ce{PbEt4}$ (tetraethyllead), and many d-element compounds, such as $\ce{(CH3)2CuLi}$.
Here I mostly avoided a huge can of worms named 'coordination compounds', which includes (mostly) interactions of d-element atoms with different electron pair donors. Some of such compounds may be argued to be mostly covalent, such as most metal carbonyles, like $\ce{Cr(CO)6}$, but some may be argued to be mostly ionic, such as $\ce{[FeF6]^{3-}}$, or ion-dipole, such as $\ce{[Co(NH3)6]^{2+}}$
Solution 4:
There is no clear separation between ionic and covalent bonds. Electronegativity values are useful in determining if a bond is to be classified as nonpolar covalent, polar covalent or ionic. The rule is that when the electronegativity difference is greater than 2.0 (Electronegativity according to Pauling), the bond is considered ionic. If the electronegativity difference is less than 0.5, then the bond is nonpolar covalent. If the electronegativity difference is between 0.5 and 1.6, the bond is considered polar covalent. For this reason $\ce{CaO}$ is ionic, while $\ce{Be3N2}$ is covalent.