You Generally Can't Apply Metamagic Feats to Invocations.
However, a warlock’s arcane repertoire is even more limited than that of a sorcerer, and his invocations are spell-like abilities, not spells.
Warlocks and other creatures with spell-like abilities might find the following feats useful.
Combat Casting: This feat works equally well with spells, invocations, or spell-like abilities.
Spell Penetration: Spell Penetration and Greater Spell Penetration have the same effect on invocations and spell-like abilities that they do on normal spells.
Sudden Metamagic Feats: These metamagic feats don't require modified spell slots, and so they work as well with spell-like abilities or invocations as they do with spells (though because spell-like abilities don't have verbal or somatic components, Sudden Silent Spell doesn’t apply and Sudden Still Spell applies only to invocations).
Creatures with spell-like abilities at a high enough level will find sudden metamagic feats less useful than the dedicated feats Empower Spell-Like Ability and Quicken Spell-Like Ability (see page 303 of the Monster Manual), as well as the Maximize Spell-Like Ability feat introduced in this chapter.
Other Metamagic Feats: Except as noted above, metamagic feats can’t generally be used to modify spell-like abilities or invocations.
Metamagic rods provide the same benefit as a metamagic feat a number of times per day. It could be argued that since there is no change in spell level when using a metamagic rod, and it emulates a sudden metamagic feat, then it should be applied to invocations. Especially since it is implied in Complete Arcane that metamagic feats that do not change spell level can be applied to spell-like abilities (credit: KRyan).
Argument aside, the SRD for metamagic rods, as it is written, does not say it applies to spell-like abilities or invocations. It states that it provides the benefit of a metamagic feat, a number of times per day - those same metamagic feats that don't apply to spell-like abilities or invocations.
What could add more depth and dimension to spell-like abilities and invocators, such as warlocks, could be to allow the use of metamagic rods. It would not be game-breaking by any means and could give more importance to often overlooked and not too expensive magic items such as metamagic rods.
Metamagic Rods: Metamagic rods hold the essence of a metamagic feat but do not change the spell slot of the altered spell. All the rods described here are use-activated (but casting spells in a threatened area still draws an attack of opportunity). A caster may only use one metamagic rod on any given spell, but it is permissible to combine a rod with metamagic feats possessed by the rod’s wielder. In this case, only the feats possessed by the wielder adjust the spell slot of the spell being cast.
Possession of a metamagic rod does not confer the associated feat on the owner, only the ability to use the given feat a specified number of times per day. A sorcerer still must take a full-round action when using a metamagic rod, just as if using a metamagic feat he possesses.
Lesser and Greater Metamagic Rods: Normal metamagic rods can be used with spells of 6th level or lower. Lesser rods can be used with spells of 3rd level or lower, while greater rods can be used with spells of 9th level or lower.