Are siloxane derivatives metal coupling agents (or alternatively, is
silane (which IS a metal coupling agent) a siloxane derivative?)?

I'll take a stab at this topic.  My understanding of a metal coupling
agent is something that allows for strong inorganic-organic bonds.  My
understanding is also that silane (SiH4) the silicon analog of methane
is not a metal coupling agent.  For the record, a siloxane bond is a
silicon-oxygen bond.  Functionalized silane derivatives such as
monoalkyl-trialkoxy silanes are metal coupling agents.  Since silane
is the simplest member of the family, it is not a derivative of any
other organosilicon compound in a traditional sense.  Siloxanes are
the Si-O-Si derivatives of silane though.  Siloxane derivatives are
not a metal coupling agent since in practical terms it is already
"happy".  The Si-O-Si-O-Si... network is incredibly stable and
unreactive.  The previously mentioned alkoxy derivative is not a
stable unreactive compound.  The compound hydrolyzes in water to form
silanol groups and alcohols in place of the alkoxy group
(eg SiR(OR')3 + 3H2O --> 3 R'OH + SiR(OH)3).  These silanol groups are
reactive and will form siloxane bonds with the metal (Si-O-M) via a
condensation reaction.  The alkyl group R will bond to the rest of the
organic phase and you now have a siloxane containing both Si-O-M and
Si-O-Si siloxane bonds, these and the carbon silicon bonds are stable
and pretty unreactive.  Therefore you now have an extremely strong
metal-organic bond.  For more information check out the Chisso corp of
Japan's informative webpage: http://www.chisso.co.jp/fine/en/ace/