With the help of enterprise wsdl, I have been trying to make upsert call in Contact where I need to relate the contact with Account . I have an external Id field on Account and trying to use it for upsert. I am getting the below Error.

<sf:exceptionCode>INVALID_TYPE</sf:exceptionCode>
           <sf:exceptionMessage>Must send a concrete entity type.</sf:exceptionMessage>
           <sf:row>-1</sf:row>
           <sf:column>-1</sf:column>

Below is the request, I am making....please let me know whats wrong here. I am making this request as sys admin.. and have all permission for this object.

<urn:upsert>
        <urn:externalIDFieldName>Account.Partner_Number_HDS__c</urn:externalIDFieldName>
            <urn:sObjects xsi:type="urn1:Contact"> <!--Zero or more repetitions:--> 
             <FirstName>EnterA</FirstName>
              <LastName>EnterB</LastName>
            <Department>Sales</Department>
            <Job_Title_HDS__c>Account Manager</Job_Title_HDS__c>
            <MobilePhone>456768985</MobilePhone>
            <Email>[email protected]</Email>
            <Account.Partner_Number_HDS__c>PA-74</Account.Partner_Number_HDS__c>

</urn:sObjects> </urn:upsert>

Thanks